Line of action
In physics, the line of action of a force F is a geometric representation of how the force is applied. It is the line through the point at which the force is applied in the same direction as the vector F→.[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.