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April 17, 2014

Homework Help: Engineering/Physics

Posted by Aaron on Sunday, February 12, 2012 at 3:30am.

A force F = 7^i + 6^j - 5^k lb. acts at the origin. Find the magnitude and
direction cosines of the moment of this force about point A (-4; 5; 6) ft

So in this case wouldn't
vector r= -4i +5j +6k
and
vector F = 7i + 6j - 5k lb?

and r X F = -61i +22j -59k = MA
[MA]= 87.67 ft-lbs

cosα = -61/87.67 => α=134░
cos▀= 22/87.67 => ▀=75.5░
cosγ= -59/87.67 => γ=132.3░

Not sure exactly where I went wrong because those angles don't make any sense.

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