Why are a door’s hinges and its doorknob placed near opposite edges of the door? Imagine trying to rotate a door by applying a force of magnitude F perpendicular to the door surface but at various distances from the hinges. You will achieve a more rapid rate of rotation for the door by applying the force near the doorknob than by applying it near the hinges.

If you cannot loosen a stubborn bolt with a socket wrench, what would you do in an effort to loosen the bolt? You may intuitively try using a wrench with a longer handle or slip a pipe over the existing wrench to make it longer. This is similar to the situation with the door. You are more successful at causing a change in rotational motion (of the door or the bolt) by applying the force farther away from the rotation axis.

In this chapter we’ll define a new physical quantity, torque, that describes the twisting or turning effort of a force. We’ll find that the net torque acting on a rigid body determines its angular acceleration, in the same way that the net force on a body determines its linear acceleration. In order to describe the effects of a force on a body the magnitude and direction of the force are important, but so is the point on the body where the force is applied.

Key terms

• Torque: a measure of tuning effect of a force
• Moment arm (or lever arm): the perpendicular distance between the line of action of the force and the pivot.