1. The problem statement, all variables and given/known data You have been asked to design a "ballistic spring system" to measure the speed of bullets. A spring whose spring constant is k is suspended from the ceiling. A block of mass M hangs from the spring. A bullet of mass m is fired vertically upward into the bottom of the block. The spring's maximum compression d is measured. Find an expression for the bullet's speed. Express your answer in terms of the variables m, M, k, d, and constant g. 2. Relevant equations Ki + Ug + Usp = Kf + Ug + Usp 3. The attempt at a solution I used conservation of momentum to find the final velocity of the bullet+block (m+M)vf=mvi + Mvi so... vf=(m/m+M)*vB where vB is the initial speed of the bullet. Next, I used: Ki + Ug + Usp = Kf + Ug + Usp (1/2)mvi^2 + (1/2)kd^2 + Mgd = (1/2)mvf^2 + (1/2)kd^2 + Mgd 0 + (1/2)kd^2 + 0 = (1/2)(m+M)[(m+M)^2*vB^2] + 0 + Mgd I found the answer to be: ((kd^2 - Mgd)(m+M))/m^2 = Vb but this was not correct...please help me.