# Homework Help: Jumping box with a spring and mass inside

1. May 24, 2014

### Zeor137

1. The problem statement, all variables and given/known data
Inside a mass M box, over a horizontal floor, there are a massless spring of elastic constant k, fixed to a body of mass m. That body is trapped to a wire in the bottom of the box. After the wire breaks the box will jump with which deformation b of the spring?
2. Relevant equations
F=m.g
F=k.b
3. The attempt at a solution
I know the answer is that below but i just can't think how would I get to it.
b > (M + 2m)g/k.

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Last edited: May 24, 2014
2. May 25, 2014

### Staff: Mentor

Hi Zeor137. I think what happens is that in the situation shown, the spring is stretched. When the wire is cut, the mass m accelerates upwards until the spring becomes compressed by almost as much as it had been stretched (less an allowance for the weight of m).

I haven't worked it out, so I can't be certain that that's the right way to view it.

3. May 25, 2014

### Staff: Mentor

Welcome to the PF. Please use NO's hint to help you work this problem. We do not allow posts here where you do not show your work toward a solution.