# Extremely Hard Conservation of Momentum Problem

1. Nov 13, 2012

### 012anonymousx

1. The problem statement, all variables and given/known data
A 0.750kg block of wood is attatched to a spring with k = 300N/m. A 0.0030kg bullet is fired into the block, and the spring is compressed 0.102m.
Calculate the velocity of the bullet before the collision.
Is this elastic or inelastic?

2. Relevant equations
How do you do it?

3. The attempt at a solution
I attempted using conservation of momentum, but no v1(prime), v2, v2(prime).
I hesitated to use conservation of energy.

2. Nov 13, 2012

### Staff: Mentor

Conservation of energy is fine after the bullet is embedded in the block. Consider the various phases of the problem: in the first phase, the bullet travels at some speed and strikes the block. An inelastic collision ensues, giving the ensemble mass some initial velocity. Conservation of momentum will help there. Then the ensemble pushes against the spring, converting its KE to PE in the spring. Conservation of energy will help there.