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How there is no work done on a book while i am holding it and moving ?

  1. Apr 15, 2013 #1
    i still don't really get how there is no work done on a book for example when am holding it and moving ?! i am a 9th grade student and when i ask this weird physics teacher of mine she keeps telling me that when i am holding the book it becomes a part of my body O__o i am not convinced at all i think that work is done on the book as it moves with me !! :)

    so ... any help ?!
     
  2. jcsd
  3. Apr 15, 2013 #2

    Doc Al

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    To do mechanical work on something you must be exerting a force in the direction of your motion. When you are holding a book, the force you exert is upwards (to support its weight). But when you walk you are moving horizontally, so no work is done on the book.

    There's certainly biological effort involved in getting your muscles to exert that force. But you are not doing any mechanical work on the book.

    If you lift the book upwards, instead of just holding it at the same height, then you'll be doing work on the book.
     
  4. Apr 15, 2013 #3

    phinds

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    In addition to what Doc Al said, if you climb stairs with the book, they you are doing work on it (AND on your body --- this is the same as what he said about lifting it upwards, I just want to be sure you see that it's a different situation than just "moving with your body" in a horizontal plane)
     
  5. Apr 15, 2013 #4
    well ... that made things clear enough
    thank u :)
     
  6. Apr 15, 2013 #5
    It sounds as if while you are holding the book motionless relative to yourself, no work is being done on the book. If you are moving the book relative to yourself, it is. For example, if the book is in your hand and your arm (not the other one, of course...) is swinging while you walk, is work then being done on the book?
     
  7. Apr 15, 2013 #6

    russ_watters

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    No. Your arms act like a pendulum in that case and gravity does work against the book, then the book does work against gravity and so on and so forth.
     
  8. Apr 16, 2013 #7
    If I were a teacher and asked the question, "If you hold a book with your palms facing up and the book on top while walking forward, are you doing work on the book?"

    I would accept the answer yes if the student mentioned the friction between your hands and the book. If they responded along the lines, "since the friction between your hands and the book is in the direction of motion, yes you are doing work on the book" that would be full credit.

    If yomna2013 felt that the act of holding the book was doing work (which is a common misunderstanding) that would be unquestionable wrong. But I do see some fuzziness depending on the exact phrasing of the question. If you view it as friction does the work, but I provide the friction (and slightly push forward), that's fine.
     
  9. Apr 16, 2013 #8
    But work IS being done on the book? I would have thought that by swinging the book, the muscles of the arm would be doing work on the book?

     
  10. Apr 16, 2013 #9

    A.T.

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    Work in general is relative. When you are standing on the ground and holding a book still, you are doing no work on the book in your frame. In the frame of someone in an moving elevator, you are doing work on the book, or the book on you.
     
  11. Apr 16, 2013 #10

    Doc Al

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    Good point. But work would only be done horizontally while you were accelerating. Once you achieved a constant walking speed, no more work is done. So you could walk for miles at that constant pace and yet be doing no work.

    If the student explained all that, and explained why no work was done against gravity, they'd get extra credit!
     
  12. Apr 16, 2013 #11

    sophiecentaur

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    The answer to this question depends totally on how 'idea' your model is. The definition of Work Done on the book requires it to change its height (PE) or speed (KE). So, if it is moving uniformly in a horizontal direction and there is no 'Force times Distance' involved, then no work is done 'on' it. This has nothing to do with the work done, internally, by the muscles of your body. A lot of 'Forces times Distances' are involved inside you.
    The operation has Zero Efficiency, in fact: Work in but no useful work out.
     
  13. Apr 16, 2013 #12

    A.T.

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    But... but... Mr. Teacher, what about the air resistance of the book. Even at constant speed your hand has to do positive work on the book, because the air is doing negative work...
     
  14. Apr 16, 2013 #13

    sophiecentaur

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    It all depends upon how 'ideal' your model is. Define that perfectly, from the start, and you will get the best answer. There will always be 'loopholes' if the goalposts are changed.
     
  15. Apr 16, 2013 #14

    russ_watters

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    Your arms essentially swing by themselves when you walk. That's why your legs get tired but your arms don't.
     
  16. Apr 16, 2013 #15

    Doc Al

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    :rofl:

    OK, extra credit for you too!
     
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