Physics Book - User contributions [en]

Types of Interactions and How to Detect Them

2026-11-09T20:03:28Z

Rmedagam6: /* Main Idea */

'''Ronika Medagam, Fall 2026'''
== Main Idea ==

In physics, an '''interaction''' occurs when two objects influence one another. These interactions can change an object's motion, transfer energy, or cause it to change shape. In introductory mechanics, interactions are primarily studied through the '''forces''' that objects exert on one another.

An important principle is that interactions are mutual: if object A interacts with object B, then object B also interacts with object A. For example, when Earth pulls a ball downward through gravity, the ball also pulls Earth toward itself through gravity. These two forces are '''equal in magnitude and opposite in direction''', but they act on '''different objects'''. This is an example of Newton's Third Law.

A helpful way to understand interactions is to ask two questions:

# '''Which objects are interacting?''' Identify the two objects involved.
# '''How are they interacting?''' Determine whether the interaction involves physical contact, gravity, electricity, or magnetism.

For example:

* '''Earth and a falling ball:''' Earth attracts the ball through a gravitational interaction. The ball also gravitationally attracts Earth.
* '''A table and a book:''' The table pushes upward on the book, while the book pushes downward on the table through a contact interaction.
* '''A rope and a cart:''' The rope pulls on the cart through tension, while the cart pulls on the rope.
* '''Air and a moving object:''' Air exerts a drag force on the object, while the object also exerts a force on the surrounding air.

=== Why Do Interactions Matter? ===

Interactions provide the physical explanation for why forces exist. Instead of simply memorizing a list of forces, we can identify the objects involved and determine how they influence one another.

This is especially useful when drawing a '''free-body diagram (FBD)'''. A free-body diagram shows all the forces acting on one selected object. To create one, first identify the other objects interacting with your chosen object. Then represent the forces those objects exert on it.

For example, consider a book resting on a table. There are two interactions involving the book:

* '''Earth and the book:''' Earth pulls the book downward through gravity.
* '''The table and the book:''' The table pushes the book upward through a contact interaction.

Therefore, the book's free-body diagram contains two forces:

* Gravitational force (weight) pointing downward.
* Normal force from the table pointing upward.

Because the book is at rest, these forces balance, resulting in zero net force.

<math>\vec{F}_{net}=0</math>

Understanding interactions is therefore the first step toward identifying forces, drawing accurate free-body diagrams, and applying Newton's laws of motion.

=== Important Distinction: Interaction Pairs ===

Newton's Third Law states that forces always occur in equal-and-opposite pairs. However, the two forces in an interaction pair act on different objects.

For example, when a table pushes upward on a book:

* The table pushes upward on the book.
* The book pushes downward on the table.

These forces do not cancel each other out on the book's free-body diagram because they act on different objects. Only forces acting on the same object are included in that object's free-body diagram.

This distinction is important when solving force and motion problems.

==Types of Interactions==
Interactions in introductory physics are often grouped into two main categories:

==Contact Interactions==

Contact interactions occur when two objects physically touch and exert forces on one another. These interactions are common in everyday situations, such as pushing an object, walking, or placing an object on a table.

The main types of contact interactions include:

===Normal Force===
The normal force is a support force exerted by a surface on an object touching it. It always acts perpendicular to the surface.

Example: A table pushes upward on a book resting on it.

How to identify it:
* Look for an object resting on or pressing against a surface.
* The normal force acts perpendicular to the surface, away from it.

===Friction===
Friction is a force that acts between surfaces in contact. It opposes relative sliding or the tendency for surfaces to slide against each other.

There are two main types:
* Static friction prevents surfaces from sliding relative to each other.
* Kinetic friction acts when two surfaces are sliding against each other.

Example: Friction between your shoes and the ground helps you walk without slipping.

How to identify it:
* Look for surfaces that are sliding or attempting to slide against each other.
* Determine which direction the surfaces are sliding or tending to slide relative to one another.

===Tension===
Tension is a pulling force transmitted through a rope, string, cable, or similar object when it is pulled tight.

Tension acts along the length of the rope and pulls away from the object attached to it.

Example: A rope pulls a bucket upward when someone lifts it from a well.

How to identify it:
* Look for ropes, strings, or cables attached to an object.
* The tension force points along the rope, away from the object.

===Compression===
Compression occurs when an object is squeezed or pressed by another object. It can cause an object to shorten, bend, or change shape.

Example: A spring becomes compressed when someone pushes its ends toward each other.

How to identify it:
* Look for objects being squeezed or pushed together.
* Consider how the contact forces act on the object.

===Spring Force===
A spring exerts a force when it is stretched or compressed. This force acts to return the spring toward its natural, unstretched position.

Example: A compressed spring pushes a block away from it.

The spring force can be modeled using Hooke's Law:

F_s = -kx

Where:
* F_s is the spring force.
* k is the spring constant.
* x is the displacement from the spring's equilibrium position.

The negative sign indicates that the spring force acts opposite the displacement.

How to identify it:
* Look for a stretched or compressed spring attached to or pushing against an object.

===Air Resistance and Drag===
Drag is a force exerted by a fluid, such as air or water, on an object moving through it. Air resistance is a specific type of drag.

Drag generally acts opposite the object's motion relative to the surrounding fluid.

Example: A parachute experiences air resistance as it falls, helping slow its descent.

How to identify it:
* Look for objects moving through air or water.
* Pay attention to problems that mention air resistance, fluid resistance, or drag.

===Collisions===
A collision occurs when two objects come into contact and exert forces on each other over a period of time. These forces can cause changes in velocity, direction, or shape.

Example: When a tennis racket hits a tennis ball, the racket exerts a force on the ball, changing its motion.

How to identify it:
* Look for objects hitting, striking, or bouncing off one another.
* Consider how the collision changes the objects' motion.

==Non-Contact Interactions==

Non-contact interactions occur when objects exert forces on one another without physically touching. These interactions can act over a distance.

There are three major types:

===Gravitational Interaction===
Gravity is an attractive interaction between objects with mass. Every object with mass attracts every other object with mass.

Near Earth's surface, gravity pulls objects downward toward Earth's center.

Example: Earth attracts a falling apple, causing it to accelerate downward.

How to identify it:
* Consider gravitational interactions whenever objects have mass.
* In most introductory mechanics problems near Earth, gravitational force is represented by the object's weight.

===Electric Interaction===
Electric interactions occur between objects that have electric charge. Depending on the charges involved, objects can attract or repel one another.

* Opposite charges attract.
* Like charges repel.

Example: A negatively charged balloon can attract a positively charged object.

How to identify it:
* Look for charged objects, static electricity, or problems involving electric charges.

===Magnetic Interaction===
Magnetic interactions occur between magnets, magnetic materials, and moving electric charges. Magnets can attract or repel depending on their poles and the materials involved.

* Opposite magnetic poles attract.
* Like magnetic poles repel.

Example: A magnet attracts a paper clip without physically touching it.

How to identify it:
* Look for magnets, magnetic poles, or magnetic materials.

==Comparing Contact and Non-Contact Interactions==

{| class="wikitable"
! Feature
! Contact Interactions
! Non-Contact Interactions
|-
| Physical contact required?
| Yes
| No
|-
| How the force occurs
| Through physical contact between objects
| Through an interaction that acts over a distance
|-
| Examples
| Normal force, friction, tension, drag
| Gravity, electric, magnetic
|-
| Everyday example
| Pushing a shopping cart
| A magnet attracting a paper clip
|}

An important distinction is that air resistance is classified as a contact interaction because air molecules physically interact with the object's surface, even though these individual collisions are too small to see.

An object can also experience multiple interactions at the same time. For example, a falling object moving through air experiences both gravitational and drag interactions.

==How to Detect Them==

Interactions are not always directly visible. However, we can identify them by examining an object's motion, surroundings, and interactions with other objects. A useful approach is to identify the object being studied and determine which other objects could be exerting forces on it.

===1. Look for Changes in Velocity===

One way to detect an interaction is by observing a change in an object's velocity. Velocity includes both speed and direction, so an object can accelerate by speeding up, slowing down, or changing direction.

According to Newton's Second Law:

<math>\vec{F}_{net} = m\vec{a}</math>

where:
* <math>\vec{F}_{net}</math> is the net force acting on the object
* <math>m</math> is the object's mass
* <math>\vec{a}</math> is its acceleration

If an object accelerates, its net force must be nonzero. This means the forces produced by the interactions acting on the object do not balance.

Example:

A planet orbiting a star continuously changes the direction of its velocity. Even if its speed remains constant, its velocity is changing. This indicates that a net gravitational force is acting toward the star.

Another example is a car slowing down after the driver applies the brakes. The car's velocity decreases because forces from interactions between the tires, road, and braking system produce a net force opposite its motion.

===2. Equilibrium Does Not Mean No Forces===

An object can experience multiple interactions even when its velocity is constant. If an object is in equilibrium, the net force acting on it is zero:

<math>\vec{F}_{net}=0</math>

This means the forces balance one another, not that no forces are present.

Example:

Consider an object falling through Earth's atmosphere at terminal velocity. Its velocity is constant, so its acceleration is zero and its net force is zero.

However, two interactions are still occurring:
* Earth exerts a gravitational force downward.
* Air exerts a drag force upward.

These forces are equal in magnitude and opposite in direction, so they cancel.

Another example is a book resting on a table. Gravity pulls the book downward, while the table exerts an upward normal force. The book remains stationary because these forces balance.

These examples demonstrate that observing no change in motion does not mean there are no interactions.

===3. Examine the Object's Surroundings===

The physical setup of a problem often provides clues about which interactions are present. Instead of only looking at how an object moves, consider what other objects are nearby or touching it.

Common clues include:

* If an object rests on or presses against a surface, consider a normal force.
* If two surfaces are sliding or attempting to slide against each other, consider friction.
* If a rope, cable, or string is attached to an object, consider tension.
* If a spring is stretched or compressed, consider a spring force.
* If an object moves through air or water, consider drag.
* If an object is near Earth or another massive object, consider gravity.
* If charged objects are involved, consider electric interactions.
* If magnets or magnetic materials are involved, consider magnetic interactions.

