Solving Moment and Cross-Product Problems with R

[bigu01]

Dear friends, and science lovers,

I am doing some statics problems with moment( cross-product).And I've been having some problems solving them.I am getting the result but in opposite sign. I mean I know we have to do R x F , but looks like I am taking R in the wrong a. For example we have to determine the moment of a force at A about point P.Should I find the vector from A to P or from P to A.Same thing in the couple forces, where we have to find the vector R between the couple forces and to multiply with one of the forces.I am getting that vector but I don't know with which force to multiply.If you can help me clear this thing on my mind I would be really thankfull.

Kind regards,

Astrit Imeri

ME student

 

[jhae2.718]

Moments are taken about a point. In the expression $\vec{M} = \vec{r} \times \vec{F}$, the vector $\vec{r}$ is from the point you are taking moments about to the point of application of the force $\vec{F}$. The following graphics (borrowed from Wiki) should illustrate this:

You can check your results using the right hand rule (assuming you're using a right-handed coordinate system!).