##### Physics I For Dummies

Explore Book*F*; and the

*lever arm.*Also called the

*moment*

*arm*, the lever arm is the perpendicular distance from the pivot point to the point at which you exert your force and is related to the distance from the axis,

*r*, byis the angle between the force and a line from the axis to the point where the force is applied.

In the figure, the lever arm, *l,* is distance *r *from the hinge to the point at which you exert your force. The torque is the product of the magnitude of the perpendicular force multiplied by the lever arm. It has a special symbol, the Greek letter tau:

For example, the lever arm in the figure is distance *r* (because this lever arm is perpendicular to the force), so

If you push with a force of 200 newtons and *r* is 0.5 meters, what’s the torque you see in the figure? In diagram A, you push on the hinge, so your distance from the pivot point is zero, which means the lever arm is zero. Therefore, the magnitude of the torque is zero. In diagram B, you exert the 200 newtons of force at a distance of 0.5 meters perpendicular to the hinge, so

The magnitude of the torque here is 100 newton-meters. But now take a look at diagram C. You push with 200 newtons of force at a distance of 2*r* perpendicular to the hinge, which makes the lever arm 2*r* or 1.0 meter, so you get this torque: