1. The problem statement, all variables and given/known data Two blocks of masses M and 3M are placed on a horizontal, frictionless surface. A light spring is attached to one of them, and the blocks are pushed together with the spring between them as shown in the figure below. A cord initially holding the blocks together is burned; after that happens, the block of mass 3M moves to the right with a speed of 2.85 m/s. Image: http://www.webassign.net/serpse/serpse7_p9-4.gif a. What is the velocity of block mass M? Assume Right is positive and left is negative I got the solution to this. It was -8.55 m/s b. Find the system's original elastic potential energy, taking M= .300 Kg This one i don't understand how you can find the elastic potential energy without knowing the spring constant/distance compressed. 2. Relevant equations Pinitial=Pfinal P=MV Us= 1/2 KX^2 3. The attempt at a solution I got the solution for part a. m1v1+m2v2=0 v1=-m2/m1 *v2 v1=-3m/m*(2.85) v1=-8.55 part b. potential energy for a spring is 1/2KX^2 The problem gives the mass M as .300kg therefore mass 3M= .900kg 1/2kx^2=1/2MV^2+1/2(3M)V^2 I took a chance and i solved it and put it in and i was right! thank you physicznoob a lot!