1. The problem statement, all variables and given/known data See: http://www.aapt.org/physicsteam/2010/upload/2010_FmaSolutions.pdf #15, 16 2. Relevant equations U_o + K_o = U + K v_CoM = vM1 + vM2 / (M1 + M2) 3. The attempt at a solution I thought that for 15 it was simply: 1/2mv^2 = mgh h = v^2/2g But this is wrong, so then I looked at the answers and saw that they dealt with CoM. How am I supposed to do this problem? And what should have hinted me toward using the CoM besides the answer choices?