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Net Work

  1. Jun 18, 2008 #1
    1. The problem statement, all variables and given/known data
    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?


    2. Relevant equations

    Wnet= F.d

    3. 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!
     
  2. jcsd
  3. Jun 19, 2008 #2

    dynamicsolo

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    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.)
     
    Last edited: Jun 19, 2008
  4. Jun 19, 2008 #3

    Dick

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    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?
     
  5. Jun 19, 2008 #4
    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 =(.
     
  6. Jun 19, 2008 #5

    Dick

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    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.
     
    Last edited: Jun 19, 2008
  7. Jun 19, 2008 #6
    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!
     
  8. Jun 19, 2008 #7

    Dick

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    No, you already have it. In the sense you are talking about the net work is 0 J. You really basically understand most of the stuff you post. It's my problem that I don't understand that you already understand.
     
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