The moment of a force about a point is a measure of the tendency of
the force to rotate a body about that point. The body does not necessarily
have to rotate about that point, but the moment defines how the force
is trying to rotate the body. A good example of a moment is when a force
applied is applied to a wrench. The wrench may not actually turn, but
the force generates a turning force (i.e. moment) around the bolt.
Moments are often referred to as torques,
and the terms can be used interchangeably. In scalar notation, a moment
about a point O is
Mo = r F
Here F is the magnitude of the force and r is the perpendicular distance
to the line of action of the force.
The orientation of the moment is in the same direction as the rotation
of the body if the body were allowed to rotate. A moment can be symbolized
as a curved vector around the rotation point.
If a force is applied at the point O or the line of action
of the force passes through point O, then the moment about point O is
zero, and the force has no tendency to rotate the body about that point.
Effect of a Force Direction