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Homework Help: Lift relative velocity problem.

  1. Oct 26, 2011 #1
    1. The problem statement, all variables and given/known data
    motor is mounted on top of lift.
    a)if p moves by y, what is the displacement of Pt A? Prove that Vp=-3Va.
    b)If motor unwidns cable at const rate 2m/s, what is vel of cable wrt lift.
    Prove vel of lift is 0.5m/s


    2. Relevant equations



    3. The attempt at a solution
    I can do part a. I have prob with part b. If motor releases cable at 2m/s, can i take it that pt p moves 2m/s. so Va is 2/-3=-0.667. Since pt a is connected to the lift, the vel of cable wrt lift is 2+0.667=2.667.
    But velocity of lift is 0.5 so the vel of cable wrt to lift is 2.5. This is getting confusing.
     

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  3. Oct 27, 2011 #2
    anyone ????
     
  4. Nov 8, 2011 #3
    this is a better picture
     

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  5. Nov 8, 2011 #4

    Simon Bridge

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    Science Advisor
    Homework Helper

    Conservation of energy is your friend here.

    What is the effective load on the motor?

    What is the force exerted to move P a distance y?
    Thus - what is the work done to do this?

    What is the work done to raise the lift, and thus point A, a distance x?

    The answer has a clue, and another method:
    Unwinding the cable at 2m/s does not mean that P moves at 2m/s.
    Remember that the motor moves with the lift. 2m/s is the rate the total length of cable increases.

    You'll note that the answer, 0.5ms, is exactly a quarter of 2m/s.
    How many cables are pulling on the lift?
    If the total length of cable increases by 2m/s, how much does each length of cable increase by?
     
    Last edited: Nov 8, 2011
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