Example 2: Determine the magnitude and directional sense of the resultant moment of the three forces about point C.

Solution: A close examination of the problem indicates that only the moment arm of force FD can be easily found from the figure, and those for the other two forces are not easily obtainable. Hence, we will use the principle of moments and break forces FA and FB into their individual components and use them to find the moment about point C.

Force FA:

Force FB:

Force FD:

The moment about point C is obtained by summing the moments of individual contributors while paying close attention to the direction of each moment. We will assume the counter-clockwise direction as the positive sense and write the equation for moment as

The negative sign indicates that the direction of moment is opposite to that assumed. In this case, the moment about point C is in the clockwise direction.

Notice that in finding the moment of each force component, what matters is the directional sense of the moment produced by that force component and not the direction of the force itself.