# Friction between rotating objects

## Main Question or Discussion Point

In physics, we learned about friction between one object sliding across another, but when we got to torque and angular velocity, etc, there was never any mention of how to calculate friction between 2 objects rotating in place. Say you have a plate on top of a table and it is rotating about the center of the plate. How would you calculate the friction between the plate and the table if the coefficient of friction is known?

## Answers and Replies

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malawi_glenn
Science Advisor
Homework Helper
change to spherical coordinates and integrate over the surface.

tiny-tim
Science Advisor
Homework Helper
change to spherical coordinates and integrate over the surface.
Yes … specifically, if you divide the plate into concentric ring-shaped bits, each bit of length 2πa will have the same behaviour under friction as a straight rod of the same mass and of length 2πa.

Thank you guys for your help. Im going to see if I can figure out this problem I was working on now