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

RPM is revolutions per minute. When talking about the rotational speed of an object you’re going to be counting its revolutions, the only question then is what relative time frame you’re interested in.

>Obviously, the distance the blades make is different on the outside than on the inside.

Since RPM measures angular momentum–how many many times the object rotates in a minute–the length of the blades is not really a factor. The axel that turns the blades will turn it at the same speed no matter how long the blades are.