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

There are some applications where you might want to know how fast the blades are moving, in that case you would use the outer most part. You can calculate it very easily if you know the rpm and radius of the windmill or whatever you are measuring.