Q: (Initially posted on December 20, 2002) FROM MENTEE AMANDA T. IN MO
What does the air (as in a specific molecule) inside of a tire do when the tire is moving? For instance, does the air move in the same direction as the tire, or does it move the opposite direction or does it stay in the same relative spot...etc?

December 23, 2002
A: FROM MENTOR CAROL TOMAN IN IL
The behavior of air in a tire could be simulated by the behavior of
water in a bowl since air and water are both fluids and in these
systems they are in round solid containers. Drop some bits of paper
or other telltales on the surface of the water and observe what
happens when you begin to rotate the bowl. At first the water stays
still as the bowl moves around it -- inertia resists the force that
wants to get the water moving. Eventually the persistent spinning
will overcome the tendency to stay at rest and the water will get
spinning at the same rate as the bowl. When you stop the bowl the
water continues to spin -- inertia at work again. Eventually the
water will stop spinning. Imagine what happens to the individual
molecules of air in your tires if you shift from drive to reverse.
And, since the interior surface of the tire is not perfectly smooth
and regular, there is also turbulence; small eddies and back-currents
probably form. Fluid dynamics is one field of science you might want
to learn more about.