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Homework Help: Torque on a compound wheel

  1. Dec 12, 2008 #1
    (sorry about another topic but this week's HW is hard @_@)

    1. The problem statement, all variables and given/known data
    A circular-shaped object has an inner radius
    of 6 cm and an outer radius of 22 cm. Three
    forces (acting perpendicular to the axis of
    rotation) of magnitudes 13 N, 28 N, and 13 N
    act on the object, as shown. The force of
    magnitude 28 N acts 40◦ below the horizontal.

    Find the magnitude of the net torque on
    the wheel about the axle through the center
    of the object. Answer in units of N · m.

    2. Relevant equations
    Torque = Fd sin (th)

    3. The attempt at a solution

    I ignored the mass because the teacher didn't mention anything about reading over the moment of intertia section in our textbook, but I think she was wrong... Anyway:

    Clockwise as the positive direction.

    Net Torque = (13 * 0.28) + (13 * 0.28) - (28 * 0.06 sin 40)
    Net torque = 6.200116816 m*N (WRONG)

    T_T I tried reading the section, but it gave me an equation (Net Torque = Moment of intertia * angular acceleration) I couldn't do anything with, as I didn't know how to solve for the angular acceleration.
  2. jcsd
  3. Dec 12, 2008 #2
    Oh the mass on the picture was 10 kg, if that even matters.

    I'll post a pic if it's needed.

    EDIT: Oops, now that I read that over, a picture is required.

    Attached Files:

    Last edited: Dec 12, 2008
  4. Dec 13, 2008 #3

    Doc Al

    User Avatar

    Staff: Mentor

    Since the question only asks for the net torque, not the resulting angular acceleration, you don't have to worry about moment of inertia.

    The outer radius is 22 cm, not 28 cm (don't add the inner and outer radii). Does the angle that the 28 N force makes with the horizontal affect the torque it creates around the axis?
  5. Dec 13, 2008 #4
    OK so the angle shouldn't even matter since it's already perpendicular to the radius...

    So then I did:

    (13 * 0.22) + (13 * 0.22) - (28 * 0.06) = 4.04 m*N

    which is still wrong....
  6. Dec 13, 2008 #5

    Doc Al

    User Avatar

    Staff: Mentor

    Looks fine to me. (Depending upon how fussy your online system is, you might need to use the correct number of significant figures.)
  7. Dec 13, 2008 #6
    my online system accepts the most possible digits as can be, which is 4.04

    two other people i've asked also approved of 4.04 X_X

    some of my friends said that this week's HW won't accept the obviously right answer, i.e. glitch/mistake in the system. i hope it's that
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