1. The problem statement, all variables and given/known data A 500g putty ball moving horizontally at 6m/s collides with and sticks to a block lying on a friction-less horizontal surface. If 25% of the kinetic energy is lost, what is the mass of the block? 2. Relevant equations initial (i) = final (f) m1v1+m2v2 = m1v1+m2v2 3. The attempt at a solution (m1v1i - m1v1f ) /v2f I'm confused :-( Somebody help me solve this. Also is the reduction of the kinetic energy relevant to solving this question?