# 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

### Pyrrhus

Hello!,

The resultant force about any point will be calculated by vectorial addition of all the forces. The moment of the resultant force about a point will be equal to the sum of the moments by each force about the same point.

This exercise sounds like a force-couple system, where you use the transport couple theorem in order to move all the force about the point P, and reduce the previous system to a equivalent one with one resultant and one couple.

Last edited: Nov 14, 2007