Line of action

In physics, the line of action (also called line of application) 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 .
The concept is essential, for instance, for understanding the net effect of multiple forces applied to a body. For example, if two forces of equal magnitude act upon a rigid body along the same line of action but in opposite directions, they cancel and have no net effect. 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.
Calculation of torque.
For the simple geometry associated with the figure, there are three equivalent equations for the magnitude of the torque associated with a force formula_1 directed at displacement formula_2 from the axis whenever the force is perpendicular to the axis:
where formula_4 is the cross-product, formula_5 is the component of formula_1 perpendicular to formula_7, formula_8 is the moment arm, and formula_9 is the angle between formula_2 and formula_1