Tuesday, May 19, 2015

Wave action

Yesterday evening I had to spend an hour or so in town while Adam played in the Huntington High School band at an event organized by the mayor. Part way through that hour I found myself up on the bridge looking out over Huntington and the Ohio River. Eventually my thoughts found their way to a dump truck and a college physics textbook. Here is how I got there.

The Ohio River here at Huntington flows east to west. This evening there was the usual wind blowing west to east. That created some small waves on the surface of the Ohio. Lots of them.

As I watched them, I decided to follow one wave crest and see how far it went. As it turned out, not far. The wave would travel a short distance and then disappear into the water. When I looked back to where I first noticed the wave, a new one was forming.

It wasn't what I expected. Then an old memory popped into my head. It was fall 1992. I was in another city, standing on a third- or fourth-floor patio near an exit ramp of an interstate highway. I saw a dump truck heading my way. The truck bed was covered by a tarp, and the tarp was flapping in the wind. I watched the air move through the tarp like a wave or a swell on water. As the truck descended the exit ramp, it neared a light pole. I figured I would focus on the light pole and watch the wave of air go through the tarp so I could judge its speed.

What did I see? The wave of air was not moving under the tarp. When I looked at the light pole and the tarp at the same time, I noticed the wave of air was stationary and the truck was moving under the wave. So instead of the wave of air moving across the truck, actually the air was standing still and the truck was moving under it.

As I pondered those waves on the Ohio River and that dump truck from nearly 23 years ago, that led to another thought in my stream of consciousness when I remembered a college physics textbook back at the house. It said water waves consist of molecules rotating in a nearly circular motion. That would explain why the same wave kept appearing in the same spot.

Here is a shot of a diagram from that textbook that explains all this:

For the curious, this is from Page 420 of "The Mechanical Universe: Introduction to Mechanics and Heat" by Richard P. Olenick, Tom M. Apostol and David L. Goodstein, © 1985, Cambridge University Press. The book went with a television course in introductory physics. It was one of my favorite programs of all time. For something from the 1980s, it had pretty good graphics that helped you follow along, even with the calculus-based math. At times you just sit back and enjoy the moving parts of the equations because you have no idea what they're doing.

To make a short story long, that's what went through my mind that evening as I watched waves on the surface of the Ohio River at beautiful downtown Huntington, the second-largest city in the Great State of West Virginia and, I believe, the sixth-largest city along the Ohio. Or, as people here seem compelled to say, the Mighty Ohio.

No comments: