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    airaws's Avatar
    airaws Posts: 3, Reputation: 1
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    #1

    Apr 11, 2009, 05:44 PM
    Finding constant speed?
    Hi my Question is regarding contant speed..

    How to find contant speed if the mass and power of that object is giving.


    For example:

    The motor for an elevator can produce 14 kW of the power. The elevator has a mass of 1100 kg, including its contents. At what constant speed will the elevator rise?
    HelpinHere's Avatar
    HelpinHere Posts: 1,062, Reputation: 144
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    #2

    Apr 11, 2009, 06:58 PM

    Well, speed, aka velocity (v) = distance (d) / time (t)
    What you provided was power and weight.

    Possible equations used:
    F=ma
    v=d/t
    P=F x v

    Power (14 kilowatts) Force (X Newtons) mass (1100 kg) acceleration (unknown) distance (unknown) time (unknown)

    Gravity is a given: -9.8 m/s, so you can find force of gravity given weight. (Z)
    14 kw = Z x V
    14 kw/Z = V


    However, that would only be the constant speed if ONLY gravity were acting upon it.
    Were you given any other information with the question?
    (This could all be wrong. I'm not 100% sure)
    airaws's Avatar
    airaws Posts: 3, Reputation: 1
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    #3

    Apr 11, 2009, 07:03 PM

    Hi.. thanks for your help...
    The question above is the one that the teacher assigned me...
    ebaines's Avatar
    ebaines Posts: 12,131, Reputation: 1307
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    #4

    Apr 13, 2009, 01:01 PM

    The work done by the motor to lift the elevator a height h is:

    W = mgh

    The power required is simply the work donen per unit time, or:

    P = dW/dt = d(mgh)/dt = mgv

    where v is the velocity of the elevator car.

    You have P = 4KW, m = 1100 Kg, and g = 9.68 m/s^2, so solve for v.
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