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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
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°
cosß= 22/87.67 => ß=75.5°
cosγ= -59/87.67 => γ=132.3°

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

  • Engineering/Physics -

    If the force acts at the origin, the lever arm r about point A is the opposite vector,
    r = 4i -5j -6k

  • Engineering/Physics -

    Ah, I knew there must be something off about my answer.

    Thank you so much!!!

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