percent of energy lost??? I'm confused... I have a case where block 1 moves with certain velocity, hits block 2 (that is at rest) and sticks to it - inelastic collision. Now, the problem is that I have to find the percent of energy (kinetic) that the first block has lost. I know that the formula is [(K initial - K final)/K(initial)]*100, but wha I'm wandering about is whether K(final) should include the mass of block 2 or not? They are actually one mass after collision, but the question asks for the lost of energy of block 1. Can you somehow explain this to me? Thank you!