# Homework Help: Use Conservation of Momentum or Conservation of KE for spring problem

1. Dec 14, 2011

### brainyasian

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

The system shown above has a spring attached to two masses. Mass I is 3 times greater in mass than Mass II. The spring is initially stretched and let go to allow the two masses to approach each other. Which of the following is the same for both masses?
a. speed
b. velocity.
c. acceleration
d. kinetic energy
e. magnitude of momentum

2. Relevant equations
Conservation of momentum
conservation of energy

3. The attempt at a solution
I know it's not the first three, but I'm not sure if you use the conservation of momentum or the conservation of energy for this problem

2. Dec 14, 2011

### Screen

Try proving each one and post what you get here.

What would cause momentum to be lost?

3. Dec 14, 2011

### brainyasian

But this is a closed system. Since no energy is lost or gained and this is a perfectly elastic collision, wouldn't both KE and momentum remain the same afterwards?

4. Dec 14, 2011

### Screen

Try writing it mathematically.

The problem is asking about the kinetic energy and momentum of each mass, not the combined total.

5. Dec 14, 2011

### brainyasian

but doesn't KE(M1) = KE(M2) because of conservation of mass and doesn't M(A)V(A) = M(B)V(B) make momentum equal?

6. Dec 14, 2011

### Screen

Read the question carefully; they haven't collided yet.