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

If something is rotating, revolutions per minute is the only measurement that makes sense because the entire length is moving at the same speed (in rpm).

Every point on the blade (in your example) moves around in a circle so the actual distance in metres, miles or whatever depends on how far that point of the blade is from the centre of the circle (the tip of the blade is moving faster in mph than the middle of it is).