1. The problem statement, all variables and given/known data [PLAIN]http://online.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?courses/phys211/oldexams/exam2/sp10/fig3.gif [Broken] A 4.0 kg circular disk slides in the x-direction on a frictionless horizontal surface with a speed of 5.0 m/s. It collides with an identical disk that is at rest before the collision. The collision is elastic. Disk 1 goes off at an angle of 60° with respect to the x-direction. Disk 2 goes off at an angle of 30° with respect to the x-direction. Treat the disks as point objects and ignore the possible rotations of the disks. 1) Why does disk two have more KE? 2) Find speed of disk 2 after collision. 2. Relevant equations Conservation of momentum/Ke since the collision is elastic. 3. The attempt at a solution My professor wants me to use center of mass reference frame (and velocity of CM) to solve this problem. I am not sure how to do this, but I do know how to calculate the Vcm before the collision. It is m1v2/(m1+m2). Vcm is same after collision so the Y velocities should cancel out. I can't quite connect that with why the KE of disk 2 is bigger, though.