# Momentum of blocks compressing spring

1. Oct 23, 2009

### Mugen Prospec

1. The problem statement, all variables and given/known data
In the figure below, block 1 (mass 1.6 kg) is moving rightward at 8 m/s and block 2 (mass 4.2 kg) is moving rightward at 2.8 m/s. The surface is frictionless, and a spring with spring constant of 1120 N/m is fixed to block 2. When the blocks collide, the compression of the spring is maximum at the instant the blocks have the same velocity. Find the maximum compression.

2. Relevant equations
mv+mv=mv
not sure what spring equation is needed.

3. The attempt at a solution Tried taking the velocity of the sytem them puting that in to kinetic energy = spring energy but that was not right.

2. Oct 24, 2009

### Andrew Mason

What is the change in velocity of each block when they are both moving at the same speed? What is that speed?

Your approach is right but not all the energy is in the spring - just the lost energy. Assume that the energy that is lost by the blocks is stored in the spring. (hint: The energy of the spring is .5kx^2 where k is the spring constant and x is the displacement from equilibrium).

AM

3. Oct 25, 2009

### Mugen Prospec

Oh ok so the total kinetic energy plus the spring energy equals the original energy of the system. I might not have said the right but I understand what your saying. I was just having a hard time figuring out where the energy went but then, like you showed, it wasnt a perfectly elastic collision.
Thank you Andrew