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Work question

  1. Nov 9, 2014 #1
    1. The problem statement, all variables and given/known data

    A student holds a book at rest on the palm of her hand. She lifts the book straight up, bringing it to rest again at a higher point.
    1. Is the net work done on the book positive, negative, or zero?
    2. Is the change in the gravitational potential energy positive, negative, or zero?
    3. Is the total mechanical energy conserved?

    2. Relevant equations


    3. The attempt at a solution
    1. I feel like the net work is zero, because as she speeds the book up the work being done is positive, but the work is negative when she begins to slow back down again. I also feel like the answer could potentially be positive though, because the speed is not constant and the displacement is upward.
    2. I feel like this is positive because the displacement of the book is upward
    3. I feel like this is true but I don't fully understand why that is.
     
  2. jcsd
  3. Nov 9, 2014 #2

    ehild

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    Gold Member

    1) is correct, the net work is zero, according to the work-energy theorem.
    2) is correct, the gravitational potential energy increases.
    3) The mechanical energy is equal to KE+PE. Is it conserved?
    The force you exert while moving the book is not a conservative force. It changes the mechanical energy.
     
  4. Nov 9, 2014 #3
    Ohhh that makes sense, thank you so much!
     
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