Things are not what they appear to be: nor are they otherwise.

- Buddha

If I apply a force that exerts a counterclockwise (+) torque on my yoyo, it will rotate: A] counterclockwise B] clockwise

If I exert a rightward force on my “semi” yoyo, it will move: A] to the right B] to the left

Does static friction do

work

on a rolling object?

Short answer: NO.

Long answer: both forces and torques do work. If the static friction

force

does + work (increasing K t ) Then the static friction

torque

does - work (decreasing K r ) (example: uphill rolling) These works exactly cancel: the effect of static friction is simply to move energy between rotation and translation. Bottom line: ignore work done by static friction!

When is 

Unlike F=ma,



can only

safely

be used in two cases: I.

II.

Rotation about a fixed, predetermined axis Rotation about the center of mass (symmetric body)

Always use a fixed axis, or the center of mass, for

 !

If you are curious and would like to see how you can get a wrong answer using



come see me at an SI session.

The real deal: angular momentum