Talk:rpm

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• Revolutions per minute
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Untitled

I move this section from the rpm page:

The theory of relativity sets limits to rotational speed for an object of a certain diameter. When the rotational speed or diameter increases, a point on the perimeter will move faster, but cannot move faster than the speed of light. If the rotational center is made to move faster, the rotating object will deform so that the perimeter moves at its allowed speed. This effect can be noted on computer hard disks.

The last sentence: "This effect can be noted on computer hard disks" must be wrong. If a hard disk is rotating with 10000 rpm, then the diameter of the disk need to be several hundred kilometer for the velocity of the perimeter to approach the speed of light. Or am I wrong? --css

Hmm. A 3.5 inch disk spinning at 10,000 RPM would be moving about 104MPH at the edge. So there's no danger of exceeding the speed of light. However, there might be some small relativistic effect even at that low speed...? I couldn't find any info on this. More likely, the issue is simply one of material deformation at high speeds. I'd be fascinated if it was the former, though. --D

I would have thought that the equivalent SI unit to rpm would be the Hz, since we are dealing in revolutions or rotations or cycles per unit time. The non-SI equivalent of rad/sec would surely be degree / sec or similar angular unit / time unit ? Ian Cairns 17:16, 4 Jul 2004 (UTC)

First - rotational speed (r/min or 1/min or r/s or 1/s) and angular speed (rad/s or deg/s) have been confused. They are separate quantities and have different units. Second - I think 1/s (1/min) is more often used as unit of rotational speed than Hz. Hz is more often used for oscillating phenomena. Aulis Eskola 20:19, 15 Jul 2004 (UTC)