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Simple Work Question

  1. Oct 25, 2006 #1
    Hi I always thought work was defined as the dot product of the force and displacement vectors.

    W = F parallel to D

    However, my physics teacher told me that it's force x DISTANCE.

    Does that mean that whenever someone lifts a weight and brings it back down is actually doing work? (displacement is zero...) :uhh:
     
  2. jcsd
  3. Oct 25, 2006 #2
    I've always known work as [tex]W = \vec {F} \cdot \vec {D}[/tex] or [tex] W = |F||D|cos\theta[/tex] theta being the angle between the force and displacement vectors.

    More info on work can be found http://en.wikipedia.org/wiki/Mechanical_work
     
  4. Oct 25, 2006 #3

    PhanthomJay

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    The person does no net work when moving the block up and then back down. As you noted, the displacement is 0. You could also argue that it takes say 100 joules of work to move it up, and -100 joules of work to bring it down. However you slice it, it all adds up to nothing.
     
  5. Oct 26, 2006 #4
    Distance that you are referring to is the distance moved the DIRECTION of the force. The person did do work: against gravity. When he lets go, it is actually gravity that does work, not him.
     
  6. Oct 26, 2006 #5

    PhanthomJay

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    ]But if he doesn't let go, and slowly lowers the weight to the floor, he still does work, but the net work he does in both directions is zip.
    He might be tired and claim he did a lot of work, but he really didn't do any. Depends on what you mean by "work".
     
  7. Oct 27, 2006 #6
    Well, if you put it that way yes. His total work against is technically negative since the direction of movement when the net work is 0. Since while he lowers down the weight, the direction of the displacement is opposite that of the force. Hence, work done is actually negative.

    Note: The man feels tired because his muscles are actually doing work by continously stretching and compressing. I won't delve further since we are already touching biology.
     
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