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Homework Help: Required tangential force to move spool

  1. Dec 2, 2014 #1
    This should be an easy one, but my answer does not seem right to me. I was hoping someone to verify for me.

    If you have a spool filled with rope that weighs 2,000 LBS on a frictionless axle. Determine the tangential force required to move the spool by pulling the rope.

    Weight of spool = 907.18 KG
    Diameter of spool = 1.22 Meters
    Angular Acceleration = 2.093 RADs per Second
    Length of spool = .74 Meters
    T = Torque
    A = Angular Acceleration
    R = Radius of spool in meters

    Explanation of calculations:

    I calculated the moment of inertia for the full spool and used it to derive the torque (knowing the angular acceleration) and then derived force out of the torque equation.


    I = 1/2*M*(L squared)
    I = .5*907.18*.547
    I = 248.11 KG*(Meters squared) Moment of inertia in Kg*Meters - squared

    If I = T/A then

    248.11*2.093 = T
    T = 519.29 Newtons*Meters

    If T = F*R then

    519.29 / .61 = F
    F = 851.29 Newtons

    As far as my tension in rope I assume it to be equal to F at 851.29 Newtons Is this correct? The value seems high to me. Please advise.

  2. jcsd
  3. Dec 2, 2014 #2


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    Science Advisor
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    Gold Member

    I haven't checked your working but...

    851 Newtons is about the same force that an 85kg man exerts on the ground due to gravity. That doesn't seem like a lot given your 2000lb spool accelerates at 2 rad/s/s. (about 114 deg/s/s)
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