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 rotations per minute

rad/s radians per second (2π radians = 360°)

°/s degrees per second

All of these are rotational velocities. (ω)

You could also measure the tangential velocity (v) of the blades. That would be the speed the blades are moving at the tips in mph, kph, m/s, etc

ω = v/r where ω is in rad/time, r is the radius, so you can convert between rotational and tangential velocity