Remember that the presence of an object does not automatically mean every possible force is acting. For example, two surfaces can touch without experiencing friction if there is no frictional force in the situation.

===4. Use an Interaction Diagram===

An interaction diagram is a useful tool for identifying forces before drawing a free-body diagram.

It shows the object being studied and the other objects interacting with it.

To create one:

# Choose the object you want to analyze.
# Identify the other objects that interact with it.
# Classify each interaction as contact or non-contact.
# Use these interactions to determine which forces act on your chosen object.

Example:

Consider a book resting on a table.

The objects involved are:
* The book
* The table
* Earth

The interactions are:
* Book and Earth: gravitational interaction.
* Book and table: contact interaction.

Therefore, the forces acting on the book are:
* Gravitational force downward.
* Normal force upward.

An interaction diagram helps prevent students from forgetting forces when creating free-body diagrams.

===5. Distinguish Between Motion and Force===

One common mistake is assuming that an object must experience a force in the direction it is moving.

However, an object's velocity and the net force acting on it are not necessarily in the same direction.

For example, consider a ball thrown vertically upward:
* While the ball moves upward, gravity acts downward.
* As the ball rises, gravity slows it down.
* At its highest point, its instantaneous velocity is zero, but gravity is still acting downward.
* As the ball falls, gravity continues acting downward, causing it to speed up.

This demonstrates that forces determine how velocity changes, not necessarily the direction in which an object is currently moving.

==Mathematical Model==

Different interactions require different mathematical models. There is no single equation that describes every type of interaction. However, Newton's Second Law provides a general relationship between the forces acting on an object and its acceleration.

===Newton's Second Law===

<math>\vec{F}_{net}=m\vec{a}</math>

where:
* <math>\vec{F}_{net}</math> is the net force acting on the object, measured in newtons (N).
* <math>m</math> is the object's mass, measured in kilograms (kg).
* <math>\vec{a}</math> is the object's acceleration, measured in meters per second squared (m/s^2).

The net force is the vector sum of all forces acting on an object:

<math>\vec{F}_{net}=\vec{F}_1+\vec{F}_2+\vec{F}_3+\cdots</math>

This means that forces must be added while accounting for their directions.

For example, if an object experiences a 10 N force to the right and a 4 N force to the left, its net force is:

<math>F_{net}=10-4=6\text{ N}</math>

The net force is 6 N to the right.

===Universal Law of Gravitation===

One example of a mathematical model for an interaction is Newton's universal law of gravitation:

<math>F_{gravity}=G\frac{m_1m_2}{r^2}</math>

where:
* <math>G</math> is the universal gravitational constant.
* <math>m_1</math> and <math>m_2</math> are the masses of the two interacting objects.
* <math>r</math> is the distance between their centers.

This equation shows that:
* Increasing either object's mass increases the gravitational force.
* Increasing the distance between the objects decreases the gravitational force.
* Gravitational force is always attractive.

For objects near Earth's surface, gravitational force is commonly approximated using:

<math>F_g=mg</math>

where:
* <math>m</math> is the object's mass.
* <math>g</math> is the acceleration due to gravity, approximately <math>9.8\text{ m/s}^2</math> near Earth's surface.

===Other Interaction Models===

Other interactions have their own mathematical models.

'''Friction:'''

For kinetic friction between two sliding surfaces:

<math>f_k=\mu_k F_N</math>

where:
* <math>f_k</math> is the kinetic friction force.
* <math>\mu_k</math> is the coefficient of kinetic friction.
* <math>F_N</math> is the normal force.

'''Spring Force:'''

For an ideal spring, Hooke's Law describes the force exerted by the spring:

<math>F_s=-kx</math>

where:
* <math>k</math> is the spring constant.
* <math>x</math> is the displacement from the spring's equilibrium position.
* The negative sign indicates that the spring force acts opposite the displacement.

'''Electric Force:'''

The electric force between two point charges can be modeled using Coulomb's Law:

<math>F_e=k_e\frac{|q_1q_2|}{r^2}</math>

where:
* <math>k_e</math> is Coulomb's constant.
* <math>q_1</math> and <math>q_2</math> are the electric charges.
* <math>r</math> is the distance between the charges.

These equations demonstrate that different interactions have different mathematical descriptions, but they can all contribute to the net force acting on an object.

==Worked Example==
Suppose a box is sliding to the right across a rough floor after being pushed and released.

What interactions are acting on the box?
* Earth and box: gravitational interaction downward
* Floor and box: normal force upward
* Floor and box: kinetic friction to the left

How can we tell?
* The box is near Earth, so gravity acts on it.
* The box is touching the floor, so a normal force acts on it.
* The box is sliding on a rough surface, so kinetic friction acts opposite the motion.

This kind of reasoning is important before writing equations or drawing a free-body diagram.

==Computational Model==
A computational model can simulate how interactions change motion over time. In many mechanics models, we:
# Identify all interactions acting on an object
# Add the forces as vectors to find the net force
# Use Newton's Second Law to calculate acceleration
# Update the object's velocity and position over small time intervals

An example is this GlowScript model of the three-body problem, where three stars interact through gravity:

[https://www.glowscript.org/#/user/zach.schrack/folder/MyPrograms/program/Lab3b 3-Body Problem]

This model shows how even a simple set of gravitational interactions can produce complex motion.

==Common Mistakes==
Some common mistakes students make when identifying interactions include:
* Assuming constant velocity means no forces are present
* Forgetting contact forces such as the normal force
* Treating "centripetal force" as a separate interaction instead of identifying the actual source of the inward force
* Listing motion instead of forces
* Forgetting that every interaction involves two objects

==Connectedness==
Interactions are central to engineering, science, and everyday life.

===Engineering===
Engineers study interactions constantly:
* Aerospace engineers study lift, drag, thrust, and gravity
* Mechanical engineers study friction, springs, tension, and collisions
* Electrical engineers study electric and magnetic interactions
* Chemical engineers study bonding forces and molecular interactions

===Everyday Life===
Many ordinary experiences depend on interactions:
* Walking depends on friction between shoes and the ground
* Cars move because of forces between tires and the road
* Phones and computers rely on electrical interactions
* The structure of matter depends on interactions at atomic and molecular scales

The physical world is shaped by interactions at every level.

==History==

The scientific understanding of interactions and forces developed over several centuries. Early philosophers attempted to explain why objects moved, fell, and interacted with one another, but many of their explanations were based on observation and reasoning rather than mathematical experiments.

===Early Ideas About Motion===

Ancient Greek philosopher Aristotle (384–322 BCE) proposed that objects required a continuous force to remain in motion. He also believed that heavier objects naturally fell faster than lighter objects. Although these ideas were influential for centuries, later experiments showed that they did not accurately describe how objects move.

===Galileo and the Study of Motion===

During the 16th and 17th centuries, Galileo Galilei (1564–1642) challenged many traditional explanations of motion through mathematical reasoning and experimentation.

Galileo studied falling objects and the effects of friction on moving objects. He recognized that an object could continue moving without a continuous applied force if opposing forces, such as friction, were absent.

His work helped establish the foundation for the modern understanding of inertia and motion.

===Isaac Newton and Classical Mechanics===

Isaac Newton (1642–1727) built upon the work of Galileo and other scientists to develop a mathematical framework for describing motion and interactions.

In 1687, Newton published his work *Philosophiæ Naturalis Principia Mathematica*, commonly known as the *Principia*. It introduced his three laws of motion and the law of universal gravitation.

Newton's three laws explained how forces affect an object's motion:
* Newton's First Law established the concept of inertia and explained that an object maintains its state of motion when the net external force is zero.
* Newton's Second Law connected the net force acting on an object to its mass and acceleration.
* Newton's Third Law explained that interacting objects exert equal and opposite forces on one another.

His law of universal gravitation showed that every object with mass attracts every other object with mass. This provided a mathematical explanation for phenomena such as falling objects, planetary orbits, and the motion of moons.

Newton's work became the foundation of classical mechanics and is still used to analyze many everyday physical interactions.

===The Development of Electromagnetism===

During the 18th and 19th centuries, scientists began studying electric and magnetic interactions more systematically.

Benjamin Franklin (1706–1790) investigated electric charge, while Charles-Augustin de Coulomb (1736–1806) developed a mathematical relationship describing the electric force between charged objects.

In the 19th century, Hans Christian Ørsted discovered that electric currents could produce magnetic effects. Michael Faraday (1791–1867) later demonstrated electromagnetic induction, showing that changing magnetic fields could produce electric currents.

James Clerk Maxwell (1831–1879) brought together many discoveries about electricity and magnetism into a unified mathematical theory. His equations showed that electricity and magnetism are connected aspects of a single electromagnetic interaction and predicted the existence of electromagnetic waves, including light.

===The Discovery of Atomic and Nuclear Interactions===

During the 20th century, experiments involving atoms and their nuclei revealed that the interactions described by classical mechanics and electromagnetism were not enough to explain everything in nature.

Scientists discovered that atoms contain positively charged protons and neutral neutrons in their nuclei, surrounded by negatively charged electrons.

Further research revealed two additional fundamental interactions:
* The strong nuclear interaction holds quarks together and binds protons and neutrons within atomic nuclei.
* The weak nuclear interaction is responsible for processes such as certain types of radioactive decay.

These discoveries expanded the understanding of interactions beyond the forces commonly studied in introductory mechanics.

===Modern Understanding of Fundamental Interactions===

Today, physics recognizes four fundamental interactions:

# Gravitational interaction: Acts between objects with mass and is described by general relativity and, in many introductory situations, Newton's law of universal gravitation.
# Electromagnetic interaction: Acts between electrically charged particles and explains electric and magnetic phenomena.
# Strong nuclear interaction: Holds quarks together and helps bind atomic nuclei.
# Weak nuclear interaction: Plays an important role in certain types of particle interactions and radioactive decay.

