Torque ( "Rotational Force" )

Remember, torque is a vector. When we write


r and F are vectors and their "cross product" or "vector product" is also a vector! We determine the direction of the "cross product" or "vector product" by the "right hand rule".

The perpendicular distance from the axis of rotation (or pivot) to the "line of action" of the force F is called the "moment arm" (sometime, the "lever arm").

Draw a vector r from the axis of rotation to the location of the force F.

To evaluate a cross product such as we must redraw the vectors r and F so they begin at a common origin:

Now rotate the first vector, r, into the second vector, F, using the fingers of your right hand (!). The new vector, tau , now points in the direction of your thumb!

The order is important in a "cross product" or a "vector product".

C = A x B

D = B x A

C = - D

A x B = - B x A

The "cross product" or "vector product" does not commute!

Torque 1

Angular Momentum
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(c) 2005, Doug Davis; all rights reserved