# Homework Help: Change in mechanical energy of a bullet

1. May 12, 2012

### Cillie

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

A 30g bullet is moving at a horizontal velocity of 500m/s come to a stop 12 cm within a solid wall. What is the change in mechanical energy of the bullet?

2. Relevant equations

Mechanical energy = 1/2mv^2+mgh

3. The attempt at a solution

The question is for 6 marks, If I had to answer it, I would say the change in mechanical energy would equal the initial kinetic and potential energy of the bullet since unless I am missing something then the bullet has no energy left inside the wall, but I am not sure, so can anyone help me or make suggestions?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. May 12, 2012

### vanihba

Potential energy in above problem is essentially gravitational energy..(you can neglect other forms they don't play much of a role here)..and anyway there is NO CHANGE in gravitational energy if the bullet went horizantally through, right?? as h is same for both initial and final situation. So basically the change in mech energy is change in kinetic energy.

Which us initial K.E. - final K.E.

final K.E. = 0 as velocity is 0.

3. May 12, 2012

### Cillie

Thanks a lot for your response.
With the information they gave us your answer is probably the one the wanted, since they didn't give the initial distance between the block and the bullet. But otherwise there would have to be a change in potential energy as the bullet would have a downwards acceleration due to gravity thus a downwards velocity and would not hit the block as the same hight it was fired at.