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Homework Help: Motion, Power, Energy Questions

  1. Feb 21, 2012 #1
    1. The problem statement, all variables and given/known data

    A load of 2,5kN is hoisted by means of a rope and a lifting machine with a rotational frequency of 40r/min and a drum diameter of 700mm.

    Calculate the following:

    1. The torque

    2. The speed of the rope or load in m/s

    3. The work done per minute

    4. The power required to lift the load

    5. The efficiency of the lifting machine if an electric motor of 5kW is required to drive the lifting machine

    2. Relevant equations

    See below

    3. The attempt at a solution

    This is my first time posting, so I hope I have got the format right. Here's my attempt at the equations

    1. T = F x r
    T = 2500N x 0.35m
    T = 875Nm

    2. V = π x d x N/60
    V = π x 0.7m x 40/60
    V = 1.465m/s

    3. WD = F x s
    WD = 2500N x (1.465 x 60)
    WD = 2500N x 87.9
    WD = 219750 joules
    WD = 219.75 kJoules

    4. P = F x V
    P = 2500N x 1.465m/s
    P = 3662.5watts
    P = 3.663kWatts

    5. Eff = OUT PWR/IN PWR %
    Eff = 3662.5w/5000w
    Eff = 0.7325
    Eff = 73.25%

    Those were the answers I got. Please correct me if I am wrong, or just give me some guidance and I can re attempt the questions.

    Thanks guys :D
    Last edited: Feb 21, 2012
  2. jcsd
  3. Feb 21, 2012 #2


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    For question 1, technically the total torque on the drum is zero. But I think they are asking for the torque which must be applied by some external motor. So I think you do have the right answer for question 1.
  4. Feb 21, 2012 #3


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    And your answer for question 2 looks good too.
  5. Feb 21, 2012 #4


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    3,4,5 all look correct as well. nice!
  6. Feb 22, 2012 #5
    Thanks BruceW, I appreciate it :D
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