# Homework Help: Elastic collision with no info on object 1

1. Oct 19, 2009

### chris097

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

Cart1, with mass m, is initially at rest and is struck by cart2, which has a mass of 337 kg and initial speed of 2.07 m/s. The collision is elastic and after the collision cart2 continues to move in its original direction at 0.900 m/s.

Find m, the mass of cart1.
Find the velocity of cart1 after the collision.

3. The attempt at a solution

I tried using standard momentum equations and isolating for m1 but I can't seeing as I don't have u1 or m1. Once I find the mass, the second part of the question should be straightforward.

2. Oct 19, 2009

### rl.bhat

Here you have to use the law of conservation of energy along with the conservation of linear momentum.

3. Oct 19, 2009

### chris097

So i would insert one into the other to solve for the unknown?

4. Oct 19, 2009

### rl.bhat

Yes.

5. Oct 19, 2009

### chris097

Does it matter which equation gets subbed into which?

6. Oct 19, 2009

### rl.bhat

m1v1 + m2v2 = m1v1' + m2v2".
In the problem v1 = 0. Substitute the known values, find v1' in terms of m1.
Put it in the energy equation to find m1.