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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It’s rpm because spinning is not the same as moving, if that makes sense. Usually we’re not talking about *distance* when we’re measuring rotations. The object isn’t going anywhere, so that’s not what we’re measuring.
You can measure the speed of a *point* on the windmill. Say you draw a red X on it somewhere on the edge. You can measure how far the X travels. This is just multiplying the circumference by the number of rotations. 😊
Also note: the distance would be very different if you measured from a different point closer to the center! That adds a complication if you try to discuss the linear “speed” of a wheel.
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