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Homework Help: The Forces of Rolling

  1. Nov 26, 2006 #1
    The question is:

    A wheel is rolling smoothly on horizontal surface. There is a constant horizontal force of magnitude 10 N. Wheel has mass of 10 kg, and radius of 0.3 m. The accel. of its centre of mass has mag. of 0.6 m/s^2.

    I am asked to find frictional force in unit-vector notation. This is what i did:
    (10N) = f - (10kg)(9.8m/s^2)sin0* = (10kg)(0.6 m/s^2)
    (10N)-f = 6.0
    -f = -4.0

    Since there is clockwise angluar acccel the friction is a negative value. So my answer is f = (-4.00 N)i

    For part b it asks for the rotational inertia of the wheel about its centre of mass. This is what i've done so far:

    (-4 N)(0.3m) = I (-0.6m/s^2/0.3m)
    -1.20 = I(-2)
    I = 0.600 rad/s

    Am i on the right track?? Please help!!!
    Last edited: Nov 26, 2006
  2. jcsd
  3. Nov 27, 2006 #2


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    Science Advisor
    Homework Helper

    Friction tends to retard the motion, and I assume the applied force and the acceleration are in one direction, with friction in the opposite direction. Your positive answer for f is because you have it in the wrong place in your equation for it to be negative. If you wrote F + f = ma, then f would be found to be negative. Your result for I has the wrong dimensions.
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