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Gravitational potential energy of a retaining wall

  1. Nov 27, 2014 #1
    1. The problem statement, all variables and given/known data
    Forty 2.0kg blocks 20.0cm thick are used to make a retaining wall in the backyard. Each row of the wall will contain 10 blocks. You may assume that the first block is placed at the reference level. How much gravitational potential energy is stored in the wall when the blocks are set in place.

    2. Relevant equations
    Eg=mgh

    3. The attempt at a solution
    To find m (mass):
    (40)(2.0kg) = 80kg
    To find height (h):
    20.0cm = 0.2m
    (0.2m)(4)= 0.8m I used 4 because the wall will be four rows high.
    Final...
    Eg= (80)(0.8)(9.8)
    = 627 J
    The answer in my book says 240J

    [Mentor's note: Thread title changed to reflect actual subject matter of post]
     
    Last edited by a moderator: Nov 27, 2014
  2. jcsd
  3. Nov 27, 2014 #2

    Orodruin

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    You need to use the location of the center of mass of the wall, not its total height.
     
  4. Nov 27, 2014 #3
    What is the location of the center of mass ?
    Is it 0.3m or 0.4m ?
     
  5. Nov 27, 2014 #4
    so...
    Eg=(80kg)(0.4m)(9.8)
    = 313.6 J
    Im still not getting the answer which is 240J.

    Can you please show your calculations? It would be really helpful!
     
    Last edited: Nov 27, 2014
  6. Nov 27, 2014 #5

    Orodruin

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    Is the centre of mass really at 0.4 m? Remember that the bottom layer is placed in such a way that it does not have potential energy. Also, I very much suspect the answer is using g ≈ 10 m/s2.
     
  7. Nov 27, 2014 #6
    I believe the height is 0.3m but when I do the calculations, I am still not getting the answer.

    Eg= (60kg)(0.3)(9.8)
    = 176.4 J
     
  8. Nov 27, 2014 #7

    haruspex

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    The 0.3m is an average over all blocks, so you must count all the blocks in the mass.
     
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