# Elastic collision between two bodies

1. Apr 13, 2014

### lookalike

1. The problem statement, all variables and given/known data
A body of mass 2.0Kg makes an elastic collision with another body at rest and continues to move in the original direction but with one-fourth of its original speed. What is the mass of the other body?

2. Relevant equations
Because it is an elastic collision I know that Energy and momentum are conserved.

So, m1v1i+ m2v2i = m1v1f+m2v2f

V1f = V2f = Vf

3. The attempt at a solution

I'm not really sure where to go because I'm not given the velocity of the second block after the collision.

2*v1i + 0 = 2*(1/4v1i) + m2v2

I'm confused because it states that the original block continues to move in the original direction. Because the blocks don't adhere to each-other or go in opposite directions I'm not sure what to do.

Last edited: Apr 13, 2014
2. Apr 13, 2014

### Simon Bridge

Lets say block A is the 2kg one initially moving and block B is the one initially stationary.
You are not told that the blocks do not stick together.
Even if they do not, it is possible for block B to end up moving faster than block A.

You are not told the mass of block B or it's final velocity, but you can write down two equations which must be true simultaneously. Write them down.

3. Apr 14, 2014

### elements

Just so you know you're told that it's an perfectly elastic collision so you know that both momentum and kinetic energy is conserved.

So why not just isolate for a variable in one equation plug it into the next equation to get the answer, while cancelling out any unknowns