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Math Help - physics

  1. #1
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    physics

    consider a elevator weighing 3200 lbs. rising with a=4.0 ft/s^2
    what is the tension in the supporting cable?
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by alderon View Post
    consider a elevator weighing 3200 lbs. rising with a=4.0 ft/s^2
    what is the tension in the supporting cable?
    The net force on the elevator is 3200x4 Newtons. This comprises the sum of
    a downward force due to gravity of 3200xg, and the upward force equal to
    the tension in the cables, so:

    3200x4 = T-3200xg,

    so:

    T = 3200(4 + g).

    Opps.. lbs not kg, so the elevator mass is ~3200*.4536 ~=1451.5 kg

    and ft/s not m/s, so ~4 ft/s 4*0.3048 ~=1.22 m/s

    so we have:

    T= 1451.5(1.22+g)~=16010 Newtons.

    I would recomend to your instructor that they stop using customary units immeadiatly
    the confusion in the system between the units of mass/weight at the very risky and
    confusing.

    RonL
    Last edited by CaptainBlack; August 22nd 2007 at 12:16 AM.
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  3. #3
    MHF Contributor
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    Quote Originally Posted by alderon View Post
    consider a elevator weighing 3200 lbs. rising with a=4.0 ft/s^2
    what is the tension in the supporting cable?
    If the elevator is going up, the tensile force on the cable must be greater than the weight of the elevator. Then this upward net force is causing the elevator to accelerate upwards. This upward net force is equal to the (mass of the elevator)*(acceleration). Or, the old reliable F = ma.

    Weight of elevator = 3200 lbs.
    So mass of elevator = 3200/32 = 100 slugs -----if g = 32 ft/sec/sec

    F = ma
    T -3200 = 100(4)
    T = 400 +3200
    T = 3600 lbs -----------answer.
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