# Difference between F=kx and F=-kx

1. Oct 14, 2012

### AryRezvani

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

I notice that when solving for k, the book often uses a positive version of the formula.

How do you know which to use?

2. Relevant equations

F=kx
F=-kx

3. The attempt at a solution

2. Oct 14, 2012

### Toranc3

Work done on the spring
F=kx positive

Work done by the spring
F=-kx negative

Remember Newton's Third Law of Motion. Re-read it if you have to.

3. Oct 14, 2012

### daivinhtran

Nope. I don't think so. F means force.
So, F=kx is the force we act on the spring
F= -kx is the force acted by the spring.
Example
Assume the x positive direction is to the right....and we stretch the spring the right, that means our force is pointed to the right....

and the force of the spring is pointed to the left.

Last edited: Oct 14, 2012
4. Oct 15, 2012

### Toranc3

Yeah your right. Once you integrate them then they are work. Sorry topic starter.