# Force / elastic potential energy of a rubber band

1. Sep 23, 2008

### Heirot

Suppose we have a rubber band of some elasticity k and of unstreched radius r0 (the band is always kept in the shape of a circle). What work is necessary to strech it to some larger radius r? How do we apply Hooke's law in this situation?

Thanks

2. Sep 24, 2008

### Heirot

I guess I misplaced my question... Sorry about that. This really isn't homework, but I'm just curious. Nevertheless... My problem is this. If we have an unstreched band of radius r0, and corresponding circumference (the length of the band) l0, and we strech it to some radius r (length l), what's the difference in potential energy? Is it 1/2 k (l - l0)^2. That would make sense due to Hooke's law. But what about the tension in the band? Is the tension dE/dr or dE/dl? The first expresion makes sense because of the circular shape. The second one follows strictly from Hooke's law. Note that the two forces are not the same. I hope this is not confusing to you as it is to me.

Thanks