# Net Work

## Homework Statement

A mass of 500 g is attached to the bottom of an unstretched spring attached to the ceiling, and has a spring constant 10 N/m. The mass is then released. What is the net work done on the mass between the point where it was released and the lowest point that it reaches?

Wnet= F.d

## The Attempt at a Solution

The point at which it is released, v=0 and the lowest point is a turning point therefore v=0. So Wnet=delta K= 0 as velocity is 0 both times. Would the work done be 0 J then? Thanks in advance for any help!

dynamicsolo
Homework Helper
This is also why there is a "work-potential energy theorem":

W = - delta_PE .

This is the work done by each force. Consider that there are two acting on the mass, gravity and the spring's restoring force, so you'll need to look at the change in potential energy for each as the mass goes from one rest point to the other.

(There is a more precise way to describe this situation using mechanical energy conservation, which may clarify things further.)

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Dick
Homework Helper
0 J? Are you serious? I think you are maybe asking too many questions and not thinking enough. This has afflicted people on this forum before. Don't let yourself be one of them. Why don't you equate gravitational potential energy change with spring potential energy change if v=0 at both points?

Sorry :\$. It's just that I'm not really good with the concept of work. I read the textbook and did all the recommended questions but this was from a practice midterm. I'll try not to post as much now and try to think them through. But I only post questions if I try them at least 3-4 times and still don't get them! My exam being on Tuesday doesn't help much as I've got to cover everything in a few days =(.

Dick
Homework Helper
Ok, but it still seems to me you could get more of these questions if you would just think harder. I believe you are pretty good. Some people do use PF as a substitute for thinking. Don't be one of them. 0 J? Really? Can you explain your thinking on that? Work=force*distance and work is relative to the force doing the work. If you think of gravity as doing the work then the result is negative the result of if you think of the spring doing the work. So maybe the correct answer is 0 J. So if net work=gravity plus spring then you are correct. Sorry for being so crabby.

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I was thinking of the work-kinetic energy theorem were Wnet= delta K. But maybe I should always stick to the equation: Ki + Ugi + Wext= Kf + Ugf + Delta E (thermal), and plug in things to see where the conversion is occuring. My textbook gives too many formulas which creates a lot of confusion!

Dick