How Does a Bullet Passing Through a Block Affect Speed and Energy?

[Memphis213]

I have been dueling with this problem for a hour or so now...

a 5.00g bullet moving with an initial speed of 400 m/s is fired into and passes through a 1.00-kg block. The block, initially at rest on a frictionless, horizontal surface, is connected to a spring with a force constant of 900 n/m. If the block moves 5.00cm to the right after impact, find (a) the speed at which the bullet emerges from the block and (b) the mechanical energy converted into internal energy in the collision.

questions I have:
1. What is converting mechanical energy?
2. Can you use conservation of energy here...i.e: whatever energy it took to compress the spring 5cm will be lost, and then you can find new speed from that? but still where is the internal energy coming from, the heat from the bullet?

thanks in advance!

 

[Doc Al]

Treat the collision and the spring compression separately. During the collision, mechanical energy is not conserved (but what is?). Some of bullet's initial KE is transformed into internal energy due to friction/deformation within the wood.

After the collsion, the block has some KE, which ends up being transformed into spring PE.