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?
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If an object has a uniform angular velocity, every point on it will have a linear velocity that depends on its distance from the centre of rotation. So yeah you can measure the speed in mph (or more reasonable units like m/s) of any point on, say, a windmill’s blade, but it’ll be a function of the radius up to that point. The angular velocity, as in, the speed at which its angle changes, that being what “rpm” denotes, is a much more comfortable measurement in this case because like I said it’s uniform. However, if I, say, swung a sword or a club in a roughly circular arc, the force of its impact is obviously determined directly by the linear velocity of the point of contact, in which case you’d want to measure and express that, but it’s still a function of its more-or-less uniform angular velocity and its length.
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