A bullet of mass 4 g moving with an initial speed of 300 m/s is fired into and passes through ta block of mass 5kg. The block, initially at rest on a frictionless horizontal surface, is connected to a spring of force constant 600 N/m. If the block moves a distance of 1.6 cm to the right after bullet passed through it, find the speed v at which the bullet emerges from the block.
KE = 1/2 m v^2
PEspring = 1/2 k x^2
KEinitial + PEinitial = KEfinal + PEfinal
p = mv
m1v1 +m2v2 = m'1v'1 + m'2v'2
The Attempt at a Solution
I tried KEbulletinitial = PEspring = KEbulletfinal by .5mv1^2 = .5kx^2 + .5mv2^2. I guess its not a conservation of energy problem. I was thinking conservation of momentum but wasn't sure how to find the momentum of the spring. I know the fspring = kx. Any hints?