CHAPTER 13

Momentum

IN THIS CHAPTER
Summary: The impulse–momentum relationship can explain how force acts in a collision. Momentum is conserved in all collisions, allowing a
prediction of objects’ speeds before and after a collision.

Key Ideas
Impulse can be expressed both as force times a time interval, and as a change in momentum.
The total momentum of a set of objects before a collision is equal to the total momentum of a set of
objects after a collision.
Momentum is a vector, so leftward momentum can “cancel out” rightward momentum.

Relevant Equations

The definition of momentum:

p = mv

The impulse–momentum theorem:

Δp =    F   Δt

Location of the center of mass:

Mxcm =  m   1    x  1    +  m   2    x  2    +  ...

If an object is moving, it has momentum. The formal definition of momentum^1 is that it’s equal to an
object’s mass multiplied by that object’s velocity. However, a more intuitive way to think about
momentum is that it corresponds to the amount of “oomph” an object has in a collision. Regardless of how
you think about momentum, the key is this: the momentum of a system upon which no net external force
acts is always conserved.