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Homework Help: Rate of Energy/Watts

  1. Oct 21, 2008 #1
    1. The problem statement, all variables and given/known data
    At what rate is a 50 kg boy using energy when he runs up a flight of stairs 10 m high, in 7 s? Provide answer in watts. (Hint: Rate of change of energy is power) Use g=10 m/s2.


    2. Relevant equations
    F=ma
    W=Fd
    P=W/T



    3. The attempt at a solution
    10/7 = .143 = A
    F = MA, so F = .143*50 = 7.15
    W = FD, so W = 7.15 *10
    P = W/T, so P = 71.5/7

    Is that right? Am I using the correct acceleration and distance and time, or is there more complicated math involved than I thought?
     
  2. jcsd
  3. Oct 21, 2008 #2
    Hi,

    "10/7 = A"

    10 is a distance in metres
    7 is a time in seconds

    so 10/7 is in metres per second. NOT an acceleration!

    Think about energy gains, not forces etc (i.e. not f=ma etc)

    Cheers
     
  4. Oct 21, 2008 #3
    I don't know how to do energy gains...
     
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