How Do You Calculate the Total Moment of Forces on a Plane?

[go2255]

I heard that the moment of F about point a is (r-r(a))xF
but i can't understand what r and r(a) are.And when there are 2 forces act though different point,how can I find the total moment.

e.g:
Four points:O(0,0,0) P(9,-3,5) Q(6,4,7) R(3,2,3.5).
OPQ is a triangular plane and R is a point on this plane.
Two forces, F(1)=i-2j+k and F(2)=3i+3j+2k act through the points P and Q respectively.
If the triangle OPQ is fixed at the point R, what is the total moment of the two forces about it??

I do it like that:(OP-OR)xF(1)+(OQ-OR)xF(2)
Is that right?

 

[tiny-tim]

hi go2255! welcome to pf!

I heard that the moment of F about point a is (r-r(a))xF
but i can't understand what r and r(a) are

r is the point of application of the force (or any point on the line of application)

r(a) is the point about which you're taking the moments

in fact, you've got it correct in your example …

I do it like that:(OP-OR)xF(1)+(OQ-OR)xF(2)
Is that right?

.And when there are 2 forces act though different point,how can I find the total moment.

you add the two moments (as vectors)