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Thread: axis of pulley?

  1. #1
    May 2005
    Christchurch kiwiland

    axis of pulley?

    I am stuck on a simple question that I belive could be a trick.
    it says -

    A 50kg body hangs at rest from a rope wrapped around a pully 15cm in diameter. what is the torque about the axis?

    the formula I have used for a similar question is

    Torque = Force x distance

    Force= Mass x Acceleration
    seeing as there is no acceleartion in this problem do i assume that Force is equal to 50kg ie 50Newtons or should i have to mulitiply by gravity (9.81) and assume this is the accelartion?

    would the distance be 15cm or .15m and where is the axis of a pulley?? please help
    Last edited by Math Help; May 1st 2005 at 04:13 PM.
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  2. #2
    Site Founder Math Help's Avatar
    Apr 2005

    Free Body Diagram

    The torque about the axis should be the force, T, of the Tension in the rope multiplied by the radius, r, of the pulley.

    If you do a free body diagram of the 50kg body you have the force, T, pulling up on the body and the weight of the body which is either 50kg*9.81m/s^2 or 50N they are equivalent.

    T = mg
    T = 50N

    The radius, r, of the pulley is either .15m or 15cm they are also equivalent.

    So the torque, J, is equal to 50N * .15m or,
    J = 7.5 Nm
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