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## Homework Statement

http://www.sumoware.com/images/temp/xzqmibbrldrafpgj.png [Broken]

At first, the system is statics. Then, a spring with spring constant k is pressed with Δx = A. At t=0, mass m and M are released so mass m and M have relative velocity to the Earth (v

_{m}and v

_{M})

Determine both velocity in k, A, m, M

## Homework Equations

E = E'

p = p'

## The Attempt at a Solution

Actually, I've got the solution in my book.

The solution is to use the conservation of energy and momentum.

But, what I don't understand is why using conservation of momentum ?

I think the momentum is not conserved, since there is force of the spring that makes both v

_{m}and v

_{M}goes in the same direction.

I imagine it. First, the system is static (initial momentum is zero). Then, the object m and M is moving in the same direction (so, I think that the final momentum is not zero)

Please help me understand the concept of conservation of momentum

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