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

Revolutions per minute x Circumference

If the circumference is 10ft, and it spins at 60 rpm that’s 600ft/minute

Also, let’s be clear, in the real world (i.e. engineering and everywhere not in the US) you do not use mph.

In the case of rotating object mph or km/hr are meaningless.

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Also:

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

That’s why MPH is meaningless.

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The only time I’ve ever heard speed in km/hr or mph being considered in rotating bodies is with turbofan engines (big plane engines) because if the outer edge of the blade exceeds the speed of sound your efficiency goes to shit.