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Potential energy problem

  1. Jan 15, 2008 #1
    [SOLVED] Potential energy problem

    A man that has a mass of 80.0 kg, wearing a backpack with a mass of 20.0 kg, climes 8848 m to the top of Mount Everest. What is his potential energy?



    The only one I know of is PE=mgh



    I added the masses, then plugged it into the equation, but I didn't get the answer.

    Edit:It was already in meters. Sorry.

    Thank you for your time.
     
    Last edited: Jan 15, 2008
  2. jcsd
  3. Jan 15, 2008 #2

    Kurdt

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    Did you convert the height in feet to meters?
     
  4. Jan 15, 2008 #3
    Sorry, it was in meters already. I apparently can't type well today.
     
    Last edited: Jan 15, 2008
  5. Jan 15, 2008 #4

    Kurdt

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    The problem is that equation only holds for short distances above the Earths surface since g depends on the height above the Earth. Have you seen any other formula for gravitational potential energy?
     
  6. Jan 15, 2008 #5

    Tom Mattson

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    Kurdt, 8848 m is a short distance above the surface of the Earth, so that is definitely not the problem.

    Twilit_Truth needs to show some work if we are to figure out what the real problem is. I'm guessing he's either making a calculational error, or doing something silly like using 32 for g, when everything is in mks units. Or possibly it's more subtle. Perhaps he is using a different datum than the answer key is using. IOW, perhaps he is setting V=0 at the surface of the Earth, while the author of the book sets V=0 at the center of the Earth or at infinity.

    Twilit_Truth: Let's see how you're doing this.
     
    Last edited: Jan 15, 2008
  7. Jan 15, 2008 #6

    Kurdt

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    haha sorry. Long time since I've done one of these. Guess I was jumping too far ahead on tracking the problem.
     
  8. Jan 15, 2008 #7
    Ok, I'll try to show you what I did. Just so you know, this is a multiple choice question.

    First off, I combined the masses of the climber and the pack, resulting in a total mass of 100. kg. Then, I multiplied it by 9.81, which is the only value that I know of for g when on the Earth. This came to 981 N. Finally, I multiplied this by the height, which is 8848 m, and got 8679888 J.

    The problem is, the answers are:

    A) 6.94E6 J
    B) 4.16E6 J
    C) 2.47E6 J
    D) 1.00E6 J

    I don't know where I went wrong. I was absent while they taught this, so I probably messed up somewhere. Thank you both for your time.
     
  9. Jan 15, 2008 #8

    hage567

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    Why are there 4 answers? Is this question supposed to be part A,B,C or D?
     
  10. Jan 15, 2008 #9
    It's a multiple choice question. Only one answer is right out of those four.
     
  11. Jan 15, 2008 #10

    hage567

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    Oh I get it.

    Anyway, I believe this is somewhat of a trick question. The question asks what is *HIS* potential energy.

    Think about it.
     
  12. Jan 15, 2008 #11
    Good eye. Thank you for the help.
     
  13. Jan 15, 2008 #12

    hage567

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    You're welcome.
     
  14. Jan 15, 2008 #13
    I did His force, and it came out to be 6943910.4 i figure that is one of your answers
    Answer A
     
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