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Work and Energy Question

  1. Nov 17, 2009 #1
    1. The problem statement, all variables and given/known data
    A weightlifter works out at the gym each day. Part of her routine is to lie on her back and lift a 44 kg barbell straight up from chest height to full arm extension, a distance of 0.54 m.

    2. Relevant equations
    If the weightlifter does 24 repetitions a day, what total energy does she expend on lifting? Assume 25 % efficiency.

    3. The attempt at a solution
    I looked at it, and could only see that the only change in energy is work. Work is 230J per repetition. I tried multiplying 230J by 24 in order to get the answer, but it was not correct.
     
  2. jcsd
  3. Nov 17, 2009 #2

    cepheid

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    What about the efficiency? Only 25% efficiency means that 3/4 of the work done is wasted, and only 1/4 of it goes to actually lifting the weight.
     
  4. Nov 17, 2009 #3
    Ya, I took the 230 J and multiplied it by 24, then by .25 but it won't accept the answer
     
  5. Nov 17, 2009 #4

    cepheid

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    That's because multiplying it by 0.25 is not the right thing to do. You should read my post again, more carefully.
     
  6. Nov 17, 2009 #5
    Ok I got the answer, its 22,000 Joules.
    Part 3 of the question: How many 480 Calorie donuts can she eat a day to supply that energy?
    I thought I could just convert the joules to calories by dividing by 4.184, then divide again by 480 to get number of donuts.
     
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