1. The problem statement, all variables and given/known data A plank of mass 5 kg is placed on a frictionless horizontal plane. Further a block of mass 1 kg is placed over the plank. A massless spring of natural length 2 m is fixed to the plank by its one end. The other end of the spring is compressed by the block by half of spring's natural length. The system is now released from rest. The spring constant is 100 N/m. What is the velocity of plank when the block leaves the plank? 2. Relevant equations 3. The attempt at a solution Since the plank acquires some velocity, there must be some friction in between the two blocks. Applying Energy conservation just after the block is released, 1/2 kx^2 = 1/2 mv^2 x=1 and m=1 v=10m/s Applying momentum conservation to the block-plank system, m x v= m x v1 + M x v2 where v1 is the velocity of the block just before leaving the plank I can't find another equation after this.