Chapter 7: Linear Momentum
7-1: Momentum and Its Relation to Force
We will use the word “momentum” to talk about linear momentum.
momentum: a vector that points in the direction of an object’s velocity, and whose
magnitude is equal to the mass of the object times its speed.
There are actually two types of momentum, linear and angular. Both types follow from
Newton’s first law: an object in motion will stay in motion unless a force acts on it.
Linear momentum, which we discuss here, is momentum an object has in the direction of
its motion. Angular momentum is the momentum an object has when it is spinning. We
will not discuss angular momentum in this chapter.
Newton came across this combination of mass times velocity often when observing and
describing the motion of objects, and scientists decided it was important enough to give it
a name. Newton stated his third law of motion in terms of momentum:
Net force = the rate of change of the quantity mass times velocity
In other words, the quantity mass times velocity will not change unless a net force acts on
it.
Let’s show how this turns into our familiar form of Newton’s third law, F = ma:
Example