A 14-g bullet strikes and embeds itself in a 0.86-kg block of wood, which rests on a horizontal, friction-less surface. The block is attached to a perfect spring whose spring constant is 95 N/m. The impact compresses the spring 10 cm. Determine the speed of the bullet before the impact.

Question

jamesj · Answer

After the spring compresses, how much spring potential energy does it have?
Or in other words, how much energy was required to make the wood and bullet compress on the spring that much?

jamesj · Answer

(btw, did you mean a 140 g bullet?)

anonymous · Answer

Nope :-P The paper says 14 g. and what equation should I use for that? F=-kx? or kx^2=PE(elastic)?

jamesj · Answer

spring potential energy is = (1/2)kx^2
You know k and x

Hence you know the energy expended.

That energy had to have come from the KE of the bullet pre-collision.

anonymous · Answer

Mmkay. I'll try that. Thank you!