Newton's Second Law: the Momentum Principle

This page describes Newton's second law, also known as the momentum principle, which relates net force to the change in Linear Momentum. This principle is used to predict the effects of forces on the motion of objects.

The Main Idea

Newton's second law, also known as the momentum principle, explains how forces cause the momentum of a system to change over time. The principle states that [math]\displaystyle{ \vec{F}_{net} = \frac{d\vec{p}_{system}}{dt} }[/math] where [math]\displaystyle{ \vec{p} }[/math] is the Linear Momentum of a system, [math]\displaystyle{ \vec{F}_{net} }[/math] is the external Net Force acting on the system from its surroundings, and t is time. Often, the system in question consists of a single particle whose motion we want to predict, although the law is also true for systems of particles and non-particle distributions of mass, such as disks. Note that both force and momentum are vector quantities, and that the change in momentum as a result of a force will always be in the direction of that force.

Here are some simple situations to help you build an intuition for the momentum principle:

If a particle travels in a straight line at a constant speed, its momentum is constant, so there is no net force acting on the particle.
If a particle travels in a straight line while speeding up, its momentum is increasing, so there must be a net force acting in the same direction as the particle's momentum.
If a particle travels in a straight line while slowing down, its momentum is decreasing, so there must be a net force acting in the opposite direction as the particle's momentum.
If a particle travels along a curving path, its momentum changes direction, and a force not parallel to the object's momentum must be acting on it.

The momentum principle has no derivation, as it is considered the definition of force. The metric unit most commonly used for force is the Newton.

A Mathematical Model

The momentum principle states that [math]\displaystyle{ \vec{F}_{net} = \frac{d\vec{p}}{dt} }[/math]. Recall that [math]\displaystyle{ \vec{p} = m\vec{v} }[/math]. Therefore, by product rule, [math]\displaystyle{ \frac{d\vec{p}}{dt} = m\frac{d\vec{v}}{dt} + \vec{v}\frac{dm}{dt} }[/math]. Usually,the mass of the particle or system is constant over time, so the second term becomes 0, and the momentum principle becomes [math]\displaystyle{ \vec{F}_{net} = m\frac{d\vec{v}}{dt} }[/math], or [math]\displaystyle{ \vec{F}_{net} = m\vec{a} }[/math], which may be a more familiar form of Newton's second law. The form [math]\displaystyle{ \vec{F}_{net} = \frac{d\vec{p}}{dt} }[/math] is preferred for several reasons:

When a particle accumulates mass that was initially at rest, such as a snowball rolling downhill, the term [math]\displaystyle{ \vec{v}\frac{dm}{dt} }[/math] is not 0, and [math]\displaystyle{ \vec{F}_{net} = m\vec{a} }[/math] is no longer accurate, while [math]\displaystyle{ \vec{F}_{net} = \frac{d\vec{p}}{dt} }[/math] is;
As a particle approaches the speed of light, [math]\displaystyle{ \vec{F}_{net} = m\vec{a} }[/math] is no longer accurate, while [math]\displaystyle{ \vec{F}_{net} = \frac{d\vec{p}}{dt} }[/math] is, as long as Relativistic Momentum is used; and
[math]\displaystyle{ \vec{F}_{net} = \frac{d\vec{p}}{dt} }[/math] has a more direct rotational analogue- that is, it will be easier to accurately learn rotational physics if you learn linear physics using this form.

That said, the relation [math]\displaystyle{ \vec{F}_{net} = m\vec{a} }[/math] can often be of use when the particle is known to have a constant mass and travel at non-relativistic speeds, particularly in combination with kinematic equations that also use [math]\displaystyle{ \vec{a} }[/math] (see Kinematics).

The momentum principle is responsible for the relationship between Impulse and Momentum.

Note that according to the momentum principle, when the external force on a system is 0 (that is, the system is closed), the rate of change of its momentum is 0. This results in Conservation of Momentum in certain situations.

Examples

1. (Simple)

At t=0, a 4kg particle is released from rest and a constant 12N force begins acting on it. How far has the particle moved after 5 seconds?

For this problem, it is easier to use [math]\displaystyle{ \vec{F}_{net} = m\vec{a} }[/math] than [math]\displaystyle{ \vec{F}_{net} = \frac{d\vec{p}}{dt} }[/math] because it allows us to solve for [math]\displaystyle{ \vec{a} }[/math] which can then be used in a kinematic equation. We know this form of Newton's second law is accurate for this problem because the particle travels at non-relativistic speeds and is of constant mass.

