Important Terms

When an object is at rotational equilibrium it is not rotating or moving, during this state there is neither no net force nor torque.

The center of mass is the point of an object at which it can be suspended without tending to rotate. This is the point at which you may pretend all of the mass of an object lies, rather than acknowledge the objects distribution of mass.

In SI units, a unit of torque is measured in newton-meter. However, because torque units are force by distance this standard is not world-wide; torque can also be measured in pounds per foot.

A line of action is the direction of force. The direction of force vector will be equal to line of action vector because forces are vector quantities.

Concurrent forces occur when the lines of action of various forces have a common point preventing an object to be set into rotation. Non-concurrent forces occur when the lines of action of various forces do not intersect; this causes the object to be set into motion.

A pivot or fulcrum is the point on which a lever is free to rotate.

Mathematical fulcrum is the point that is used when solving a torque problem, such as a rigid-body static problem, where forces intersect and can also be the physical fulcrum.

Physical fulcrum is the point that an object moves about (see pivot/fulcrum above)

A couple is a pair of equal parallel forces that are opposite in direction. The torque of a couple is found by multiplying one of the forces by the perpendicular distance between them.