Does the outside edge of a ceiling fan blade move faster than the inside edge?

It does, but its angular velocity may be tampering with your sense of its speed…

By Elizabeth Earley

Imagine two circular running tracks — one encircles the other. A person is running a lap on the smaller track while another person is running a lap on the longer track. For both people to run one lap in the same amount of time, the person running on the longer track would have to run faster than the person on the shorter track.

Similarly, the outside edge of a fan blade travels further than the inside edge during the time of one revolution. So the answer is: “Yes, the outside edge must be moving faster,” says Warren Seering, the Weber-Shaughness Professor of Mechanical Engineering and professor of engineering systems.

But that’s just the simple answer. “A perception that they’re going at the same speed could be related to the concept of rotation,” Seering adds. He explains that the motion of an object is a combination of translation and rotation. The fan blades are all rotating at the same speed, but different points on a blade translate at different speeds. These speeds are, in fact, related. A point on a blade translates at a speed that equals the product of the speed at which the blades are rotating and the distance from the speeding point to the center or axis of the blades. The further a  point is from the center, the faster it travels.

So the simple answer is yes, even though they’re rotating together, the outside edge of a ceiling fan blade moves faster than its inside edge.

Thanks to 37-year-old Curtis Elsis from Orlando, Fla., (and his friends from the firehouse) for this question.

Posted: October 01, 2013

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