Eli5: How is the speed of a rotating object defined (e.g. a windmill) if you don‘t use rpm. Where do you measure the speed in mph?

Question

I had a look at a huge windmill and was wondering how fast the blades are spinning. Obviously, the distance the blades make is different on the outside than on the inside. Is the speed of a rotating object always measured in rpm and not in mph?

In: 3

Answer 1

The tip speed is a function of the RPM and your distance from the center spindle.

For most slower applications it’s not particularly important, but it does become a concern for higher-speed uses. Propeller tips do strange things when they exceed the speed of sound, and there’s a fundamental material strength limit to how fast you can accelerate something in a circle.