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Homework Help: Conservation of energy/ efficiency question.

  1. Jan 25, 2009 #1
    1. The problem statement, all variables and given/known data

    A climber's gravitational potential energy increases from 14000 J to 21 000 J while climbing a cliff. She expends 18 000 J of energy during this activity. What is the efficiency of this process?


    2. Relevant equations

    Efficiency= useful output/ total input

    Total energy before = total energy after

    3. The attempt at a solution

    Well, I've been able to solve all other efficiency/ conservatino of energy questions but for some reason I just don't understand this one. I don't understand what to consider as the input or the useful output energy. Thanks for your help!
     
  2. jcsd
  3. Jan 25, 2009 #2

    LowlyPion

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    What was her change in potential energy? Isn't that the useful output?

    The work gone into increasing potential energy?
     
  4. Jan 25, 2009 #3
    Okay well then useful output would be 7 000 J / 14 000 J (input), and efficiency would be 50 % ? But the answer says it is 61 %.
     
  5. Jan 25, 2009 #4

    LowlyPion

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    No. She expended 18,000 J not 14,000.
     
  6. Jan 25, 2009 #5
    Oh! Okay. Still doesn't work with the answer though ah.
     
  7. Jan 25, 2009 #6

    LowlyPion

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    It is 1 - the answer.

    I think the answer may be wrong?
     
  8. Jan 25, 2009 #7
    Yeah we have an exam review booklet which said that was the answer, but I just found the same exam on the internet and it said yours was right, so the booklet answer was wrong. Thanks!
     
  9. Jan 25, 2009 #8

    LowlyPion

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    That's a relief. Wouldn't make sense otherwise.

    Good luck.
     
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