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The magnitude of torque

  1. Oct 4, 2007 #1
    [SOLVED] the magnitude of torque

    1. The problem statement, all variables and given/known data
    Determine the magnitude of the torque about A by evaluating lABxFl (the magnitude of vector AB cross vector F) where AB = -15i + 12j and F= -200lbs (cos theta)i - 200lbs (sin theta)j

    2. Relevant equations
    cross product and maybe the magnitude of torque

    3. The attempt at a solution
    so i did the cross product and ended up with
    k = (-15(-200 sin theta))-(12(-200cos theta)) whick i simplified
    to 3000(sin theta) + 2400(cos theta)

    so k = 3000(sin theta) + 2400(cos theta)

    so am I done or should I take the sqrt of the values = to k ??
    the problem i have with this is theta is undefined and i would end up with a more complicated expression.
    so what do i do
     
  2. jcsd
  3. Oct 5, 2007 #2

    HallsofIvy

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    Staff Emeritus
    Science Advisor

    That is not "k= ". What you have is the coefficient of k (unit z-vector). The magnitude is the value of that coefficient. If you wanted to be real technical, the "magnitude" is the square root of the square of that: its absolute value and so just what you have as long as it is positive.
     
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