# Homework Help: Dynamics Question

1. Jun 18, 2017

### brk51

1. The problem statement, all variables and given/known data

2. Relevant equations

F_spring = ks and F_spring potential = 1/2ks^2
3. The attempt at a solution

So I recognize that @ theta = 0 it is a 3/4/5 triangle. The length of the spring is at .5m...furthermore the given spring force at that angle is 100 N. So why is it wrong to do 100 = (1/2) (500) (s)^2 ==>> s = .632m... Why should I use F= ks

Everything else I understand...

2. Jun 18, 2017

### person123

Maybe I don't understand what you're trying to do, but it looks like you're plugging in the force of the spring where the potential energy should go.

If you know that the spring applies 500 newtons for every meter its stretched, if it's applying 100 newtons, it must have been stretched 0.2 meters (and therefore its normal length is 0.3 meters). That would allow you to find the potential energy in the spring. (Again, if I'm misunderstanding the problem, please tell me).