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Homework Help: Winch power requirement

  1. Sep 19, 2010 #1
    1. The problem statement, all variables and given/known data
    I need to write an equation to find the necessary power requirements of a winch used to raise and lower a lift of an undetermined mass and a counterweight that is half the mass of the lift. The maximum speed of the lift is 3m/s, and the lift travels a total of 30m.


    2. Relevant equations
    So far I have p=w*s where p is power, w is the work done and s is speed in m/s. Substituting the known value for speed I get p=w*3

    Then I have w=f*d where f is force and d is distance in m. Substituting the value for d i get w=f*30

    Next I have f=m*a where m is mass and a is acceleration in m/s squared. So I get a bit stuck on this part, should a be acceleration due to gravity? in which case I would have f=(m+1/2m)*9.81...?

    Then I want to put all this into one formula, so I THINK this is right..
    p={[(m+1/2m)*a]*d}*s

    3. The attempt at a solution

    which if i substitute in the correct values, the equation should look like this, with only the value of m being unknown...
    P={[(m+m/2)*9.81]*30m}*3m/s

    Is that the neatest way to set out the formula, and have I gotten the right formulas? I'm fairly sure I've done it right, just wanted to double check. Thanks in advance for having a read =)
     
  2. jcsd
  3. Sep 19, 2010 #2

    rl.bhat

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    So far I have p=w*s where p is power, w is the work done and s is speed in m/s

    The above equation is wrong.
    It should be P = f*v, where f is the force applied and v is the velocity.
     
  4. Sep 19, 2010 #3
    Okay thanks, so it should instead be - P=[(m+m/2)*9.81]*3 ?
    I thought there would be a difference between the velocity and the maximum speed given for the lift.
     
    Last edited: Sep 19, 2010
  5. Sep 19, 2010 #4
    One more question, if the value for M was known the answer to P would be in watts correct?
     
  6. Sep 19, 2010 #5

    rl.bhat

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    Yes.
     
  7. Sep 19, 2010 #6

    rl.bhat

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    The acceleration of the lift can be found as follows.

    2mg - T = 2m*a....(1)
    T - mg = m*a.....(2)

    Solvr these to equations to find a.
     
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