# Homework Help: Elastic collision between 2 different masses

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1. Nov 11, 2015

### Yeldarb

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

2. Relevant equations
p = p1 +p2
p = mv

3. The attempt at a solution
See picture, using equations for the velocity and using 90-tan-1(2) for the angle

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2. Nov 12, 2015

### ehild

Do not forget that momentum is a vector quantity. Both of its components are conserved.
Why do you think that θ2=90-tan-1(2)?

3. Nov 12, 2015

### Yeldarb

Does this mean that I need to use a px and a py equation? Does what I got help me at all? The picture shows tan(theta1)=2, so theta1= tan-1(2) and since the angle between the two should be 90, 90-theta1 =theta2

Thanks

4. Nov 12, 2015

### SammyS

Staff Emeritus
What makes you think the angle between their directions is 90° ?

5. Nov 12, 2015

### ehild

Yes, you need an equation for px and an other one for py. NO, the two angles do not add up to 90°. It happens if the two masses are equal.