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 you want the speed of the object you have to measure in rpm. You can only measure in kph if you want to measure the speed of a surface on the object (e.g. the rotational speed at the equator of the earth)