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**Torque** is the ability of a force to cause a body to rotate
about
a particular axis.

In Figure 8.1, a force, *F* acts on an object at the
point
*B* . The object is pivoted so that it rotates about the point,
*O* . The magnitude of the torque caused by *F* about the
axis through *O* is:

= *Fl*_{}

*l*_{} = *l*sin

**Note: **

- We can also write torque as
= *Fl*_{}=*Fl*sin =*F*_{}*l*(1) *F*_{}=*F*sin is the component of*F*perpendicular to*OB*. - The units of torque are
*N**m*. - Torque is defined to be
**positive**if it tends to rotate the object counterclockwise, and**negative**if it tends to rotate the object clockwise. - If an object is acted on by more than one force the total
torque is the sum of the torques due to the individual forces:
= + +... (2) *F*_{1}*l*_{1}sin , etc. Be sure to get the signs right for each individual torque.

10/9/1997