In physics, the line of action of a force F expresses the geometry of how F is applied. It is the line through the point at which F is applied and along the direction in which F is applied.[1]
The concept is essential, for instance, for understanding the net effect of multiple forces applied to a body. As an example, if two forces of equal magnitude act upon a rigid body along the same line of action but in opposite directions, then they have no net effect—loosely speaking, they cancel one another out. But if, instead, their lines of action are not identical, but merely parallel, then their effect is to create a moment on the body, which tends to rotate it.
One practical application of the concept is in animation and character posing.