# Law of conservation of energy problem

1. Apr 6, 2009

### leapoldstotch

1. The problem statement, all variables and given/known data
A .5 kg block is dropped from 15m. A 5 gram bullet hits the block at 233 m/s. At what height must the block be hit to completely stop the blocks freefall momentarily.

2. Relevant equations
p=mv
1/2mv^2=mgh

3. The attempt at a solution
bullets ke
1/2(.005)(233)^2=.5(9.8)(h)
When the bullets kenetic energy is equal to the blocks potential it will momentarily pause in mid air. Atleast i thought.

bullet's ke = 135.72 joules= blocks pe = 27.698m
The block would have to be 27 meters above the ground to have as much potential energy as the bullet described.

Then i was thinking of solving it using momentum but im not sure how that would work out.

2. Apr 6, 2009

### rl.bhat

Equating momentums of the block and the bullet, you can find the velocity of the block.
Then by using kinematics you can find h.

3. Apr 6, 2009

### tiny-tim

Hi leapoldstotch!
Energy is not conserved in a collision unless the question tells you it is …

in this case, the bullet will embed in the block, and have the same speed as it, so this is a perfectly inelastic collision and energy is obviously not conserved.

You can only use conservation of momentum (for the collision itself).