Posted by Brian on .
Two blocks of masses M and 5M 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 seen in the figure below:
img404.imageshack.us/img404/8349/sbpic0906.png
1) A cord initially holding the blocks together is burned; after this the block of mass 5M moves to the right with a speed of v1=2.26 m/s. What is the velocity of the block of mass M?
I found the answer for this to be 11.3 m/s (which is correct).
2) Calculate the original elastic energy in the spring if M = 0.340 kg.
I don't know how to do this one! please help. I tried using 1/2mv1^2 + 1/2mv2^2 but no luck :(

Physics 
Brian,
Nevermind I figured it out... :)