Which force component to use for moment of a force

[shaan_aragorn]

This question may sound silly to all of you brainy guys out there but I am one silly guy.
While calculating the moment of a force how do you choose which component (horizontal or vertical) to choose? I know that M=F*d. Its confusing when authors sometimes use the veritical and sometimes the other component.
Also please let me know if there is a nice little websites where such basic conepts could be found.

 

[FredGarvin]

The component you need is whatever component is perpendicular to the radius you are using as the distance. For example, imagine a circle. If you draw a radius from the center to the 3 o'clock position, then the vertical component will produce the torque. If you draw a radius from the center to the 6 o'clock position, then it would be the horizontal component. The radius dictates the component used.

When in doubt, hyper physics is a great website:
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

 

[mathmike]

inother words you would use the distance that is shortest that makes a rright angle from the line of action to the position of application