# Quick Question - Moments about a force

1. Nov 13, 2007

### jmcmillian

Hi there, quick conceptual question just to make sure I know what I am doing...

I am given two forces, F1 and F2, in vector form with given magnitudes.

(I) Find the resultant force about point P (given).
(II) I am also asked to find the Moments of the resultant force about point P.

(I) I establish position vectors from P to each point (I'll call it A for F1 and B for F2) and multiply by magnitudes. Then add to get the resultant force about P.

(II) Then for the second part, I cross my resultant force about P with the "r" for the resultant force, which would be the distance between F1 and F2 from P?

It's that last sentence where you can see I'm unclear at. Any help would be greatly appreciated!

2. Nov 14, 2007

### Shooting Star

If the forces are given in vector forms, i.e., the i, j, k components are given, just add them to get the resultant. Resultant force about a point does not mean anything. For finding the resultant, you need not multiply anything.

For the 2nd part, you have to find a point on the line of action of the resultant. If posn vector of that pt is r wrt to origin O, then the moment about P is (rOP) X R, where OP is the posn vector of pt P, and R is the resultant.