# Work done by spring forces - positive or negative

by lemonpie
Tags: spring force, work
 P: 4,512 Keep in mind that there are two forces. The force of the spring on some object and the force of the object on the the spring. Secondly, a positive force along X is in the positive X direction. $$F=-kx$$ Which one is this? Is it the force on the spring or by the spring? On and By are your two key words. Always think of them in bold font and you should get your signs correct. Edit: there is another idea to keep in mind dealing with vector quantities that can be confusing at first. So if the above is not enough, and you are using force as a vector, there can be one more thing to keep in mind.
 Sci Advisor HW Helper P: 4,300 I think the problem is in the calculation of your work. The potential energy of the spring is $U = \frac12 k x^2$ from which you can determine the spring constant. Quote Wikipedia: "(Note that potential energy of a spring is always non-negative.)". The work you are doing to stretch the spring is getting stored in the spring as potential energy. I.e. you are doing work on the block so if you let W be the work done by the spring on the block then U = -W (you are adding energy to the spring while W is like energy released by the spring in the process). Does that make sense?