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Homework Help: Torque = F*r*sin(theta)

  1. Jun 9, 2009 #1
    1. The problem statement, all variables and given/known data
    For the equation torque = F*r*sin(theta), is theta obtained from either angle:
    F makes with r (or perpendicular component of F makes with r, or the perpendicular component of r makes with F)?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jun 9, 2009 #2


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    Gold Member

    Torque can be written as [tex]\tau = \vec{r} \times \vec{F}[/tex] so one must measure the angle between r and F.

    So find r then measure the angle created by F. So basically we're looking at the perpendicular component of F.
    It would make no sense if it was the component of the moment arm r.
  4. Jun 9, 2009 #3


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    Science Advisor
    Homework Helper
    Gold Member

    I find it best when using T = Frsin theta to use theta as the included angle between F and r, then assign a positive or negative value to it depending on whether the torque is cw or ccw. It is often easier, however, depending on the problem, to calculate torque as the product of the force times the perpendicular distance from the line of action of the force to the point about which you are calculating the torque, with appropriate plus or minus sign.
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