Modern physics describes the electromagnetic, strong, and weak interactions using quantum field theories. Gravity is described by Einstein's general theory of relativity, which explains it in terms of the curvature of spacetime.

Although scientists have made significant progress in understanding these interactions, research continues into how the fundamental interactions might be connected and how gravity can be incorporated into a quantum theory.

The study of interactions has therefore evolved from explaining everyday motion to understanding the behavior of matter and energy throughout the universe.

==Further Reading==
* Fundamental Interactions in Physics
* Introductory mechanics and electromagnetism textbooks
* PHET simulations for visualizing physical interactions

==External Links==
* [https://phet.colorado.edu/ PhET Simulations]

Types of Interactions and How to Detect Them

2026-11-09T19:41:09Z

Rmedagam6: Major changes to history

'''Ronika Medagam, Fall 2026'''
==Main Idea==
In physics, an **interaction** occurs when two objects influence one another. These interactions can change an object's motion, transfer energy, or cause it to change shape. In introductory mechanics, interactions are primarily studied through the **forces** that objects exert on one another.

An important principle is that interactions are mutual: if object A interacts with object B, then object B also interacts with object A. For example, when Earth pulls a ball downward through gravity, the ball also pulls Earth toward itself through gravity. These two forces are **equal in magnitude and opposite in direction**, but they act on **different** objects.

A helpful way to understand interactions is to ask two questions:

1. **Which objects are interacting?** Identify the two objects involved.
2. **What evidence tells us an interaction is occurring?** Look for contact, gravitational attraction, electric or magnetic effects, or changes in an object's motion or shape.

For example:
- **Earth and a falling ball:** Earth attracts the ball through a gravitational interaction. The ball also gravitationally attracts Earth.
- **A table and a book:** The table pushes upward on the book, while the book pushes downward on the table through a contact interaction.
- **A rope and a cart:** The rope pulls on the cart through tension, while the cart pulls on the rope.
- **Air and a moving object:** Air exerts a drag force on the object, while the object also exerts a force on the surrounding air.

### Why Do Interactions Matter?

Interactions provide the physical explanation for why forces exist. Instead of simply memorizing a list of forces, we can identify the objects involved and determine how they influence one another.

This is especially useful when drawing a **free-body diagram**. A free-body diagram shows all the forces acting on one selected object. To create one, first identify the other objects interacting with your chosen object. Then represent the forces those objects exert on it.

For example, if you are analyzing a book resting on a table, you should identify two interactions:
- Earth interacting gravitationally with the book.
- The table interacting through contact with the book.

These interactions produce the two forces shown on the book's free-body diagram: gravity downward and the normal force upward.

Understanding interactions is therefore the first step toward identifying forces and applying Newton's laws of motion.

==Types of Interactions==
Interactions in introductory physics are often grouped into two main categories:

==Contact Interactions==

Contact interactions occur when two objects physically touch and exert forces on one another. These interactions are common in everyday situations, such as pushing an object, walking, or placing an object on a table.

The main types of contact interactions include:

===Normal Force===
The normal force is a support force exerted by a surface on an object touching it. It always acts perpendicular to the surface.

Example: A table pushes upward on a book resting on it.

How to identify it:
* Look for an object resting on or pressing against a surface.
* The normal force acts perpendicular to the surface, away from it.

===Friction===
Friction is a force that acts between surfaces in contact. It opposes relative sliding or the tendency for surfaces to slide against each other.

There are two main types:
* Static friction prevents surfaces from sliding relative to each other.
* Kinetic friction acts when two surfaces are sliding against each other.

Example: Friction between your shoes and the ground helps you walk without slipping.

How to identify it:
* Look for surfaces that are sliding or attempting to slide against each other.
* Determine which direction the surfaces are sliding or tending to slide relative to one another.

===Tension===
Tension is a pulling force transmitted through a rope, string, cable, or similar object when it is pulled tight.

Tension acts along the length of the rope and pulls away from the object attached to it.

Example: A rope pulls a bucket upward when someone lifts it from a well.

How to identify it:
* Look for ropes, strings, or cables attached to an object.
* The tension force points along the rope, away from the object.

===Compression===
Compression occurs when an object is squeezed or pressed by another object. It can cause an object to shorten, bend, or change shape.

Example: A spring becomes compressed when someone pushes its ends toward each other.

How to identify it:
* Look for objects being squeezed or pushed together.
* Consider how the contact forces act on the object.

===Spring Force===
A spring exerts a force when it is stretched or compressed. This force acts to return the spring toward its natural, unstretched position.

Example: A compressed spring pushes a block away from it.

The spring force can be modeled using Hooke's Law:

F_s = -kx

Where:
* F_s is the spring force.
* k is the spring constant.
* x is the displacement from the spring's equilibrium position.

The negative sign indicates that the spring force acts opposite the displacement.

How to identify it:
* Look for a stretched or compressed spring attached to or pushing against an object.

===Air Resistance and Drag===
Drag is a force exerted by a fluid, such as air or water, on an object moving through it. Air resistance is a specific type of drag.

Drag generally acts opposite the object's motion relative to the surrounding fluid.

Example: A parachute experiences air resistance as it falls, helping slow its descent.

How to identify it:
* Look for objects moving through air or water.
* Pay attention to problems that mention air resistance, fluid resistance, or drag.

===Collisions===
A collision occurs when two objects come into contact and exert forces on each other over a period of time. These forces can cause changes in velocity, direction, or shape.

Example: When a tennis racket hits a tennis ball, the racket exerts a force on the ball, changing its motion.

How to identify it:
* Look for objects hitting, striking, or bouncing off one another.
* Consider how the collision changes the objects' motion.

==Non-Contact Interactions==

Non-contact interactions occur when objects exert forces on one another without physically touching. These interactions can act over a distance.

There are three major types:

===Gravitational Interaction===
Gravity is an attractive interaction between objects with mass. Every object with mass attracts every other object with mass.

Near Earth's surface, gravity pulls objects downward toward Earth's center.

Example: Earth attracts a falling apple, causing it to accelerate downward.

How to identify it:
* Consider gravitational interactions whenever objects have mass.
* In most introductory mechanics problems near Earth, gravitational force is represented by the object's weight.

===Electric Interaction===
Electric interactions occur between objects that have electric charge. Depending on the charges involved, objects can attract or repel one another.

* Opposite charges attract.
* Like charges repel.

Example: A negatively charged balloon can attract a positively charged object.

How to identify it:
* Look for charged objects, static electricity, or problems involving electric charges.

===Magnetic Interaction===
Magnetic interactions occur between magnets, magnetic materials, and moving electric charges. Magnets can attract or repel depending on their poles and the materials involved.

* Opposite magnetic poles attract.
* Like magnetic poles repel.

Example: A magnet attracts a paper clip without physically touching it.

How to identify it:
* Look for magnets, magnetic poles, or magnetic materials.

==Comparing Contact and Non-Contact Interactions==

{| class="wikitable"
! Feature
! Contact Interactions
! Non-Contact Interactions
|-
| Physical contact required?
| Yes
| No
|-
| How the force occurs
| Through physical contact between objects
| Through an interaction that acts over a distance
|-
| Examples
| Normal force, friction, tension, drag
| Gravity, electric, magnetic
|-
| Everyday example
| Pushing a shopping cart
| A magnet attracting a paper clip
|}

An important distinction is that air resistance is classified as a contact interaction because air molecules physically interact with the object's surface, even though these individual collisions are too small to see.

An object can also experience multiple interactions at the same time. For example, a falling object moving through air experiences both gravitational and drag interactions.

==How to Detect Them==

Interactions are not always directly visible. However, we can identify them by examining an object's motion, surroundings, and interactions with other objects. A useful approach is to identify the object being studied and determine which other objects could be exerting forces on it.

===1. Look for Changes in Velocity===

One way to detect an interaction is by observing a change in an object's velocity. Velocity includes both speed and direction, so an object can accelerate by speeding up, slowing down, or changing direction.

According to Newton's Second Law:

<math>\vec{F}_{net} = m\vec{a}</math>

where:
* <math>\vec{F}_{net}</math> is the net force acting on the object
* <math>m</math> is the object's mass
* <math>\vec{a}</math> is its acceleration

If an object accelerates, its net force must be nonzero. This means the forces produced by the interactions acting on the object do not balance.

Example:

A planet orbiting a star continuously changes the direction of its velocity. Even if its speed remains constant, its velocity is changing. This indicates that a net gravitational force is acting toward the star.

Another example is a car slowing down after the driver applies the brakes. The car's velocity decreases because forces from interactions between the tires, road, and braking system produce a net force opposite its motion.

===2. Equilibrium Does Not Mean No Forces===

An object can experience multiple interactions even when its velocity is constant. If an object is in equilibrium, the net force acting on it is zero:

<math>\vec{F}_{net}=0</math>

This means the forces balance one another, not that no forces are present.

Example:

Consider an object falling through Earth's atmosphere at terminal velocity. Its velocity is constant, so its acceleration is zero and its net force is zero.

However, two interactions are still occurring:
* Earth exerts a gravitational force downward.
* Air exerts a drag force upward.

These forces are equal in magnitude and opposite in direction, so they cancel.

Another example is a book resting on a table. Gravity pulls the book downward, while the table exerts an upward normal force. The book remains stationary because these forces balance.

These examples demonstrate that observing no change in motion does not mean there are no interactions.

===3. Examine the Object's Surroundings===

The physical setup of a problem often provides clues about which interactions are present. Instead of only looking at how an object moves, consider what other objects are nearby or touching it.

Common clues include:

* If an object rests on or presses against a surface, consider a normal force.
* If two surfaces are sliding or attempting to slide against each other, consider friction.
* If a rope, cable, or string is attached to an object, consider tension.
* If a spring is stretched or compressed, consider a spring force.
* If an object moves through air or water, consider drag.
* If an object is near Earth or another massive object, consider gravity.
* If charged objects are involved, consider electric interactions.
* If magnets or magnetic materials are involved, consider magnetic interactions.

