Homework Help: Bullet fired at a block of wood

1. Jun 19, 2010

Motorboar

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

An 19-g bullet is shot vertically into an 5-kg block. The block lifts upward 5 mm. The bullet penetrates the block in a time interval of 0.001 s. Assume the force on the bullet is constant during penetration. The impulse on the block due to capture of the bullet is closest to:

2. Relevant equations
Impulse = P_2 - P_1

3. The attempt at a solution
1/2(mbullet + mblock)v2^2 = (mbullet + mblock)gy
v2 = sqrt(2gy)
v1 = ((mbullet + mblock)/mbullet)*v2
Impulse = m2v2 - m1v1

Comes out wrong, and didn't use the time given. I modeled my solution after a ballistic pendulum problem. Any pointers?

2. Jun 19, 2010

bjd40@hotmail.com

try to do it using conservation of momentum, instead of conservation of energy. total mechanical energy is not conserved here, as the block got permanent deformations by the bullet.

3. Jun 19, 2010

Staff: Mentor

This is good. Note that you are asked to find the impulse on the block, so what is the change in momentum of the block?

The time is irrelevant for this question. (But I suspect there are other parts to this problem.)