# Ball momentum physics problem

1. Dec 1, 2005

### fabbo

a ball of mass 200g falls to the gound and just before striking the ground its speed is 5m/s. It rebounds with a velocity of 4m/s. What is the magnitude and direction of the change in momentum.

I have the momentum before the rebound as -1kg/m/s and after as 0.8kg/m/s but I don't know how to work out the change or the direction? Can anyone help me do it?

2. Dec 1, 2005

<<<<<up>>>>>

3. Dec 1, 2005

### Staff: Mentor

The change (in anything) is final value - initial value. Keep track of your signs and you'll be able to tell the direction.

4. Dec 1, 2005

### fabbo

does this mean the change is 0.8 - - 1 giving 1.8kg/m/s?

5. Dec 1, 2005

### Staff: Mentor

Absolutely. And what does the sign tell you about the direction of the change in momentum?

Note added: Be careful in expressing your units. The units of momentum are kg-m/s (not kg/m/s).

Last edited: Dec 1, 2005
6. Dec 1, 2005

### vaishakh

Fabbo-the change in mommentum is the product of change in velocity and mass. don't jump to answer the change in mommentum without doing this step. this i think makes it easier for you to understand the change in mommentum. but remember even this that this isonly applicable when the body does not changes its mass.
earlier the velocity is 5units and then the velocity is 4 units in the opposite direction. thus they have opposite signs. rest do alone!

7. Dec 1, 2005

### Staff: Mentor

What makes you think he skipped this step?