Remember that the presence of an object does not automatically mean every possible force is acting. For example, two surfaces can touch without experiencing friction if there is no frictional force in the situation.

===4. Use an Interaction Diagram===

An interaction diagram is a useful tool for identifying forces before drawing a free-body diagram.

It shows the object being studied and the other objects interacting with it.

To create one:

# Choose the object you want to analyze.
# Identify the other objects that interact with it.
# Classify each interaction as contact or non-contact.
# Use these interactions to determine which forces act on your chosen object.

Example:

Consider a book resting on a table.

The objects involved are:
* The book
* The table
* Earth

The interactions are:
* Book and Earth: gravitational interaction.
* Book and table: contact interaction.

Therefore, the forces acting on the book are:
* Gravitational force downward.
* Normal force upward.

An interaction diagram helps prevent students from forgetting forces when creating free-body diagrams.

===5. Distinguish Between Motion and Force===

One common mistake is assuming that an object must experience a force in the direction it is moving.

However, an object's velocity and the net force acting on it are not necessarily in the same direction.

For example, consider a ball thrown vertically upward:
* While the ball moves upward, gravity acts downward.
* As the ball rises, gravity slows it down.
* At its highest point, its instantaneous velocity is zero, but gravity is still acting downward.
* As the ball falls, gravity continues acting downward, causing it to speed up.

This demonstrates that forces determine how velocity changes, not necessarily the direction in which an object is currently moving.

==Mathematical Model==

Different interactions require different mathematical models. There is no single equation that describes every type of interaction. However, Newton's Second Law provides a general relationship between the forces acting on an object and its acceleration.

===Newton's Second Law===

<math>\vec{F}_{net}=m\vec{a}</math>

where:
* <math>\vec{F}_{net}</math> is the net force acting on the object, measured in newtons (N).
* <math>m</math> is the object's mass, measured in kilograms (kg).
* <math>\vec{a}</math> is the object's acceleration, measured in meters per second squared (m/s^2).

The net force is the vector sum of all forces acting on an object:

<math>\vec{F}_{net}=\vec{F}_1+\vec{F}_2+\vec{F}_3+\cdots</math>

This means that forces must be added while accounting for their directions.

For example, if an object experiences a 10 N force to the right and a 4 N force to the left, its net force is:

<math>F_{net}=10-4=6\text{ N}</math>

The net force is 6 N to the right.

===Universal Law of Gravitation===

One example of a mathematical model for an interaction is Newton's universal law of gravitation:

<math>F_{gravity}=G\frac{m_1m_2}{r^2}</math>

where:
* <math>G</math> is the universal gravitational constant.
* <math>m_1</math> and <math>m_2</math> are the masses of the two interacting objects.
* <math>r</math> is the distance between their centers.

This equation shows that:
* Increasing either object's mass increases the gravitational force.
* Increasing the distance between the objects decreases the gravitational force.
* Gravitational force is always attractive.

For objects near Earth's surface, gravitational force is commonly approximated using:

<math>F_g=mg</math>

where:
* <math>m</math> is the object's mass.
* <math>g</math> is the acceleration due to gravity, approximately <math>9.8\text{ m/s}^2</math> near Earth's surface.

===Other Interaction Models===

Other interactions have their own mathematical models.

'''Friction:'''

For kinetic friction between two sliding surfaces:

<math>f_k=\mu_k F_N</math>

where:
* <math>f_k</math> is the kinetic friction force.
* <math>\mu_k</math> is the coefficient of kinetic friction.
* <math>F_N</math> is the normal force.

'''Spring Force:'''

For an ideal spring, Hooke's Law describes the force exerted by the spring:

<math>F_s=-kx</math>

where:
* <math>k</math> is the spring constant.
* <math>x</math> is the displacement from the spring's equilibrium position.
* The negative sign indicates that the spring force acts opposite the displacement.

'''Electric Force:'''

The electric force between two point charges can be modeled using Coulomb's Law:

<math>F_e=k_e\frac{|q_1q_2|}{r^2}</math>

where:
* <math>k_e</math> is Coulomb's constant.
* <math>q_1</math> and <math>q_2</math> are the electric charges.
* <math>r</math> is the distance between the charges.

These equations demonstrate that different interactions have different mathematical descriptions, but they can all contribute to the net force acting on an object.

==Worked Example==
Suppose a box is sliding to the right across a rough floor after being pushed and released.

What interactions are acting on the box?
* Earth and box: gravitational interaction downward
* Floor and box: normal force upward
* Floor and box: kinetic friction to the left

How can we tell?
* The box is near Earth, so gravity acts on it.
* The box is touching the floor, so a normal force acts on it.
* The box is sliding on a rough surface, so kinetic friction acts opposite the motion.

This kind of reasoning is important before writing equations or drawing a free-body diagram.

==Computational Model==
A computational model can simulate how interactions change motion over time. In many mechanics models, we:
# Identify all interactions acting on an object
# Add the forces as vectors to find the net force
# Use Newton's Second Law to calculate acceleration
# Update the object's velocity and position over small time intervals

An example is this GlowScript model of the three-body problem, where three stars interact through gravity:

[https://www.glowscript.org/#/user/zach.schrack/folder/MyPrograms/program/Lab3b 3-Body Problem]

This model shows how even a simple set of gravitational interactions can produce complex motion.

==Common Mistakes==
Some common mistakes students make when identifying interactions include:
* Assuming constant velocity means no forces are present
* Forgetting contact forces such as the normal force
* Treating "centripetal force" as a separate interaction instead of identifying the actual source of the inward force
* Listing motion instead of forces
* Forgetting that every interaction involves two objects

==Connectedness==
Interactions are central to engineering, science, and everyday life.

===Engineering===
Engineers study interactions constantly:
* Aerospace engineers study lift, drag, thrust, and gravity
* Mechanical engineers study friction, springs, tension, and collisions
* Electrical engineers study electric and magnetic interactions
* Chemical engineers study bonding forces and molecular interactions

===Everyday Life===
Many ordinary experiences depend on interactions:
* Walking depends on friction between shoes and the ground
* Cars move because of forces between tires and the road
* Phones and computers rely on electrical interactions
* The structure of matter depends on interactions at atomic and molecular scales

The physical world is shaped by interactions at every level.

==History==

The scientific understanding of interactions and forces developed over several centuries. Early philosophers attempted to explain why objects moved, fell, and interacted with one another, but many of their explanations were based on observation and reasoning rather than mathematical experiments.

===Early Ideas About Motion===

Ancient Greek philosopher Aristotle (384–322 BCE) proposed that objects required a continuous force to remain in motion. He also believed that heavier objects naturally fell faster than lighter objects. Although these ideas were influential for centuries, later experiments showed that they did not accurately describe how objects move.

===Galileo and the Study of Motion===

During the 16th and 17th centuries, Galileo Galilei (1564–1642) challenged many traditional explanations of motion through mathematical reasoning and experimentation.

Galileo studied falling objects and the effects of friction on moving objects. He recognized that an object could continue moving without a continuous applied force if opposing forces, such as friction, were absent.

His work helped establish the foundation for the modern understanding of inertia and motion.

===Isaac Newton and Classical Mechanics===

Isaac Newton (1642–1727) built upon the work of Galileo and other scientists to develop a mathematical framework for describing motion and interactions.

In 1687, Newton published his work *Philosophiæ Naturalis Principia Mathematica*, commonly known as the *Principia*. It introduced his three laws of motion and the law of universal gravitation.

Newton's three laws explained how forces affect an object's motion:
* Newton's First Law established the concept of inertia and explained that an object maintains its state of motion when the net external force is zero.
* Newton's Second Law connected the net force acting on an object to its mass and acceleration.
* Newton's Third Law explained that interacting objects exert equal and opposite forces on one another.

His law of universal gravitation showed that every object with mass attracts every other object with mass. This provided a mathematical explanation for phenomena such as falling objects, planetary orbits, and the motion of moons.

Newton's work became the foundation of classical mechanics and is still used to analyze many everyday physical interactions.

===The Development of Electromagnetism===

During the 18th and 19th centuries, scientists began studying electric and magnetic interactions more systematically.

Benjamin Franklin (1706–1790) investigated electric charge, while Charles-Augustin de Coulomb (1736–1806) developed a mathematical relationship describing the electric force between charged objects.

In the 19th century, Hans Christian Ørsted discovered that electric currents could produce magnetic effects. Michael Faraday (1791–1867) later demonstrated electromagnetic induction, showing that changing magnetic fields could produce electric currents.

James Clerk Maxwell (1831–1879) brought together many discoveries about electricity and magnetism into a unified mathematical theory. His equations showed that electricity and magnetism are connected aspects of a single electromagnetic interaction and predicted the existence of electromagnetic waves, including light.

===The Discovery of Atomic and Nuclear Interactions===

During the 20th century, experiments involving atoms and their nuclei revealed that the interactions described by classical mechanics and electromagnetism were not enough to explain everything in nature.

Scientists discovered that atoms contain positively charged protons and neutral neutrons in their nuclei, surrounded by negatively charged electrons.

Further research revealed two additional fundamental interactions:
* The strong nuclear interaction holds quarks together and binds protons and neutrons within atomic nuclei.
* The weak nuclear interaction is responsible for processes such as certain types of radioactive decay.

These discoveries expanded the understanding of interactions beyond the forces commonly studied in introductory mechanics.

===Modern Understanding of Fundamental Interactions===

Today, physics recognizes four fundamental interactions:

# Gravitational interaction: Acts between objects with mass and is described by general relativity and, in many introductory situations, Newton's law of universal gravitation.
# Electromagnetic interaction: Acts between electrically charged particles and explains electric and magnetic phenomena.
# Strong nuclear interaction: Holds quarks together and helps bind atomic nuclei.
# Weak nuclear interaction: Plays an important role in certain types of particle interactions and radioactive decay.

