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Homework Help: Potential energy and work

  1. Oct 11, 2009 #1
    1. The problem statement, all variables and given/known data

    Six identical books, 4.0 cm thick and each with a mass of .80 Kg, lie individually on a flat table. How much work would be needed to stack the books one on top of the other.

    2. Relevant equations
    U= mgy

    3. The attempt at a solution

    total mass = .8kg x 6 = 4.8
    G= 9.8
    y= .004 * 6 = .024

    (4.8)(9.8)(.024) = 1.12 J

    the answer is 4.7 J. What am I doing wrong?
  2. jcsd
  3. Oct 11, 2009 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    y is not simply 6 x brick thickness.

    The bricks are each raised a different height, in order to make them into a single stack of 6 bricks.
  4. Oct 11, 2009 #3
    Youre doing work against gravity right? You have to lift one book to put it on the other book, a certain height. When you add a third book, has the height you have to lift it changed?
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