Quick Question - Moments about a force

[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!

 

[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 (r – OP) X R, where OP is the posn vector of pt P, and R is the resultant.

 

[ogb p]

Hi there,

It seems like you are on the right track with your approach to finding the resultant force about point P. To clarify, the position vectors you establish from P to each point (A and B) should be the distance and direction from P to each force, not just the magnitude. Then, you can use vector addition to find the resultant force about P.

For the second part, you are correct that you need to use the cross product to find the moment of the resultant force about point P. The "r" vector should be the distance and direction from point P to the line of action of the resultant force, which can be found by drawing a perpendicular line from P to the line of action of the resultant force.

I hope this helps clarify things for you. Let me know if you have any other questions. Happy calculating!