# i am confused with physics

Once again i am turning to this site to cure my incompetence and my splitting headache.

In a strange form of billiards, the cue ball is one third the mass of the other balls, which are stripy. There is no spin, and I hit a stripy ball centrally with the cue ball (travelling at 1.4ms −1) such that the cue ball rebounds in the opposite direction with half of its initial speed. What is the speed of the stripy ball?
Original post by TheKidAtTheBack
Once again i am turning to this site to cure my incompetence and my splitting headache.

In a strange form of billiards, the cue ball is one third the mass of the other balls, which are stripy. There is no spin, and I hit a stripy ball centrally with the cue ball (travelling at 1.4ms −1) such that the cue ball rebounds in the opposite direction with half of its initial speed. What is the speed of the stripy ball?

You need to use conservation of momentum.
Let the cue ball mass be m, and start by writing an expression for the total momentum before and after the collision.
Original post by Sinnoh
You need to use conservation of momentum.
Let the cue ball mass be m, and start by writing an expression for the total momentum before and after the collision.

I still don't understand what a) is being asked and b) what you are saying
Please note that I am genuinely incompetent so you will need to explain things as if talking to a 9 year old
Original post by TheKidAtTheBack
I still don't understand what a) is being asked and b) what you are saying
Please note that I am genuinely incompetent so you will need to explain things as if talking to a 9 year old

Aight.

There's a stripy ball. At the beginning, it's at rest. It gets hit with the cue ball. The cue ball was traveling at the speed given in the question.

All you're told about what happens next, is that the cue ball bounced back off the striped ball and that it's going half as fast backwards as it did forwards.

You're asked to find out, given this information, how fast the striped ball must be going.

The thing that lets you find this out is the law of conservation of momentum. Momentum is velocity multiplied by mass. Whatever the total momentum of both balls was at the start, must be the same afterwards.

You're not told what the mass of either ball actually is, but I don't think that will be an issue.

So if the mass of the striped ball is m, and the mass of the cue ball is a third of that, what's the momentum at the start?