# 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?

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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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RPM is the easiest way to do it, and if you have the diameter of what is spinning you can easily use it to determine the absolute speed of any part of it at any point.

You absolutely could measure the speed other ways. You could say that the tip of the fan blade is moving 30 miles per hour compared to the stationary center. It is just that doing so doesn’t tend to be particularly useful most of the time.

RPM is the easiest way to do it, and if you have the diameter of what is spinning you can easily use it to determine the absolute speed of any part of it at any point.

You absolutely could measure the speed other ways. You could say that the tip of the fan blade is moving 30 miles per hour compared to the stationary center. It is just that doing so doesn’t tend to be particularly useful most of the time.

The tip speed is a function of the RPM and your distance from the center spindle.

For most slower applications it’s not particularly important, but it does become a concern for higher-speed uses. Propeller tips do strange things when they exceed the speed of sound, and there’s a fundamental material strength limit to how fast you can accelerate something in a circle.

The tip speed is a function of the RPM and your distance from the center spindle.

For most slower applications it’s not particularly important, but it does become a concern for higher-speed uses. Propeller tips do strange things when they exceed the speed of sound, and there’s a fundamental material strength limit to how fast you can accelerate something in a circle.

RPM is the easiest way to do it, and if you have the diameter of what is spinning you can easily use it to determine the absolute speed of any part of it at any point.

You absolutely could measure the speed other ways. You could say that the tip of the fan blade is moving 30 miles per hour compared to the stationary center. It is just that doing so doesn’t tend to be particularly useful most of the time.

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.

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.

The tip speed is a function of the RPM and your distance from the center spindle.

For most slower applications it’s not particularly important, but it does become a concern for higher-speed uses. Propeller tips do strange things when they exceed the speed of sound, and there’s a fundamental material strength limit to how fast you can accelerate something in a circle.