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Doing Work

  1. Feb 7, 2005 #1
    Hi all,
    There's one controversial question, regarding work. If no physical work is done while holding a heavy object and carrying it horizontally, why then do we spend so much energy.
     
  2. jcsd
  3. Feb 7, 2005 #2

    Jir

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    imagine this:
    (this part is a stick)
    --------------------
    |
    |
    |<-----fishing line with weight on it
    O

    obviously the stick bends (and is already bent because of its own weight). however if you replace the stick with your arm, your arm will not bend (only slightly but we can ignore that), instead it will contract muscles and use up chemical energy.

    There is a slight difference in both cases, with the human arm holding the weight up the weight will never come closer to the ground (even if the gravity increases hundredfold). Untill the energy reserves are depleted ofcourse. whilst the stick bends bends and bends, untill it snaps.

    Basicly both stick and arm use the same amount of energy, just in different forms. However they do not produce work as work requires a distance over which something moves. Work is just a definition. If something does not produce work it does not mean it does not use up energy.

    I hope im correct with all of that.
     
  4. Feb 7, 2005 #3

    Doc Al

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    Staff: Mentor

    While no work is being done on the object, your muscles still must contract and relax to maintain the tension required to hold the object. This requires energy (chemical energy), which ends up as heat.
     
  5. Feb 7, 2005 #4
    This is somewhat controversial. The stick bends while the arm does not, yet they spend the same amount of energy. Why do they spend the same?
     
  6. Feb 7, 2005 #5

    Doc Al

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    The stick, an inanimate object, expends no energy in holding up the weight. Jir's explanation is incorrect.
     
  7. Feb 7, 2005 #6

    Jir

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    First of all, they both carry the same weight. Secondly im asuming they both weight the same (the stick and arm). The arm counteracts the gravity force in a different way the stick does though.

    Stick bends (elastic energy), Arm uses muscles (chemical energy)

    "Jir's explanation is incorrect." yeh, instead of spend it should be exert. (as above)

    edit:
    what i mean is, the forces they both exert are the same. however as using chemical energy is less efficient in this case, the stick has less energy sticking in the holding of the weight. That should be just around right i think.
     
    Last edited: Feb 7, 2005
  8. Feb 7, 2005 #7

    ZapperZ

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    I could mess up with your head even more by saying,

    "Run up a flight of stairs, and then run back down - your muscles may be aching and you might be breathing hard, but you've done no work!"

    but I won't!

    [Ain't I a stinker?] :)

    Zz.
     
  9. Feb 7, 2005 #8
    Your teacher means to say "gravity does no work on the object" which I hope you will agree makes perfect sense. Of course, walking involves constant acceleration, deceleration and torques, so of course carrying an object takes work.
     
  10. Feb 8, 2005 #9

    cepheid

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    ^Careful though...no work is done on the object while carrying it. That's the whole point of this discussion. So what work does it take to carry the object? (I'm not disputing the fact that your body will expend energy while doing so).
     
  11. Feb 8, 2005 #10
    Work is defined as the integral of force with respect to distance, so you cannot carry something without doing work on it. Cepheid, I feel you may be confusing gravity, which is a path independent force (back to the same place, no work done) and dissipative real world forces which are always path dependent.

    If you are trying to ask about why holding an object in place is not considered physical work but is considered to be "work" in an ordinary language sense, then simply consult the definitions for the two different types of work.
     
  12. Feb 8, 2005 #11

    ZapperZ

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    This isn't entirely true. Work is defined as the integral of the dot product of the applied force and the displacement. It means that if the applied force is perpendicular to the displacement, no work is done (example: moving charge in a uniform magnetic field). So yes, you CAN have something underdoing a displacement (carrying something) and yet, no work is done.

    Zz.
     
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