physics
posted by Anonymous .
Can someone tell me if my answers are correct?
A .145 kg block of wood hangs from the ceiling by a string. A bullet with a mass of .004 kg and a speed of 540 m/s strikes the block. After the collision, the block with the bullet embedded rises 42 cm up. a) What is the velocity of the block and the bullet immediately after the collision? b) What percentage of the kinetic energy was lost when the bullet struck the block?
For part a) i have 14.5 m/s and b) 97.3%. I have a feeling this might be wrong though. Pls help!

(a) M = mass of block m = mass of bullet
(M+m)gH = (1/2)(M+m)V^2
V = sqrt(2gH), where H = 0.42 m
You answer is indeed wrong.
(b) Compare (1/2)*m*Vbullet^2 with
(1/2)(M+m)V^2
The difference divided by the bullet initial KE is the fraction of kinetic energy lost.