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Energy Problem

  1. Mar 31, 2007 #1
    One kilogram of fat is equivalent to 30 MJ of energy. The efficiency of converting fat to mechanical energy is about 20%.

    a) Suppose a 75 kg person runs up the stairs of tall building with a vertical height of the 450 m, how much work is done by the person?

    b) If all the energy used to do the work comes from "burning" fat, how much fat is used up by the exercise?

    c) If the person took 45 min. to run up the stairs, what was their power output.

    My proof:

    A. m= 75kg
    d= 450m
    w=75kg(9.8N/kg)*450m
    w=3.3*10^5J

    PLZ HELP ME IN B AND C AND TELL ME A IS RIGHT OR WRONG.
     
  2. jcsd
  3. Mar 31, 2007 #2

    Dick

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    A is fine. So 1kg of fat can provide .20*30MJ of mechanical work. What fraction is w of this? That's the fraction of a kg that you will burn. For C, power is w/time. If you express time in seconds then what units is the power in?
     
  4. Mar 31, 2007 #3
    HOW CAN I FIND B?
    So 75kg of fat can provide (.20*30MJ)75 KG
    WHAT'S NEXT?
     
  5. Mar 31, 2007 #4

    Dick

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    You don't have to shout. You have (.20*30MJ)/kg. So (.20*30MJ/kg)*x kg=3.03*10^5J. Solve for x. What's the problem? MJ=10^6J if that helps.
     
  6. Mar 31, 2007 #5
    Anyone Explain Me
     
  7. Mar 31, 2007 #6

    Dick

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    Think proportions. 3.03*10^5J is to (.20)*30MJ as x kg is to 1kg. Where x kg is the amount of fat you burn. Solve for x. If you don't get that, I give up.
     
  8. Mar 31, 2007 #7
    can u give me answer b and c?
     
  9. Mar 31, 2007 #8

    Dick

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    No. Think.
     
  10. Mar 31, 2007 #9
    Anyone Explain Me
     
  11. Mar 31, 2007 #10

    Dick

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    You mean "Anyone Give Me The Answer". And I don't think anyone will. But you might get lucky... It's really easy.
     
  12. Mar 31, 2007 #11
    it's not easy for me
     
  13. Mar 31, 2007 #12

    Dick

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    Point taken. But A was actually harder. How do you solve a problem like 2/3=x/4. That is the problem you are facing. Just with odder numbers and units attached.
     
  14. Mar 31, 2007 #13
    thxxxxxxxx
    i got u
    MJ=10^6J
    (.20*30MJ/kg)*x kg=3.3*10^5J
    so 330000/6000000
    Answer=55g
     
  15. Mar 31, 2007 #14

    Dick

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    Absolutely right.
     
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