Two blocks of mass M and 3M are placed on a horizontal, frictionless surface. A light spring is attached t one of them and the blocks are pushed together with the spring between them. The cord ties the two blocks together. The cord is burned and the block of mass 3M moves to the right with a speed of v.
(a) What is the velocity of the block of mass M?
(b) Find the system's original elastic potential energy.