At 12:28 PM 9/20/01 -0400, Michael Edmiston wrote:
Can anyone think of good ways to describe the domino example?
1) RPF was fond of the following example:
A long line of cars is stopped at a traffic light. The light turns
green. The car in front starts to go. Then the car behind it. There is a
"wave" of car-starts that propagates backwards from the intersection --
even though each particular car moves only forward.
This does not, by itself, illustrate conservation, but it sets up the
following: .....
2) Consider the propagation of the following sound wave: Let's examine the
air outside a piston + closed cylinder. This is synonymous with an
"infinite baffle" loudspeaker. Suddenly shove the cylinder _into_ the
cylinder. Ignore what happens inside the cylinder. Outside, we have just
launched a _rarefaction_ wave. It will propagate outward. It will carry
energy outward at the speed of sound. The air molecules will, of course,
be moving every which way, but the _average_ motion will be slightly
inward, as the air fills in the hole left by the retreating piston.
So the average air-flow is in one direction (inward), and the energy-flow
is in the opposite direction (outward). If we use the sound for
signalling, the information-flow is also outward.