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Carrying a heavy box - work done?

  1. Jan 18, 2009 #1
    this is not homework,
    different internet articles and online textbooks suggest different answers.


    Calculate work done:
    a.) Person carries a box with constant velocity some distance.
    b.) Person runs with constant velocity some distance.
     
  2. jcsd
  3. Jan 18, 2009 #2

    ZapperZ

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    Then you need to provide these sources that show these "different answers". We have no way of knowing if these sources are correct, if you've read them correctly, or if they are simply different situations.

    If not, we too will simply be another "internet article" that gives you a different answer. How will you know which one is correct? Go to another forum and ask again?

    Please note that in this forum, if you wish to discuss something that you've read or encountered, then a full reference to that source must be made for the discussion to have any meaning. If not, we are simply going by hearsay.

    Zz.
     
  4. Jan 18, 2009 #3
    If you carried a box at constant velocity, the net work on that box is zero (since the velocity was constant, acceleration was zero, and net force = 0 .. so work = F*distance = 0).

    What confuses many people is: You do burn Calories running at a constant velocity. Your body did indeed do work. And you will indeed find this on any excerise resource listing the amount of work (Calories or Joules) different activities take.


    The problem is this: due to biology and how our muscles work, it takes our muscles energy to exert a constant force ... even if we are not moving or accelerating. But if you went in there and modelled all the tiny pices of your body, work still equals force * distance. The equations are not violated.

    This is akin to when students get to conservation of energy. Energy is not "destroyed" in situations with friction. If we modelled all the interactions, we'd see energy is indeed still conserved. It is just difficult to show in an easy way in an introductory lecture. Usually the hand-wavy explaination of "the energy goes into heat" is stated and we hope the students trust us enough till they learn more and can approach it in more depth if they are still interested.


    EDIT:
    Oh, I just realized another possibility. If you carry the box upstairs, or down stairs (or a hill, or slope, etc) then non-zero work is done on the box even if carried at a constant velocity. Is that what you meant?
     
  5. Jan 18, 2009 #4
    JustinLevy,

    thank you. i will get back to that.




    ZapperZ,

    to find sources im talking about just pick up any textbook or find any article on the WWW, Wikipedia for example.


    i can answer your questions,
    but i asked my question 1st, so, if you know the answer, can you please tell us?




    i do not wish to discuss internet articles,
    im asking simple question, can you please answer it?


    anyway, here is reference:
    http://www.google.com/search?hl=en&q=physics+work+force+&btnG=Search
     
    Last edited: Jan 18, 2009
  6. Jan 18, 2009 #5

    russ_watters

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    That's not good enough. What ZZ said in that first quote is correct: we have no way of knowing what the real issue is here unless we see the sources you speak of. PM one of us the actual sources where you've seen this contradiction and we'll comment/reopen the thread. Otherwise, Justin's answer is probably sufficient as it point's out the most common misunderstandings people have about this issue.
     
  7. Jan 18, 2009 #6

    HallsofIvy

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    I just looked at 3 different textbooks, two websites other than Wikipedia, and Wikipedia. They all said the same thing. I did not find that "different internet articles and online textbooks suggest different answers."



    In what world do you ask for help and then put conditions on what help you will accept?

    And all the references there say the same thing that Justin Levy said. There is still no indication that "different internet articles and online textbooks suggest different answers."
     
  8. Jan 18, 2009 #7
    Energy, Work and Human Locomotion

    There is this question I was about to answer, however it got locked in the meantime.

    Energy is the ability to do work or to cause change.

    There is quite a bit of work going on there, some positive, some negative, but work done is most certainly not zero. Total work done is most definitely some positive number proportional to how much energy it took to 'cause change'.


    Obviously, as this person suggests, there is a confusion. Formula given, and the first answer says the "work done _on the box" is zero. Then later he says, some work must have been done as energy is consumed during exercise. I agree with the second part of this answer. Since energy is the ability to do work, then mere fact that it takes energy to perform human locomotion means some work gets done. Therefore, work done is not zero.


    I'm not really sure what reference says, questions are not phrased equally. This is where you can get different answers to the "same" question. In most cases mistake is in the question when it was not specific enough, the rest of confusion comes out of assumptions, as usual.

    Some are assuming "work done" means "work done by gravity", others assume it means "work done by person", and by making this assumption they will actually both fail to consider total work done and answer the question properly.
     
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