Modern physics describes the electromagnetic, strong, and weak interactions using quantum field theories. Gravity is described by Einstein's general theory of relativity, which explains it in terms of the curvature of spacetime.

Although scientists have made significant progress in understanding these interactions, research continues into how the fundamental interactions might be connected and how gravity can be incorporated into a quantum theory.

The study of interactions has therefore evolved from explaining everyday motion to understanding the behavior of matter and energy throughout the universe.

==Further Reading==
* Fundamental Interactions in Physics
* Introductory mechanics and electromagnetism textbooks
* PHET simulations for visualizing physical interactions

==External Links==
* [https://phet.colorado.edu/ PhET Simulations]

Types of Interactions and How to Detect Them

2026-11-09T19:38:49Z

Rmedagam6: revamped content, added explanations, diagrams, etc

'''Ronika Medagam, Fall 2026'''
==Main Idea==
In physics, an **interaction** occurs when two objects influence one another. These interactions can change an object's motion, transfer energy, or cause it to change shape. In introductory mechanics, interactions are primarily studied through the **forces** that objects exert on one another.

An important principle is that interactions are mutual: if object A interacts with object B, then object B also interacts with object A. For example, when Earth pulls a ball downward through gravity, the ball also pulls Earth toward itself through gravity. These two forces are **equal in magnitude and opposite in direction**, but they act on **different** objects.

A helpful way to understand interactions is to ask two questions:

1. **Which objects are interacting?** Identify the two objects involved.
2. **What evidence tells us an interaction is occurring?** Look for contact, gravitational attraction, electric or magnetic effects, or changes in an object's motion or shape.

For example:
- **Earth and a falling ball:** Earth attracts the ball through a gravitational interaction. The ball also gravitationally attracts Earth.
- **A table and a book:** The table pushes upward on the book, while the book pushes downward on the table through a contact interaction.
- **A rope and a cart:** The rope pulls on the cart through tension, while the cart pulls on the rope.
- **Air and a moving object:** Air exerts a drag force on the object, while the object also exerts a force on the surrounding air.

### Why Do Interactions Matter?

Interactions provide the physical explanation for why forces exist. Instead of simply memorizing a list of forces, we can identify the objects involved and determine how they influence one another.

This is especially useful when drawing a **free-body diagram**. A free-body diagram shows all the forces acting on one selected object. To create one, first identify the other objects interacting with your chosen object. Then represent the forces those objects exert on it.

For example, if you are analyzing a book resting on a table, you should identify two interactions:
- Earth interacting gravitationally with the book.
- The table interacting through contact with the book.

These interactions produce the two forces shown on the book's free-body diagram: gravity downward and the normal force upward.

Understanding interactions is therefore the first step toward identifying forces and applying Newton's laws of motion.

==Types of Interactions==
Interactions in introductory physics are often grouped into two main categories:

==Contact Interactions==

Contact interactions occur when two objects physically touch and exert forces on one another. These interactions are common in everyday situations, such as pushing an object, walking, or placing an object on a table.

The main types of contact interactions include:

===Normal Force===
The normal force is a support force exerted by a surface on an object touching it. It always acts perpendicular to the surface.

Example: A table pushes upward on a book resting on it.

How to identify it:
* Look for an object resting on or pressing against a surface.
* The normal force acts perpendicular to the surface, away from it.

===Friction===
Friction is a force that acts between surfaces in contact. It opposes relative sliding or the tendency for surfaces to slide against each other.

There are two main types:
* Static friction prevents surfaces from sliding relative to each other.
* Kinetic friction acts when two surfaces are sliding against each other.

Example: Friction between your shoes and the ground helps you walk without slipping.

How to identify it:
* Look for surfaces that are sliding or attempting to slide against each other.
* Determine which direction the surfaces are sliding or tending to slide relative to one another.

===Tension===
Tension is a pulling force transmitted through a rope, string, cable, or similar object when it is pulled tight.

Tension acts along the length of the rope and pulls away from the object attached to it.

Example: A rope pulls a bucket upward when someone lifts it from a well.

How to identify it:
* Look for ropes, strings, or cables attached to an object.
* The tension force points along the rope, away from the object.

===Compression===
Compression occurs when an object is squeezed or pressed by another object. It can cause an object to shorten, bend, or change shape.

Example: A spring becomes compressed when someone pushes its ends toward each other.

How to identify it:
* Look for objects being squeezed or pushed together.
* Consider how the contact forces act on the object.

===Spring Force===
A spring exerts a force when it is stretched or compressed. This force acts to return the spring toward its natural, unstretched position.

Example: A compressed spring pushes a block away from it.

The spring force can be modeled using Hooke's Law:

F_s = -kx

Where:
* F_s is the spring force.
* k is the spring constant.
* x is the displacement from the spring's equilibrium position.

The negative sign indicates that the spring force acts opposite the displacement.

How to identify it:
* Look for a stretched or compressed spring attached to or pushing against an object.

===Air Resistance and Drag===
Drag is a force exerted by a fluid, such as air or water, on an object moving through it. Air resistance is a specific type of drag.

Drag generally acts opposite the object's motion relative to the surrounding fluid.

Example: A parachute experiences air resistance as it falls, helping slow its descent.

How to identify it:
* Look for objects moving through air or water.
* Pay attention to problems that mention air resistance, fluid resistance, or drag.

===Collisions===
A collision occurs when two objects come into contact and exert forces on each other over a period of time. These forces can cause changes in velocity, direction, or shape.

Example: When a tennis racket hits a tennis ball, the racket exerts a force on the ball, changing its motion.

How to identify it:
* Look for objects hitting, striking, or bouncing off one another.
* Consider how the collision changes the objects' motion.

==Non-Contact Interactions==

Non-contact interactions occur when objects exert forces on one another without physically touching. These interactions can act over a distance.

There are three major types:

===Gravitational Interaction===
Gravity is an attractive interaction between objects with mass. Every object with mass attracts every other object with mass.

Near Earth's surface, gravity pulls objects downward toward Earth's center.

Example: Earth attracts a falling apple, causing it to accelerate downward.

How to identify it:
* Consider gravitational interactions whenever objects have mass.
* In most introductory mechanics problems near Earth, gravitational force is represented by the object's weight.

===Electric Interaction===
Electric interactions occur between objects that have electric charge. Depending on the charges involved, objects can attract or repel one another.

* Opposite charges attract.
* Like charges repel.

Example: A negatively charged balloon can attract a positively charged object.

How to identify it:
* Look for charged objects, static electricity, or problems involving electric charges.

===Magnetic Interaction===
Magnetic interactions occur between magnets, magnetic materials, and moving electric charges. Magnets can attract or repel depending on their poles and the materials involved.

* Opposite magnetic poles attract.
* Like magnetic poles repel.

Example: A magnet attracts a paper clip without physically touching it.

How to identify it:
* Look for magnets, magnetic poles, or magnetic materials.

==Comparing Contact and Non-Contact Interactions==

{| class="wikitable"
! Feature
! Contact Interactions
! Non-Contact Interactions
|-
| Physical contact required?
| Yes
| No
|-
| How the force occurs
| Through physical contact between objects
| Through an interaction that acts over a distance
|-
| Examples
| Normal force, friction, tension, drag
| Gravity, electric, magnetic
|-
| Everyday example
| Pushing a shopping cart
| A magnet attracting a paper clip
|}

An important distinction is that air resistance is classified as a contact interaction because air molecules physically interact with the object's surface, even though these individual collisions are too small to see.

An object can also experience multiple interactions at the same time. For example, a falling object moving through air experiences both gravitational and drag interactions.

==How to Detect Them==

Interactions are not always directly visible. However, we can identify them by examining an object's motion, surroundings, and interactions with other objects. A useful approach is to identify the object being studied and determine which other objects could be exerting forces on it.

===1. Look for Changes in Velocity===

One way to detect an interaction is by observing a change in an object's velocity. Velocity includes both speed and direction, so an object can accelerate by speeding up, slowing down, or changing direction.

According to Newton's Second Law:

<math>\vec{F}_{net} = m\vec{a}</math>

where:
* <math>\vec{F}_{net}</math> is the net force acting on the object
* <math>m</math> is the object's mass
* <math>\vec{a}</math> is its acceleration

If an object accelerates, its net force must be nonzero. This means the forces produced by the interactions acting on the object do not balance.

Example:

A planet orbiting a star continuously changes the direction of its velocity. Even if its speed remains constant, its velocity is changing. This indicates that a net gravitational force is acting toward the star.

Another example is a car slowing down after the driver applies the brakes. The car's velocity decreases because forces from interactions between the tires, road, and braking system produce a net force opposite its motion.

===2. Equilibrium Does Not Mean No Forces===

An object can experience multiple interactions even when its velocity is constant. If an object is in equilibrium, the net force acting on it is zero:

<math>\vec{F}_{net}=0</math>

This means the forces balance one another, not that no forces are present.

Example:

Consider an object falling through Earth's atmosphere at terminal velocity. Its velocity is constant, so its acceleration is zero and its net force is zero.

However, two interactions are still occurring:
* Earth exerts a gravitational force downward.
* Air exerts a drag force upward.

These forces are equal in magnitude and opposite in direction, so they cancel.

Another example is a book resting on a table. Gravity pulls the book downward, while the table exerts an upward normal force. The book remains stationary because these forces balance.

These examples demonstrate that observing no change in motion does not mean there are no interactions.

===3. Examine the Object's Surroundings===

The physical setup of a problem often provides clues about which interactions are present. Instead of only looking at how an object moves, consider what other objects are nearby or touching it.

Common clues include:

* If an object rests on or presses against a surface, consider a normal force.
* If two surfaces are sliding or attempting to slide against each other, consider friction.
* If a rope, cable, or string is attached to an object, consider tension.
* If a spring is stretched or compressed, consider a spring force.
* If an object moves through air or water, consider drag.
* If an object is near Earth or another massive object, consider gravity.
* If charged objects are involved, consider electric interactions.
* If magnets or magnetic materials are involved, consider magnetic interactions.

