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Confused about answer to problem.

  1. Sep 23, 2012 #1
    1. The problem statement, all variables and given/known data

    A 1.57 kg brick measures 20.0 cm x 8.00 cm x 5.50 cm. Taking the zero of potential energy when the brick lies on its broadest face. (a) What is the potential energy when the brick is standing on end? Note: You can treat the brick as though all its mass is concentrated at its center. (b) What is the potential energy when it's balanced on its 8 cm edge?


    2. Relevant equations

    Ug=mgh

    3. The attempt at a solution

    I could not figure out the first part of the question because I don't know what "standing on end" means.

    As for B:

    m = 1.57 kg
    g = 9.81
    h = 0.08 m

    Ug = (1.57)(9.81)(0.08) = 1.23 J

    That is the answer I get, but Mastering Physics tells me that there was a rounding error and the answer is 1.17 J and I cannot figure out where I was wrong. This is not supposed to be a hard problem. Maybe I'm thinking too hard about it??
     
    Last edited: Sep 23, 2012
  2. jcsd
  3. Sep 23, 2012 #2
    Check the difference of CM between 20.0 cm x 8.00 cm surface facing the floor and that of
    8.00 cm x 5.50 cm.

    I guess it is between the broadest and the least broad. There are 3 dimensions of faces.
     
  4. Sep 23, 2012 #3
    Thanks for replying! Could you explain to me what you mean by check the difference? I'm a little slow...
     
  5. Sep 23, 2012 #4
    If it is standing with height of 20cm, then the height of CM is 10cm.
    Likewise if it is standing with height of 5.5cm, then the CM must be (5.5/2)cm
     
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