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**Homework Statement**

A system is composed of two blocks of mass m1 and m2 connected

by a massless spring with spring constant k. The blocks slide on a

frictionless plane. The unstretched length of the spring is L. Initially

m2 is held so that the spring is compressed to L/2 and m1 is forced

against a stop. m2 is released at t = 0.

Find the motion of the center of mass of the system as a function

of time.

I understand that when string is back to full Length L, M2 has speed v=0.5L*sqrt(K/m2).

And that is the moment when M1 loses contact with the stop and start moving with M2.

**My question 1.**

So once M1 leaves the stop, to

**keep momentum constant**, the center of mass should have speed

m2*v/ (m1+m2)

Am I right? I am asking because I read some other solution which states M1 would move at the same speed v as m2, then momentum is not conserved.

**My question 2.**

Is energy also conserved? In above, if both M1 and M2 moves in speed m2*v/(m1+m2), or both in speed v, in either case energy is not conserved. What is causing energy to be not conservative here??

Thanks a lot!