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Work, displacement, and gravity problem!

  1. Mar 17, 2009 #1
    1. The problem statement, all variables and given/known data

    A 1.4 kg book is lying on a 0.80 m-high table. You pick it up and place it on a bookshelf 2.2 m above the floor.

    1) How much work does your hand do on the book?

    2.) How much work does gravity do on the book? [solved]

    2. Relevant equations

    Force*displacement=work

    f=ma

    3. The attempt at a solution

    Ok. I was able to find the displacement to be 1.4 m. Since acceleration due to gravity is 9.8 (and negative in this case) i multiplied it by the mass 1.4 kg to get the force. Then I took the answer (-13.7) and multiplied that by the displacement which gave me the correct answer of -19 J.

    The problem is that I', not sure how to find the amount of work the hand does on the book. The displacement is the same, but I don't know how to find the force. Any help would be appreciated.
     
  2. jcsd
  3. Mar 17, 2009 #2
    Draw a free body diagram for the book to see the forces acting on it.

    Another way to look at it is that in your system energy has to be conserved (work is a measure of energy)
     
  4. Mar 17, 2009 #3
    Your hand is providing all the work agianst gravity required to lift the book to it's new position. So the work your hand does is directly related to the change in potential energy. Can you see how?
     
  5. Mar 17, 2009 #4
    I think I'm starting to get it now. Thank you.
     
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