# Bullet in a block hitting a spring

1. Nov 2, 2010

### preluderacer

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

An 8-g bullet is shot into a 4.0-kg block, at rest on a frictionless horizontal surface. The bullet remains lodged in the block. The block moves into a spring and compresses it by 5.0 cm. The force constant of the spring is 2200 N/m. In the figure, the initial velocity of the bullet is closest to:

3. The attempt at a solution

what I did was set (mbullet+mblock)vblock=1/2(k)x^2 I solved the the velocity of the block and got 0.686 m/s. Then I set mbullet(v)bullet=(m1+m2)vblock and solved for the vbullet and got 343N. This answer doesnt seem quite right. What am I doing wrong?

2. Nov 2, 2010

### ehild

The left-hand side of your equation is momentum, the right-hand side is energy. They can not be equal.

ehild

3. Nov 2, 2010

### preluderacer

i think i got it 1/2(m+M)v^2=1/2(k)(x)^2

Last edited: Nov 2, 2010
4. Nov 2, 2010

### ehild

Write out the kinetic energy of the block with the bullet inside.

ehild