Remember that the presence of an object does not automatically mean every possible force is acting. For example, two surfaces can touch without experiencing friction if there is no frictional force in the situation.

===4. Use an Interaction Diagram===

An interaction diagram is a useful tool for identifying forces before drawing a free-body diagram.

It shows the object being studied and the other objects interacting with it.

To create one:

# Choose the object you want to analyze.
# Identify the other objects that interact with it.
# Classify each interaction as contact or non-contact.
# Use these interactions to determine which forces act on your chosen object.

Example:

Consider a book resting on a table.

The objects involved are:
* The book
* The table
* Earth

The interactions are:
* Book and Earth: gravitational interaction.
* Book and table: contact interaction.

Therefore, the forces acting on the book are:
* Gravitational force downward.
* Normal force upward.

An interaction diagram helps prevent students from forgetting forces when creating free-body diagrams.

===5. Distinguish Between Motion and Force===

One common mistake is assuming that an object must experience a force in the direction it is moving.

However, an object's velocity and the net force acting on it are not necessarily in the same direction.

For example, consider a ball thrown vertically upward:
* While the ball moves upward, gravity acts downward.
* As the ball rises, gravity slows it down.
* At its highest point, its instantaneous velocity is zero, but gravity is still acting downward.
* As the ball falls, gravity continues acting downward, causing it to speed up.

This demonstrates that forces determine how velocity changes, not necessarily the direction in which an object is currently moving.

==Mathematical Model==

Different interactions require different mathematical models. There is no single equation that describes every type of interaction. However, Newton's Second Law provides a general relationship between the forces acting on an object and its acceleration.

===Newton's Second Law===

<math>\vec{F}_{net}=m\vec{a}</math>

where:
* <math>\vec{F}_{net}</math> is the net force acting on the object, measured in newtons (N).
* <math>m</math> is the object's mass, measured in kilograms (kg).
* <math>\vec{a}</math> is the object's acceleration, measured in meters per second squared (m/s^2).

The net force is the vector sum of all forces acting on an object:

<math>\vec{F}_{net}=\vec{F}_1+\vec{F}_2+\vec{F}_3+\cdots</math>

This means that forces must be added while accounting for their directions.

For example, if an object experiences a 10 N force to the right and a 4 N force to the left, its net force is:

<math>F_{net}=10-4=6\text{ N}</math>

The net force is 6 N to the right.

===Universal Law of Gravitation===

One example of a mathematical model for an interaction is Newton's universal law of gravitation:

<math>F_{gravity}=G\frac{m_1m_2}{r^2}</math>

where:
* <math>G</math> is the universal gravitational constant.
* <math>m_1</math> and <math>m_2</math> are the masses of the two interacting objects.
* <math>r</math> is the distance between their centers.

This equation shows that:
* Increasing either object's mass increases the gravitational force.
* Increasing the distance between the objects decreases the gravitational force.
* Gravitational force is always attractive.

For objects near Earth's surface, gravitational force is commonly approximated using:

<math>F_g=mg</math>

where:
* <math>m</math> is the object's mass.
* <math>g</math> is the acceleration due to gravity, approximately <math>9.8\text{ m/s}^2</math> near Earth's surface.

===Other Interaction Models===

Other interactions have their own mathematical models.

'''Friction:'''

For kinetic friction between two sliding surfaces:

<math>f_k=\mu_k F_N</math>

where:
* <math>f_k</math> is the kinetic friction force.
* <math>\mu_k</math> is the coefficient of kinetic friction.
* <math>F_N</math> is the normal force.

'''Spring Force:'''

For an ideal spring, Hooke's Law describes the force exerted by the spring:

<math>F_s=-kx</math>

where:
* <math>k</math> is the spring constant.
* <math>x</math> is the displacement from the spring's equilibrium position.
* The negative sign indicates that the spring force acts opposite the displacement.

'''Electric Force:'''

The electric force between two point charges can be modeled using Coulomb's Law:

<math>F_e=k_e\frac{|q_1q_2|}{r^2}</math>

where:
* <math>k_e</math> is Coulomb's constant.
* <math>q_1</math> and <math>q_2</math> are the electric charges.
* <math>r</math> is the distance between the charges.

These equations demonstrate that different interactions have different mathematical descriptions, but they can all contribute to the net force acting on an object.

==Worked Example==
Suppose a box is sliding to the right across a rough floor after being pushed and released.

What interactions are acting on the box?
* Earth and box: gravitational interaction downward
* Floor and box: normal force upward
* Floor and box: kinetic friction to the left

How can we tell?
* The box is near Earth, so gravity acts on it.
* The box is touching the floor, so a normal force acts on it.
* The box is sliding on a rough surface, so kinetic friction acts opposite the motion.

This kind of reasoning is important before writing equations or drawing a free-body diagram.

==Computational Model==
A computational model can simulate how interactions change motion over time. In many mechanics models, we:
# Identify all interactions acting on an object
# Add the forces as vectors to find the net force
# Use Newton's Second Law to calculate acceleration
# Update the object's velocity and position over small time intervals

An example is this GlowScript model of the three-body problem, where three stars interact through gravity:

[https://www.glowscript.org/#/user/zach.schrack/folder/MyPrograms/program/Lab3b 3-Body Problem]

This model shows how even a simple set of gravitational interactions can produce complex motion.

==Common Mistakes==
Some common mistakes students make when identifying interactions include:
* Assuming constant velocity means no forces are present
* Forgetting contact forces such as the normal force
* Treating "centripetal force" as a separate interaction instead of identifying the actual source of the inward force
* Listing motion instead of forces
* Forgetting that every interaction involves two objects

==Connectedness==
Interactions are central to engineering, science, and everyday life.

===Engineering===
Engineers study interactions constantly:
* Aerospace engineers study lift, drag, thrust, and gravity
* Mechanical engineers study friction, springs, tension, and collisions
* Electrical engineers study electric and magnetic interactions
* Chemical engineers study bonding forces and molecular interactions

===Everyday Life===
Many ordinary experiences depend on interactions:
* Walking depends on friction between shoes and the ground
* Cars move because of forces between tires and the road
* Phones and computers rely on electrical interactions
* The structure of matter depends on interactions at atomic and molecular scales

The physical world is shaped by interactions at every level.

==History==
Many foundational ideas about interactions were developed by Isaac Newton (1642-1727). His laws of motion described how forces affect motion, and his law of universal gravitation provided a mathematical model for gravitational interactions.

Later scientists expanded the study of interactions by developing models for electric, magnetic, atomic, and nuclear forces. Over time, physics has built a broader understanding of how objects influence one another across many scales.

==Further Reading==
* Fundamental Interactions in Physics
* Introductory mechanics and electromagnetism textbooks
* PHET simulations for visualizing physical interactions

==External Links==
* [https://phet.colorado.edu/ PhET Simulations]

Types of Interactions and How to Detect Them

2026-11-09T19:34:27Z

Rmedagam6:

Newton's First Law of Motion

2026-11-09T19:21:33Z

Rmedagam6:

Claimed by

This page describes Newton's first law of motion, the first of his three famous laws of motion published in his work Principia Mathematica.

==The Main Idea==

Newton's first law states that an object at rest will stay at rest and an object in motion will stay in motion with the same speed and direction of travel unless the object is acted upon by an unbalanced external force.

For example:

- A book resting on a desk will remain there until someone moves it.
- A soccer ball will remain at rest until someone kicks it.
- A spacecraft moving through space can continue traveling without needing its engines to constantly push it forward.

Newton's first law states that it is the natural tendency for objects to remain on their current course. The tendency of matter to obey this law is called [[Inertia]], so it is also sometimes called the Law of Inertia.

A common misconception is that objects have the tendency to slow down on their own. However, Newton's first law explains that this is not true. Decelerating objects have a force acting on them opposite to their direction of motion. For example, a rolling ball slows down due to friction opposing the motion of the ball.

===How It Originated===

This law is an extension of the works of Galileo. Galileo developed the idea that an object would stay in motion linearly at a constant speed unless a force acts on it. Newton enhanced this concept by also adding that objects at rest will stay at rest unless acted upon by an unbalanced force.

Newton defined this force as proportional to the acceleration of the object. As an action is being exerted onto an object, it will change the objects state. Systems and bodies change their state of motion through interactions with their surroundings.

===A Mathematical Model===

The first law states that if the [[Net Force]] acting on the object is zero, then its velocity <math>\vec{v}</math> will not change over time. Velocity is a vector, which has both direction and magnitude, therefore if the Net Force is zero, neither the direction or magnitude can be changing. In other words, if the net force acting on an object is zero, it will not accelerate.

This idea can be expressed in the following manner:
:<math>
\sum \mathbf{\vec{F}} = 0\; \Leftrightarrow\; \frac{\mathrm{d} \mathbf{\vec{v}} }{\mathrm{d}t} = 0.
</math>

Newton's first law applies to:

1) Objects at rest (<math>|\vec{v}| = 0</math>), which will stay at rest unless a nonzero force acts upon it.

2) Objects in motion (<math>|\vec{v}| \neq 0</math>), which will continue to be in motion with the same velocity, proceeding in the same straight line, unless a nonzero force acts upon it.

===A Computational Model===

We can use a computational model (Glowscript code) to visualize Newton's First Law. If the net force acting on an object is zero, neither it's speed nor direction will change. The Glowscript program linked below demonstrates a ball rolling at constant speed in a straight path to the right.

https://www.glowscript.org/#/user/ajordan84/folder/MyPrograms/program/wikipage/edit

We can use another computational model to demonstrate the balls behavior when acted upon by an unbalanced force. Notice how the ball accelerates when a force is applied.

https://www.glowscript.org/#/user/ajordan84/folder/MyPrograms/program/wiki2

==Examples==

Because of the qualitative nature of Newton's first law, some of these example problems are conceptual questions rather than mathematical calculations.

