# I thought springs can only pull

1. Feb 18, 2015

### reyrey389

http://faculty.mint.ua.edu/~pleclair/ph125/Misc/exam2_form_a_solns.pdf

On page 1, problem 1, where it shows a mass M and another mass 3M connected by a cord and spring. Then the cord is burned off, and then the spring "pushes the blocks away" I get the momentum conservation and how to work the problem, but I don't get why the springs would behave like this and push the blocks? If the blocks were at rest initially, then the spring is neither stretched nor compressed, it is at its relaxed state and so it has no energy at all. But even if it did, this would come from kinetic energy of the blocks moving to the left/right, and stretching the spring to the left/right, so then the moment the cord is cut , wouldn't the spring *pull* the blocks towards each other ?

Thanks

2. Feb 18, 2015

### A.T.

Why?

3. Feb 18, 2015

### davenn

You didn't read the problem correctly did you ....

Reread the second sentence ... what does it say ?

cheers
Dave

4. Feb 18, 2015

### sk1105

The idea that a spring pulls back to its relaxed state after being stretched is merely one particular case of a more general statement about springs. Namely, if the length of a spring is perturbed from its equilibrium length, the spring exerts a restoring force, opposite to the perturbation, that attempts to recover the equilibrium situation. The perturbation can, of course, be in either direction, so the restoring force can be a push or a pull.

5. Feb 18, 2015

### reyrey389

Yeah I read it wrong, from what I understand, the blocks were initially pushed towards each other and this was balanced by the spring force, to give a net force of zero, and to keep the blocks at rest. But the forces that oppose the spring force pushing the blocks away, is it only the tension in the cord , or is there an additional applied force from the persons hand squeezing them together ? It makes more sense this way, cause if the blocks are at rest, I can't picture how Tension alone can pull them towards each other. So shouldn't F_applied + Tension = Spring Force (Initially) or is it only Tension = Spring Force

6. Feb 18, 2015

### davenn

the blocks are pushed close together, compressing the spring between them. Whilst they are in that state, the cord is attached between the 2 blocks
to keep them together ( and the spring compressed). The cord is under tension, the compressed spring has stored energy
when the cord breaks, the energy in the spring is released, returning it to its uncompressed state and also pushing the blocks apart

Dave