The magnitude of torque

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[SOLVED] the magnitude of torque

Homework Statement


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

Homework Equations


cross product and maybe the magnitude of torque

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
 

Answers and Replies

  • #2
HallsofIvy
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Homework Statement


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

Homework Equations


cross product and maybe the magnitude of torque

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
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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