===Simple===

Question 1: Suppose you want to push a box across a table in a straight line at a constant speed. What force, if any, would you have to exert on the box? (Describe it qualitatively- not enough information is supplied for a numerical answer.)

Answer: The moving box would experience 3 forces (besides any you exert on it): gravity, normal force from the table, and friction with the table and air. Gravity points downwards, normal force points upwards, and friction opposes the direction of motion. Because of the nature of normal force, the normal force takes on whatever magnitude necessary to cancel gravity. The only unbalanced force is therefore friction. Since your objective is to keep the box moving in a straight line at a constant speed (that is, at a constant velocity), the net force acting on the box must be 0 according to Newton's first law. The force you exert should therefore balance the friction force by being equal in magnitude and opposite in direction.

Question 2: Is a change in position an indicator of interaction?

Answer: On its own, a change in position is not enough to indicate an interaction because an object can have a nonzero velocity (that is, have a changing position) even with no forces acting on it, so long as that velocity is constant. With some additional information, however, a change in position can indicate an interaction. For example, if an object is initially at rest and is later found at another position, its velocity must have changed and it must have been acted on by a nonzero unbalanced external force.

===Medium===

Question: A 16kg traffic light is suspended by two cables, each 22<math>^\circ</math> from horizontal, as shown below:

[[File:Newtonsfirsttrafficlight.png]]

What is the tension in each of the cables?

Solution: Because the traffic light is at rest and not accelerating, by Newton's first law, any forces acting on it must be balanced (that is, the net force acting on it must be 0). The forces acting on the traffic light are gravity and tension in the 2 cables. The horizontal components of the cables' tension forces must be equal in magnitude, or the traffic light would be accelerating to the left or the right. Combining that information with the fact that the two cables are inclined by the same amount leads to the conclusion that the tension in the two cables must be the same. Finally, we know that the combined vertical components of the two cables' tension forces must equal the traffic light's weight, or the light would be accelerating vertically. This allows us to create the following equation:

<math>2T\sin(22) = 16*9.8</math>

<math>T = \frac{16*9.8}{2\sin(22)} = 209.3</math>N.

===Difficult===

(Requires knowledge of [[Static Friction]].)

Question: Suppose there exists a car of mass 9000 kg that is moving at a constant speed of 90 m/s in an easterly direction. The car is being buffeted by a strong wind, which exerts a 1000N force on it in the northerly direction. From this information, you know that the coefficient of static friction between the road and teh car's tires <math>\mu_s</math> must be at least what value?

Solution: Because the car is travelling at a constant speed without changing direction, by Newton's first law, any forces acting on it must be balanced (that is, the net force acting on it must be 0). The forces acting on the car are gravity, the normal force from the road, the wind, and static friction with the road. Gravity acts in the downward direction, the normal force acts in the upward direction, the wind acts in a northerly direction, and static friction acts in a southerly direction. We know that the magnitude of the normal force must be equal to the magnitude of the gravitational force, or the car would be accelerating vertically. That is, the magnitude of the normal force <math>N</math> must be 9,000kg * 9.8m/s = 88200N. We also know that the magnitude of the static friction force must be equal to the magnitude of the wind force, or the car would be accelerating along the north-south axis. That is, the magnitude of the friction force <math>f_s</math> must be 1000N.

<math>f_s \leq \mu_s * N</math>

<math>\mu_s \geq \frac{f_s}{N}</math>

<math>\mu_s \geq \frac{1000}{88200} = .0113</math>

==Connectedness==

Newton's first law is applicable to any situation where the net force on an object is 0 and its velocity remains constant. We can also apply Newton's First Law to the opposite situation: the net force is nonzero and therefore the velocity is non constant. There are nearly limitless examples of such situations, as well as nearly limitless applications. A few can be found below.

===Scenario: Tablecloth Party Trick===

A classic demonstration of Newton's first law is a party trick in which a tablecloth is yanked out from underneath an assortment of dinnerware, which barely moves and remains on the table. The tablecloth accelerates because a strong external force- a person's arm- acts on it, but the only force acting on the dinnerware is kinetic friction with the sliding tablecloth. This force is significantly weaker, and if the tablecloth is pulled quickly enough, does not have enough time to impart a significant impulse on the dinnerware. This trick demonstrates Newton's first law because the dinnerware begins at rest and remains so because no significant forces act on it.

===Scenario: Objects in Space===

In space, some intergalactic objects exist so far away from other bodies of matter that gravitational forces acting on them are negligible in magnitude. These objects continue to move in a straight line at a constant speed for very long periods due to Newton's first law.

===Scenario: Turning car===

You have probably experienced a situation in which you were driving or riding in a car when the driver takes a sharp turn. As a result, you were pressed against the side of the car to the outside of the turn. This is the result of Newton's first law; when the car changed direction, your body's natural tendency to continue moving in a straight line caused it to collide with the side of the car (or your seatbelt), which then applied enough normal force to cause your body to turn along with the car.

===Scenario: Soccer Ball on a Field===

Imagine kicking a soccer ball across a field. If the field were perfectly smooth with no air resistance or friction, the soccer ball would continue to roll forever at the same speed at which it began rolling and in the same direction it was originally kicked. However, due to friction from the field and air resistance (external forces that oppose the ball's motion), the ball slows down and eventually stops rolling.

==History==

The nature of the tendencies of matter regarding motion has been the subject of much thought throughout human history. Aristotle (384–322 BCE) famously believed that all objects have a "natural place" towards which they tend: heavy objects belong on the earth and therefore tend to move downwards, while lighter substances such as smoke belong in the sky and therefore tend to move upwards. Once an object reached its natural place, Aristotle believed, it would remain there at rest. Aristotle believed objects could not continue to move forever without being acted on by a force to keep it in motion, which is consistent with any observations he could have made on the surface of the earth, although today, we know this to be the result of [[Friction]].

Galileo Galilei (1564-1642), who studied the motion of celestial bodies, was the first to propose that perpetual motion was actually the natural state of objects, and that forces such as friction were necessary to bring them to rest or otherwise change their velocities. Galileo performed an experiment with two ramps and a bronze ball. The two ramps were set up at the same angle of incline, facing each other. Galileo observed that if a ball was released on one of the ramps from a certain height, it would roll down that ramp and up the other and reach that same height. He then experimented with altering the angle of the second ramp. He observed that even when the second ramp was less steep than the first, the ball would reach the same height it was dropped from. (Today, this is known to be the result of conservation of energy.) Galileo reasoned that if the second ramp were removed entirely, and the ball rolled down the first ramp and onto a flat surface, it would never be able to reach the height it was dropped from, and would therefore never stop moving if conditions were ideal. Galileo was essentially the first person to propose the idea that we now know as Newton's first law.

Isaac Newton (1643-1727) confirmed Galileo's idea with his own experiments and published it along with his 2 other laws in his 1687 work Principia Mathematica. Newton refined Galileo's concept by expanding it to add that objects at rest will stay at rest, unless acted upon by a force. Although he gave credit to Galileo, today the law is known by Newton's name.

Newton's first law appears in Principia (which was written in Latin) as follows:

''Lex I: Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.''

Translated to English, this reads:

"Law I: Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed."

Newton continued experimenting and found many fundamental ideas used in modern Physics today.

==See Also==
*[[Inertia]]
*[[Velocity]]
*[[Acceleration]]
*[[Newton's Second Law of Motion]]
*[[Newton's Third Law of Motion]]
*[[Galileo Galilei]]

===External links===

Simple explanation of Newton's first law: https://www.youtube.com/watch?v=1XSyyjcEHo0

Science of NFL Football: https://www.youtube.com/watch?v=08BFCZJDn9w

Real world application of Newton's First Law: https://www.youtube.com/watch?v=8zsE3mpZ6Hw

Everything you want to know about Newton's First Law of Motion: http://swift.sonoma.edu/education/newton/newton_1/html/newton1.html

NASA page on Newton's first law: https://www.grc.nasa.gov/www/k-12/airplane/newton1g.html

==References==

http://teachertech.rice.edu/Participants/louviere/Newton/law1.html

http://education.seattlepi.com/galileos-experiments-theory-rolling-balls-down-inclined-planes-4831.html

https://science.howstuffworks.com/innovation/scientific-experiments/newton-law-of-motion2.htm

Matter and Interactions: Modern Mechanics. Volume One. 4th Edition.

[[Category: Interactions]]

Newton's First Law of Motion

2026-11-09T19:09:07Z

Rmedagam6:

Claimed by Ronika Medagam Fall 26

This page describes Newton's first law of motion, the first of his three famous laws of motion published in his work Principia Mathematica.

==The Main Idea==

Newton's first law states that an object at rest will stay at rest and an object in motion will stay in motion with the same speed and direction of travel unless the object is acted upon by an unbalanced external force.

For example:

- A book resting on a desk will remain there until someone moves it.
- A soccer ball will remain at rest until someone kicks it.
- A spacecraft moving through space can continue traveling without needing its engines to constantly push it forward.

Newton's first law states that it is the natural tendency for objects to remain on their current course. The tendency of matter to obey this law is called [[Inertia]], so it is also sometimes called the Law of Inertia.

A common misconception is that objects have the tendency to slow down on their own. However, Newton's first law explains that this is not true. Decelerating objects have a force acting on them opposite to their direction of motion. For example, a rolling ball slows down due to friction opposing the motion of the ball.

===How It Originated===

This law is an extension of the works of Galileo. Galileo developed the idea that an object would stay in motion linearly at a constant speed unless a force acts on it. Newton enhanced this concept by also adding that objects at rest will stay at rest unless acted upon by an unbalanced force.

Newton defined this force as proportional to the acceleration of the object. As an action is being exerted onto an object, it will change the objects state. Systems and bodies change their state of motion through interactions with their surroundings.

