The object rotates about an axis, which we will call the
Torque
is a measure of how much a force acting on an object causes that object
to rotate.pivot point, and will label 'O'. We will call the force 'F'.
The distance from the pivot point to the point where the force acts is
called the moment arm, and is denoted by 'r'. Note that this
distance, 'r', is also a vector, and points from the axis of rotation
to the point where the force acts. (Refer to Figure 1 for a pictoral representation
of these definitions.) |
Figure 1 Definitions |

Torque is defined as

= **r F **sin().

Using the
**right hand rule**, we can find the direction of the torque on an object.
If we put our fingers in the direction of **r**, and curl them to the
direction of **F**, then the thumb points in the direction of the torque.

Imagine pushing a door to open it. The force of your push (**F**)
causes the door to rotate about its hinges (the pivot point, O). How hard
you need to push depends on the distance you are from the hinges (**r**)
(and several other things, but let's ignore them now). The closer you are
to the hinges (i.e. the smaller **r** is), the harder it is to push.
This is what happens when you try to push open a door on the wrong side.
The torque you created on the door is smaller than it would have been had
you pushed the correct side (away from its hinges).

Note that the force applied, **F**, and the moment
arm, **r**, are independent of the object. Furthermore, a force applied
at the pivot point will give zero torque since there is the moment arm
**r** = 0.

Another way of
explaining the above equation is that torque is the product of the magnitude
of the force and the perpendicular distance from the force to the axis
of rotation (i.e. the pivot point).
Let the force acting on an object be broken up into its tangential,
Ftan, and radial, Frad, components
(see Figure 2). (Note that the tangential component of force, Ftan
is There is an alternate method of calculating torque. |
Figure 2 Tangential and radial components of force F |

There may
be more than one force acting on an object, and each of these forces may
act on different point on the object. Then, each force will cause a torque.
**The net torque is the sum of the individual torques.**

Rotational Equilibrium is analogous to translational equilibrium, where
the sum of the forces are equal to zero. **In rotational equilibrium,
the sum of the torques is equal to zero.** In other words, there is no
net torque on the object.

Example
Illustrating the Right Hand Rule

Example
Problem on Torque

Continue to:
Torque and Angular Acceleration

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