# FHSST Physics/Momentum/Change

# Change in Momentum[edit]

If either an object's mass or velocity changes then its momentum too will change. If an object has an initial velocity and a final velocity , then its change in momentum, , is

= |

## Worked Example 35 Change in Momentum[edit]

**Question:** A rubber ball of mass 0.8*kg* is dropped and strikes the floor at a velocity of . It bounces back with an initial velocity of . Calculate the change in momentum of the rubber ball caused by the floor.

**Answer:**

Step 1 :

Analyse the question to determine what is given. The question explicitly gives

- the ball's mass,
- the ball's initial velocity, and
- the ball's final velocity

all in the correct units.

Do not be confused by the question referring to the ball bouncing back with an ``initial velocity of . The word *initial* is included here since the ball will obviously slow down with time and is the speed immediately after bouncing from the floor.

Step 2 :

What is being asked? We are asked to calculate the change in momentum of the ball,

We have everything we need to find . Since the initial momentum is directed downwards and the final momentum is in the upward direction, we can use the algebraic method of subtraction discussed in the vectors chapter.

Step 3 : Firstly, we choose a positive direction. Let us choose down as the positive direction. Then substituting,

__Down is the positive direction__

where we remembered in the last step to include the direction of the change in momentum in words.