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 yet another version of the "right hand rule".

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!

Angular Momentum

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(c) 2002, Doug Davis; all rights reserved