A rotational force, also known as a torque, depends upon the force and where that force is applied;

torque = moment arm x force.

The lever arm or moment arm is the perpendicular distance from the force to the axis of rotation.

Consider the torque exerted on this wrench by a force applied in different directions or in different places.


We can also write this as


= torque

= r F sin

where r is the distance between the pivot point and where the force F is applied.


= F d


d = moment arm

the perpendicular distance from the pivot point to the line of action of the force F,

And, since

sin = sin (180o - )

That is, the angle can be either of the angles between position vector r and the force F.

= r F sin


Moment of Inertia

Newton's Second Law
Return to ToC, Ch10, Rotation about a Fixed Axis

(c) Doug Davis, 2001; all rights reserved