Types of Interactions and How to Detect Them
Eren Cetinok, Spring 2026
Main Idea
In physics, an interaction is a mutual influence between two objects or systems that can change motion, energy, or both. In introductory mechanics, interactions are usually identified through the forces they produce. If object A interacts with object B, then object B also interacts with object A.
A helpful way to study interactions is to ask:
- What two objects are interacting?
- How can we tell that the interaction is happening?
For example:
- Earth and a falling ball interact gravitationally.
- A table and a book interact through contact.
- A rope and a cart interact through tension.
- Air and a moving object interact through drag.
Types of Interactions
Interactions in introductory physics are often grouped into two main categories:
Contact Interactions
These occur when two objects touch each other.
Examples:
- Normal force
- Friction
- Tension
- Compression
- Spring force
- Air resistance
- Collisions
Non-Contact Interactions
These occur when two objects do not need to be touching.
Examples:
- Gravitational interactions
- Electric interactions
- Magnetic interactions
A useful rule is that if two objects are touching, there is often a contact interaction. If they are not touching, then the interaction may be gravitational, electric, or magnetic.
How to Detect Them
Interactions are not always directly visible, so we often detect them by looking for evidence.
1. Change in Velocity
A force can be detected from a change in velocity, either in magnitude or direction. This is described by Newton's Second Law:
[math]\displaystyle{ \vec{F}_{net} = m\vec{a} }[/math]
If an object accelerates, then one or more interactions must be acting on it.
Example: A planet orbiting a star changes the direction of its velocity continuously, which tells us a gravitational interaction is present.
2. Equilibrium Does Not Mean No Forces
If an object is in equilibrium, then its net force is zero, but that does not mean there are no interactions.
Example: Consider an object falling through Earth's atmosphere at terminal velocity. Its velocity is constant, so its acceleration is zero and the net force is zero. However, gravity is still acting downward. Therefore, there must also be an upward air resistance force balancing gravity.
This example shows that even when there is no visible acceleration, interactions may still be present.
3. Contact Clues
In many problems, the physical setup helps identify interactions:
- If an object rests on a surface, there is usually a normal force.
- If a rough surface is involved, friction may be present.
- If a rope, cable, or string is attached, tension may be present.
- If an object is moving through air or fluid, drag may be present.
Mathematical Model
Different types of interactions use different mathematical models. There is no single formula that describes all interactions.
One example is the universal law of gravitation:
[math]\displaystyle{ F_{gravity} = G \frac{m_1 m_2}{r^2} }[/math]
where:
- [math]\displaystyle{ G }[/math] is the gravitational constant
- [math]\displaystyle{ m_1 }[/math] and [math]\displaystyle{ m_2 }[/math] are the masses of the two interacting objects
- [math]\displaystyle{ r }[/math] is the distance between their centers
Other interactions, such as friction, spring forces, and electric forces, each have their own mathematical models.
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:
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