# Change in momentum?

1. Nov 22, 2011

### Count Duckula

ok so my physics textbooks crap, it explains something in a tiny paragraph without any examples (I'm getting new ones). I have this question I'm stuck on. There is 2 balls that both have the mass of 1.5kg, one is not moving and one is moving at 4m/s - so ball A is about to collide with ball B. the questions are: the momentum of ball A just after collision, momentum of ball B just after collision and velocity of ball B after the collision?

do I use force= change in momentum/time taken for the change equation. so for the first two im guessing the question is to find the change in momentum which is force x time taken for change. but what is the time taken for the change - is it 4m/s???? or do I need a different equation to find out the time it took for the change and then use that to find out the change in momentum?

2. Nov 22, 2011

### grzz

What is the resultant force on BOTH balls when they collide with each other?

3. Nov 22, 2011

### Count Duckula

I don't know, all the information for the question I have is what I have given...

.... oh... under the question theres another line that says, 'after the collision both balls move to the right but the velocity of ball A is now 1m/s

... I should be able to figure out these questions now, I think... maybe if you could do the first for me? so I have something to work from, as I have no example to get an idea with.

4. Nov 22, 2011

### grzz

when the two balls collide with each other, ball A gives a force to ball B and ball B gives an equal and opposite force to ball A. Hence the resultant force on the two balls taken together is zero.

Hence momentum is conserved.

i.e. initial momentum = final momentum.

Try to work out this equation and you get the answer required.