# Homework Help: Two colliding particles, finding ratio of M/m

Tags:
1. Jan 21, 2016

### irhum31

1. The problem statement, all variables and given/known data

A particle of mass m and initial velocity v0 collides elastically with a particle of unknown mass M coming from the opposite direction as shown in the left-hand sketch on the next page. After the collision, m has velocity v0/2 at right angles to the incident direction, and M moves off in the direction shown in the sketch. Find the ratio M/m.

2. Relevant equations

None given, although I'd assume the elastic condition collision equations.

3. The attempt at a solution
I don't even know where to begin.

2. Jan 21, 2016

### PeroK

Welcome to Physics forums.

Have you ever heard of the conservation laws?

3. Jan 21, 2016

### irhum31

I'm sorry if this question is trivial. It's just I cannot seem to wrap my head around how I can use momentum conservation with kinetic energy conservation to find this ratio, especially when I have no idea what the other velocity is.

4. Jan 21, 2016

### PeroK

So you do know where to begin! I can't see any diagram. Where is M moving after the collision?

If you don't know a quantity, then use a variable. Eventually, you should be able to cancel it out of your equations.

What can you say about conservation of momentum in this collision?

5. Jan 21, 2016

### irhum31

Aah, sorry. Here's the diagram. It's the one to the left under 6.9. (https://goo.gl/Pc87AW)

Momentum is conserved. Don't think there's more to be said than that, is there?

6. Jan 21, 2016

### PeroK

Hint: Momentum is a vector quantity.

I'm going offline now. I'm sure someone else will pick I up.