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While driving home at mph with my daughters the other...

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Dear Tom and Ray:



While driving home at 70 mph with my daughters the other day, I began to wonder if the air inside a rotating tire is actually rotating with the tire, or is it stagnant inside the tire? And if it is rotating with the tire, how long does it continue to move once the tire has stopped rotating? Anxiously awaiting an answer. -- Tim

RAY: Tim, wherever you work, they're not giving you nearly enough hours.

TOM: Actually, it's a very interesting fluid-dynamics question, which we put to Car Talk's Staff Theoretical Physicist, Wolfgang Reuckner (who is also moonlighting at Harvard University -- at least until they catch him).

RAY: Wolf says that the short answer is yes, the air in the tire rotates with the rest of the tire. Even though air is about 50 times less viscous than water, it's got enough viscosity (or internal friction) to be moved by the walls of the tires.

TOM: The air closest to the tire walls would get dragged along first, and then that layer of air would drag the next layer of air, and so on. Exactly how long would it take to get all of the air going?

RAY: "It's possible to do that calculation," says Wolf, "but it would have to wait until after my racquetball game and four faculty meetings." He says it's safe to assume that once you've been cruising along at a steady speed for a short time, the air is rotating at almost the same speed as the tire. It'll always be a little bit slower than the tire, due to the frictional loss from viscosity.

TOM: And when the car comes to a halt, the air inside the tire keeps rotating for a while, too. Again, it's viscosity that slows it down. How quickly the air slows down depends on how quickly the tire slows down. If you come to a gradual stop, the air might be very close to still by the time the tire stops. If you come to a screeching halt, the air would keep rotating after you stop.

RAY: How much longer? "It's possible to do that calculation," says Wolf, "but it would have to wait until after my faculty lunch, four tenure reviews and an all-college colloquium on Squirrel Breeding in Academic Settings." He did say that he'd guess that it would continue to move for no longer than 10 or 20 seconds.
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