1. The problem statement, all variables and given/known data http://www.pdx.edu/physics/sites/www.pdx.edu.physics/files/Nov-19th_version_Comprehensive Exam I_Nov-2006.pdf 2. Relevant equations 3. The attempt at a solution This problem is driving me nuts. Its the problem number 1 in the afternoon session exam provided in the link above. It's about conservation of linear momentum. I have analysed the first situation and got the equation V2=V(1-F/2) *given that (1-f)^1/2 = 1-f/2 as f << 1. This is given in the Feynman's exercise book. I can't for the life of me analyse the second situation. Could someone please at least tell me what the equation for the conservation of Kinetic Energy be?