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Help for a short question

  1. Dec 8, 2004 #1
    A man holding a book is walking, what's the work done on the book by him?
    Is this should be accounted into the work done by friction but not by the man? Cheers
  2. jcsd
  3. Dec 8, 2004 #2
    What is work? It is the force exerted times the displacement in the direction of this force. So what is/are the force(s) exerted and what is the displacement in the direction of this/these force(s)?
  4. Dec 8, 2004 #3
    How is the man holding the book ??
    Hint:- He is overcoming the wt. of the book to hold on to it.

    This should give you some idea about the direction of motion and the book's displacement.
  5. Dec 9, 2004 #4
    Yes I know the work done by his hand is 0.
    But the book is moving, the force applied is the reaction force of his motion, so what's the work done?
  6. Dec 9, 2004 #5
    I think you can best find the work done considering the kinetic energy and noticing that friction (between the book and hand) does no work as they do not move relative to each other.
  7. Dec 10, 2004 #6

    But isn't the book is moving relative to the ground?
    There must have some force applied.
  8. Dec 10, 2004 #7


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    No. Since there are no horizontal forces involved at all, the book won't need to be pushed along in the direction it is going. (Assuming the man walks with constant velocity).

    The only forces on the book are the gravitational force downwards and the force by the man upwards.
  9. Dec 10, 2004 #8


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    Because the man is holding the book while he is walking, you can consider it part of the system. Thus no work is done individually on the book. And if the man is moving with a constant velocity then no work is done at all. The only forces acting on the book are the gravitational and the normal force of the hand on the book.
    Last edited: Dec 10, 2004
  10. Dec 10, 2004 #9
    :biggrin: :biggrin: :biggrin:
  11. Dec 10, 2004 #10


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    Umm.. I do not recollect ever mentioning Galileo in that response. Actually I am sure I did not.... because that does not even have relation to him.
  12. Dec 10, 2004 #11
    How about the force by man is vertically upward, and the man is running?
    The friction is able to hold the book on the hand of the man,i.e. not falling+ staying at one position of the hand, what's the work done by the man?
  13. Dec 11, 2004 #12

    Well you took it away after I mentioned it.... o:) so sure you do not remember it... (You were referring to the PF member Galileo though)
  14. Dec 11, 2004 #13
    No matter whether the book is accelerating or non-accelerate, the man is still giving work to it.
  15. Dec 11, 2004 #14
    Ofcourse it matters if the book is accelerating or not, do you remember F=ma? If the book moves with a uniform velocity the resultant force on it is zero (Newtons first law, aka Galileo's principle). So the work done on it is also zero.
  16. Dec 14, 2004 #15


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    I see that now :biggrin: tricky tricky
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