Difference between F=kx and F=-kx

[AryRezvani]

Homework Statement

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

How do you know which to use?

Homework Equations

F=kx
F=-kx

The Attempt at a Solution

 

[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.

 

[daivinhtran]

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.

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.

 

[Toranc3]

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.

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