Homework Help: Physics/ Engineering

Posted by Aaron on Saturday, February 11, 2012 at 9:30pm.

Calculate the moment about a point (5; 4; 3) ft. caused by the forces
P = 3^i - 2^j - 4^k lb. and Q = 5^i + 7^j - 2^k lb. acting at the points
(7; 2; 2) ft. and (3; 2; 5) ft., respectively.

What I did was this;

vector r1= (7,2,2) - (5,4,3) = 2i-2j-5k

vector r2= (3,2,5) - (5,4,3) = -8i-6j+2k

Vector M1= (2i-2j-5k) X P = -7i-7j+2k
Magnitude= 7.55 ft-lbs

Vector M2= (-8i-6j+2k) X Q = -2i-6j+85k
Magnitude= 86.23 ft-lbs

Moment about the given point = (7.55) + (86.23) = 93.78 ft-lbs

I'm unsure if this is right

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