[math]\displaystyle{ \vec{F}_{net} = m\vec{a} }[/math] can be rearranged into [math]\displaystyle{ \vec{a} = \frac{\vec{F}_{net}}{m} }[/math]

[math]\displaystyle{ a = \frac{12}{4} = 3 }[/math] (direction doesn't matter for this problem.)

[math]\displaystyle{ x_f = \frac{1}{2}at^2 + v_it + x_i = \frac{1}{2}(3)(5^2) = \frac{75}{2} }[/math] = 37.5 m.

2. (Intermediate)

A cylindrical rocket ship of mass [math]\displaystyle{ M }[/math] and speed [math]\displaystyle{ v }[/math] is cruising through outer space when it encounters a stationary dust cloud. The dust cloud has a number density of [math]\displaystyle{ N }[/math] particles per unit volume, and each dust particle has a mass of [math]\displaystyle{ m }[/math]. The rocket ship has a sticky surface on its front face that allows it to accumulate all of the dust that it hits, absorbing it into its mass. The radius of the rocket ship's circular face is [math]\displaystyle{ R }[/math]. What force does the rocket's thruster need to exert on it in order to preserve its speed as it moves through the dust cloud? (Assume that the consumption of fuel has a negligible impact on the mass of the rocket.)

Advanced Note: in order to use [math]\displaystyle{ \frac{dm}{dt} }[/math] for the accumulation of mass, the accumulated mass (i.e. the dust) must initially be at rest, like in the above problem. Otherwise, one must treat the accumulation of dust particles as a series of collisions, or change one's frame of reference so that the accumulated mass is initially at rest.

3. (Difficult)

A particle of mass [math]\displaystyle{ m }[/math] and speed [math]\displaystyle{ v }[/math] moves in a circular path. Its angular frequency is [math]\displaystyle{ \Omega }[/math]. (Angular frequency is the rate at which the angle of the particle's position is changing in radians per unit time.) Using the momentum principle, show that

a) a nonzero net force is acting on the particle,

b) the magnitude of the force is given by [math]\displaystyle{ f = mv\Omega }[/math], and

c) the direction of the force is inwards towards the center of the circle.

a) The particle's momentum vector is changing over time; its magnitude is constant (mv), but its direction changes as its position along the circular path changes. It is always tangential to the circular path. Since [math]\displaystyle{ \vec{p} }[/math] changes over time, a nonzero net force must be acting on the particle. In fact, the only way a particle of constant mass can have a net force of 0 acting on it is if it is travelling along a straight line at a constant speed, or if it is at rest.

b)

c) As one can see from the diagram above, [math]\displaystyle{ d\vec{p} }[/math] is pointing towards the center of the circle. If the particle is exactly at the top of the circle, its momentum is to the right (or left if the particle is travelling counterclockwise), and as [math]\displaystyle{ d\theta }[/math] approaches 0, [math]\displaystyle{ d\vec{p} }[/math] approaches a downward direction, which is towards the center of the circle.

For more information, see Centripetal Force and Curving Motion. Our answer agrees with the formulas given on that page.

Connectedness

Newton's Second Law is applicable to any situation where a force is applied and a system's momentum changes. There are nearly limitless examples of such situations. One can be found below.

Scenario: car brakes

When brakes are applied to a car, they apply a force in the direction opposite to the car's direction of travel. This causes the car's momentum, which is initially forward, to change over time in the backward direction, until it becomes 0, at which point the car has come to a stop.

History

In his 1687 book Principia Mathematica, Isaac Newton (1643-1727) published his three laws of motion. Although his first and third laws were inspired by other scientists' work, his second law was entirely original. Below was the law as it appeared in his book:

Original Latin:

“Lex II: Mutationem motus proportionalem esse vi motrici impressae, et fieri secundum lineam rectam qua vis illa imprimitur.”

This was translated closely in Motte's 1729:

“Law II: The alteration of motion is ever proportional to the motive force impress'd; and is made in the direction of the right line in which that force is impress'd.”

In other words, the rate of change of momentum of a body is proportional to the force impressed on the body, and happens in the direction of that force.

This law was the first time force had ever been defined, and it is a definition that is still used today. As a result of Newton's contribution to the science of forces, the unit of force most commonly still used today is named after him.

Newton's Second Law: the Momentum Principle

Contents

The Main Idea

A Mathematical Model

Examples

1. (Simple)

2. (Intermediate)

3. (Difficult)

Connectedness

Scenario: car brakes

History

See also

Further reading

External links

References

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