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

Maybe not entirely related, but almost all modern cars measure vehicle speed (and outputs that to the speedometer) at the wheels through ABS system. You do need to change the wheel size value in the ECU when changing to bigger or smaller wheels to make sure the measurments stays accurate