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Homework Help: Given mass, distance, and time, calculate power

  1. Sep 19, 2010 #1
    1. The problem statement, all variables and given/known data

    How much power does a weightlifter expend when lifting a 50kg barbell a distance of 1.2m in a time interval of 1.5s?

    2. Relevant equations

    Power=work done/time interval (in Newton meters per second or Watts)
    Work=Fd (in Newton meters or Joules)

    3. The attempt at a solution
    Power = Fd/t = (m)(a)(d)/t = 50kg(9.8 m/s2)(1.2m)/1.5s = 392 Nm/s = 392 W

    This is how I solved the problem, but my professor said the correct answer is 40 Watts!
    Work = force x distance; work = 50 kg x 1.2 m, work = 60
    Power = work /time; 60/1.5 = 40 Watt

    Can someone please verify which way is correct??? I am sticking by my answer of 392 Watts
  2. jcsd
  3. Sep 19, 2010 #2


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    Homework Helper

    '50kg' is a mass and not a force, it must be correctly multiplied by acceleration (gravity in this case) to convert it to a force. You are correct.
  4. Sep 19, 2010 #3
    Ok, so you are sure the correct answer is 392 Watts? I feel like I will be shunned for going against the professor, but I have to go with my gut on this one.

  5. Sep 19, 2010 #4


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    I didn't check over your answer, but I am quite sure that 'kg' is a unit of mass and not force.
  6. Sep 19, 2010 #5
    I agree. Thanks!
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