# Rotation at speed c

1. Aug 15, 2012

### QuantumHop

I'm assuming the maximum number of revolutions per second for a disk is defined as speed c divided by the circumference of the disk, eg a disk with a circumference of half a meter is allowed to rotate twice as fast per second as a disk with a circumference of one meter.

C = circumfrence of the disk
c = speed of light
mrps = maximum revolutions per second (not meters per second)

So is the value of mrps nice and simple : mrps = c / C

Or are there more complicated relativistic affects, for example does the circumference shrink due to length contraction.

2. Aug 15, 2012

### pervect

Staff Emeritus
The relationship that the velocity in the lab frame is 2*pi*r* revolutions / second, where r is the radius in the lab frame, doesn't change in the lab frame. The circumference of the disk in its own "frame" (which is not really a frame!) is different (larger) than 2*pi*r however. See any of the threads about the Ehrenfest paradox.

3. Aug 15, 2012

### QuantumHop

I thought something strange was going to happen but wasn't sure, thanks for the info.