Bullet, Collision, Block

1. Jan 31, 2009

roeb

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

A wooden block of mass M resting on a frictionless horizontal surface is attached to a rigid rod of length l and of negligible mass. The rod is pivoted at the other end. A bullet of mass m traveling parallel to the horizontal surface and perpendicular to the rod with speed v hits the block and becomes embedded in it.

What fraction of the original kinetic energy is list in the collision?

2. Relevant equations

KE = 1/2 m v^2
L = cross(r,p)

3. The attempt at a solution

The only way I can think of doing this problem is using linear momentum conservation.
Say if pi = m*v, pf = (m+M)v'
v' = v * m / (M+m)

final / initial Kinetic Energy = m / (M+m)

I believe that I should be using conservation of angular momentum. According to part a of this problem (not stated) the angular momentum of the bullet+block = mvl.

If I were to use cons. of angular momentum, I don't really know to formulate it.
Initial Li = 0
Final Lf = mvl

Does anyone have any hints?

2. Jan 31, 2009

davieddy

fraction lost = (init - final)/init
=1 - m/(m+M)
=M/(m+M)