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Force, moment and torque

  1. Jul 13, 2007 #1
    I have a ruler which is connected to a spring that causes the ruler to move in a circular motion around the spring.
    When I measure the force with a newtonmeter at the top of the ruler it is less than it is at midpoint.

    So my question is, why is it that the force exerted decreases as the point where force is being measured from increases.
    I thought it might have something to do with torque, however
    the force is created in the center of circular motion and not at point r, so i don't know if this equation applies or not:
    t=f x r.
  2. jcsd
  3. Jul 13, 2007 #2
    Well, it would help me if you could explain the setup in a little more detail. It sounds as if you have a spring (coil spring?) attached to the ruler very near the center of the rotation. If it's literally at the exact point, then it can't exert a torque, as you've suggested, but that means the ruler wouldn't rotate, either. The fact that it does rotate means there is a non-zero torque, so your spring must be attached at some non-zero r from the center of rotation. r might be very small, but it can't be zero if the spring is exerting a torque, which is necessary for rotational motion.

    That said, once you agree that there is a torque on the ruler, then the first part of your question becomes easier to answer. The same torque produces the forces you measure at the two points on the ruler, so the force will be given in each case as f = t/r (assuming you're measuring the force perpendicular to the ruler). If the second r is twice the first, then the force measured there will be half what you measured at the first r.

    Make sense?
    - Bruce
  4. Jul 14, 2007 #3
    Yep that makes sense thanks
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