Physics
posted by Jon .
A 0.500 kg block is sitting on a horizontal, frictionless surface. The block is connected to a horizontal spring with a force constant of 124 N/m. The other end of the horizontal spring rests against a wall. When a 100.0 g arrow is fired into the wooden block, the spring compresses by 25 cm.
(a) What is the maximum potential energy of the spring?
Ee (at max. compression)= 1/2kx^2
works out to be 3.88 J
Please help with the following..
(b) What was the speed of the arrow and block just after collision?
(c) What was the initial kinetic energy of the arrow?
(d) Explain any difference between (a) and (c).
Thanks for any assistance!

the initial KE of the block/arrow must be the same energy as the PE of the spring, so solve for speed from that.
The arrow hit the block, momentum was conserved
massarrow*speedarrow=(massarrow+block)speed above.
so use that speed to get the KE of the arrow.
The difference? It was an inelastic collision, what happened to the energy? 
so what is the answer to b) and c)