===A Mathematical Model===

The first law states that if the [[Net Force]] acting on the object is zero, then its velocity <math>\vec{v}</math> will not change over time. Velocity is a vector, which has both direction and magnitude, therefore if the Net Force is zero, neither the direction or magnitude can be changing. In other words, if the net force acting on an object is zero, it will not accelerate.

This idea can be expressed in the following manner:
:<math>
\sum \mathbf{\vec{F}} = 0\; \Leftrightarrow\; \frac{\mathrm{d} \mathbf{\vec{v}} }{\mathrm{d}t} = 0.
</math>

Newton's first law applies to:

1) Objects at rest (<math>|\vec{v}| = 0</math>), which will stay at rest unless a nonzero force acts upon it.

2) Objects in motion (<math>|\vec{v}| \neq 0</math>), which will continue to be in motion with the same velocity, proceeding in the same straight line, unless a nonzero force acts upon it.

===A Computational Model===

We can use a computational model (Glowscript code) to visualize Newton's First Law. If the net force acting on an object is zero, neither it's speed nor direction will change. The Glowscript program linked below demonstrates a ball rolling at constant speed in a straight path to the right.

https://www.glowscript.org/#/user/ajordan84/folder/MyPrograms/program/wikipage/edit

We can use another computational model to demonstrate the balls behavior when acted upon by an unbalanced force. Notice how the ball accelerates when a force is applied.

https://www.glowscript.org/#/user/ajordan84/folder/MyPrograms/program/wiki2

==Examples==

Because of the qualitative nature of Newton's first law, some of these example problems are conceptual questions rather than mathematical calculations.

===Simple===

Question 1: Suppose you want to push a box across a table in a straight line at a constant speed. What force, if any, would you have to exert on the box? (Describe it qualitatively- not enough information is supplied for a numerical answer.)

Answer: The moving box would experience 3 forces (besides any you exert on it): gravity, normal force from the table, and friction with the table and air. Gravity points downwards, normal force points upwards, and friction opposes the direction of motion. Because of the nature of normal force, the normal force takes on whatever magnitude necessary to cancel gravity. The only unbalanced force is therefore friction. Since your objective is to keep the box moving in a straight line at a constant speed (that is, at a constant velocity), the net force acting on the box must be 0 according to Newton's first law. The force you exert should therefore balance the friction force by being equal in magnitude and opposite in direction.

Question 2: Is a change in position an indicator of interaction?

Answer: On its own, a change in position is not enough to indicate an interaction because an object can have a nonzero velocity (that is, have a changing position) even with no forces acting on it, so long as that velocity is constant. With some additional information, however, a change in position can indicate an interaction. For example, if an object is initially at rest and is later found at another position, its velocity must have changed and it must have been acted on by a nonzero unbalanced external force.

===Medium===

Question: A 16kg traffic light is suspended by two cables, each 22<math>^\circ</math> from horizontal, as shown below:

[[File:Newtonsfirsttrafficlight.png]]

What is the tension in each of the cables?

Solution: Because the traffic light is at rest and not accelerating, by Newton's first law, any forces acting on it must be balanced (that is, the net force acting on it must be 0). The forces acting on the traffic light are gravity and tension in the 2 cables. The horizontal components of the cables' tension forces must be equal in magnitude, or the traffic light would be accelerating to the left or the right. Combining that information with the fact that the two cables are inclined by the same amount leads to the conclusion that the tension in the two cables must be the same. Finally, we know that the combined vertical components of the two cables' tension forces must equal the traffic light's weight, or the light would be accelerating vertically. This allows us to create the following equation:

<math>2T\sin(22) = 16*9.8</math>

<math>T = \frac{16*9.8}{2\sin(22)} = 209.3</math>N.

===Difficult===

(Requires knowledge of [[Static Friction]].)

Question: Suppose there exists a car of mass 9000 kg that is moving at a constant speed of 90 m/s in an easterly direction. The car is being buffeted by a strong wind, which exerts a 1000N force on it in the northerly direction. From this information, you know that the coefficient of static friction between the road and teh car's tires <math>\mu_s</math> must be at least what value?

Solution: Because the car is travelling at a constant speed without changing direction, by Newton's first law, any forces acting on it must be balanced (that is, the net force acting on it must be 0). The forces acting on the car are gravity, the normal force from the road, the wind, and static friction with the road. Gravity acts in the downward direction, the normal force acts in the upward direction, the wind acts in a northerly direction, and static friction acts in a southerly direction. We know that the magnitude of the normal force must be equal to the magnitude of the gravitational force, or the car would be accelerating vertically. That is, the magnitude of the normal force <math>N</math> must be 9,000kg * 9.8m/s = 88200N. We also know that the magnitude of the static friction force must be equal to the magnitude of the wind force, or the car would be accelerating along the north-south axis. That is, the magnitude of the friction force <math>f_s</math> must be 1000N.

<math>f_s \leq \mu_s * N</math>

<math>\mu_s \geq \frac{f_s}{N}</math>

<math>\mu_s \geq \frac{1000}{88200} = .0113</math>

==Connectedness==

Newton's first law is applicable to any situation where the net force on an object is 0 and its velocity remains constant. We can also apply Newton's First Law to the opposite situation: the net force is nonzero and therefore the velocity is non constant. There are nearly limitless examples of such situations, as well as nearly limitless applications. A few can be found below.

===Scenario: Tablecloth Party Trick===

A classic demonstration of Newton's first law is a party trick in which a tablecloth is yanked out from underneath an assortment of dinnerware, which barely moves and remains on the table. The tablecloth accelerates because a strong external force- a person's arm- acts on it, but the only force acting on the dinnerware is kinetic friction with the sliding tablecloth. This force is significantly weaker, and if the tablecloth is pulled quickly enough, does not have enough time to impart a significant impulse on the dinnerware. This trick demonstrates Newton's first law because the dinnerware begins at rest and remains so because no significant forces act on it.

===Scenario: Objects in Space===

In space, some intergalactic objects exist so far away from other bodies of matter that gravitational forces acting on them are negligible in magnitude. These objects continue to move in a straight line at a constant speed for very long periods due to Newton's first law.

===Scenario: Turning car===

You have probably experienced a situation in which you were driving or riding in a car when the driver takes a sharp turn. As a result, you were pressed against the side of the car to the outside of the turn. This is the result of Newton's first law; when the car changed direction, your body's natural tendency to continue moving in a straight line caused it to collide with the side of the car (or your seatbelt), which then applied enough normal force to cause your body to turn along with the car.

===Scenario: Soccer Ball on a Field===

Imagine kicking a soccer ball across a field. If the field were perfectly smooth with no air resistance or friction, the soccer ball would continue to roll forever at the same speed at which it began rolling and in the same direction it was originally kicked. However, due to friction from the field and air resistance (external forces that oppose the ball's motion), the ball slows down and eventually stops rolling.

==History==

The nature of the tendencies of matter regarding motion has been the subject of much thought throughout human history. Aristotle (384–322 BCE) famously believed that all objects have a "natural place" towards which they tend: heavy objects belong on the earth and therefore tend to move downwards, while lighter substances such as smoke belong in the sky and therefore tend to move upwards. Once an object reached its natural place, Aristotle believed, it would remain there at rest. Aristotle believed objects could not continue to move forever without being acted on by a force to keep it in motion, which is consistent with any observations he could have made on the surface of the earth, although today, we know this to be the result of [[Friction]].

Galileo Galilei (1564-1642), who studied the motion of celestial bodies, was the first to propose that perpetual motion was actually the natural state of objects, and that forces such as friction were necessary to bring them to rest or otherwise change their velocities. Galileo performed an experiment with two ramps and a bronze ball. The two ramps were set up at the same angle of incline, facing each other. Galileo observed that if a ball was released on one of the ramps from a certain height, it would roll down that ramp and up the other and reach that same height. He then experimented with altering the angle of the second ramp. He observed that even when the second ramp was less steep than the first, the ball would reach the same height it was dropped from. (Today, this is known to be the result of conservation of energy.) Galileo reasoned that if the second ramp were removed entirely, and the ball rolled down the first ramp and onto a flat surface, it would never be able to reach the height it was dropped from, and would therefore never stop moving if conditions were ideal. Galileo was essentially the first person to propose the idea that we now know as Newton's first law.

Isaac Newton (1643-1727) confirmed Galileo's idea with his own experiments and published it along with his 2 other laws in his 1687 work Principia Mathematica. Newton refined Galileo's concept by expanding it to add that objects at rest will stay at rest, unless acted upon by a force. Although he gave credit to Galileo, today the law is known by Newton's name.

Newton's first law appears in Principia (which was written in Latin) as follows:

''Lex I: Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.''

Translated to English, this reads:

"Law I: Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed."

Newton continued experimenting and found many fundamental ideas used in modern Physics today.

==See Also==
*[[Inertia]]
*[[Velocity]]
*[[Acceleration]]
*[[Newton's Second Law of Motion]]
*[[Newton's Third Law of Motion]]
*[[Galileo Galilei]]

===External links===

Simple explanation of Newton's first law: https://www.youtube.com/watch?v=1XSyyjcEHo0

Science of NFL Football: https://www.youtube.com/watch?v=08BFCZJDn9w

Real world application of Newton's First Law: https://www.youtube.com/watch?v=8zsE3mpZ6Hw

Everything you want to know about Newton's First Law of Motion: http://swift.sonoma.edu/education/newton/newton_1/html/newton1.html

NASA page on Newton's first law: https://www.grc.nasa.gov/www/k-12/airplane/newton1g.html

==References==

http://teachertech.rice.edu/Participants/louviere/Newton/law1.html

http://education.seattlepi.com/galileos-experiments-theory-rolling-balls-down-inclined-planes-4831.html

https://science.howstuffworks.com/innovation/scientific-experiments/newton-law-of-motion2.htm

Matter and Interactions: Modern Mechanics. Volume One. 4th Edition.

[[Category: Interactions]]