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Torque Lesson

torque animation

A force applied in such a way that it creates a change in the rate of rotation (angular velocity) of an object.  It is defined as the product of the applied Force and the perpendicular Length from some point (typically measured from the pivot point).  This length measurement is often called the "lever arm".  Torque can actually be defined by three separate formulas.   The first two are most common for use in high school level physics where the third (dealing with a cross product) is more often used in college level physics.

In this case the lever arm is drawn and measured so that it is perpendicular to the force.

In this case the force is broken down into components with one of them being perpendicular to the lever arm.  The value for the perpendicular component is used as the force.

In this case the force being applied and the lever arm are naturally perpendicular to each other.  Any of the three formulas would work fine in this case.

The units for torque are usually Nm (not a joule) or lb.ft (pound feet).

A net torque () is analogous to a net force (Fnet) when examining its influence on motion.  applies to rotational motion the same way Fnet applies to linear motion.  Newton's laws still apply with only a few modifications to the words.

Newton's First Law (adapted to torque)
In the absence of a net torque () an object will continue with its present rotational (angular) velocity.  In other words, if it is not rotating, it will continue to not rotate.  If it is rotating it will keep the same rate of rotation.   It will have no rotational (angular) acceleration.
Newton's Second Law (adapted to torque)
If there is a net torque ( ), the object will have a rotational (angular) acceleration.   It will either be increasing or decreasing its rate of rotation. The amount of the object's acceleration will be related to its rotational inertia (moment of inertia) which is the rotational version of mass (regular inertia).
Newton's Third Law (remains unchanged for torque)
When one object applies a force on another object the second object applies an equal and opposite force back on the first object.

A few important notes about torque: