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Torque about the axle of a rotating disk

  1. Jun 8, 2009 #1
    1. The problem statement, all variables and given/known data
    "The 19.8 cm diameter disk rotates on an axle through its center. F1=18.1 N, F2=26.6 N, F3=26.6 N, F4=18.1 N, and d=4.64 cm. What is the net torque about the axle? "

    http://i4.photobucket.com/albums/y111/kathy_felldown/kn-pic1314.png

    I have two questions about this problem:
    (1) How do I know whether each force results in a negative or positive torque? I know that the torque conventions are positive if counterclockwise, and negative if clockwise, but the problem doesn't indicate the directions of the forces.
    (2) Which forces result in no torque? I learned that if a force is parallel or acting on the axis (in this case, there are no forces acting directly on the axis), that force doesn't produce any torque. How do I denote which force is acting parallel to the axis, since its a disk?


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jun 8, 2009 #2
    Forces acting parallel to the axis don't produce torque?

    1. The problem statement, all variables and given/known data
    I learned that if a force is acting parallel, or acting on the axis of rotation, it doesn't produce torque. If a force is acting at an angle to the axis, does that mean that only ONE component of that force (ie. the y or x component) actually produces torque?

    How do I know which one is the force component that doesn't produce torque, if it's not a straightforward situation like an opening door, and instead, a more ambiguous situation like a rotating disk?


    2. Relevant equations



    3. The attempt at a solution
     
  4. Jun 8, 2009 #3

    berkeman

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    Staff: Mentor

    Re: Forces acting parallel to the axis don't produce torque?

    That's why you use the vector form of the equation for calculating the torque. Can you show us that vector equation?

    Torque = ??
     
  5. Jun 8, 2009 #4

    berkeman

    User Avatar

    Staff: Mentor

    (I merged your two threads asking a common question)

    Also, we would need to see the drawing from the problem to help you with the question on which forces do what.
     
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