Tuesday, July 15, 2003

Question of the Day: Wave Motion In Water 

Came home last night and found C studying for an Earth Science class, the unit on oceans. Her book was open to a series of pictures, a cartoon, of a boat's motion as it was passed by an ocean wave. The picture showed the boat tracing a clockwise circular path (with wave motion from left to right.) This immediately seemed wrong to me--a transverse wave such as the sinusoidal ocean wave transmits motion perpendicular to the wave path, not along the wave path--the boat should just go up and down. I spent some time with my nose in the physics book without coming up with a satisfactory explanation for the alleged circular motion. After several hours spent contemplating various combinations of vectors, I resorted to 'ask the group' among the folks at work. This was not such a long shot as it might seem ... they're engineers, mostly mechanical, who specialize in fluid systems. Finally, H, one of the few (shudder) chemEs, found, this site, which had a great animation:

and a simple explanation. Basically, waves in water are both transverse and longitudinal waves, not just transverse (e.g. sinusoidal.) The lesson? Start with just one bad assumption among a whole bunch of good assumptions, mix in a few facts, and you too can spend a whole evening trying to make the model work out.

"Facts, schmacts. You can prove anything that's even remotely true with facts." -Homer Simpson

(And I don't want to hear any complaints if all this just seems like a thin excuse to link to that mesmerizing gif ... just sit back, relax, and watch the blue dot ... you're getting sleepy ... focus on the blue dot ... your eyes are closing ... )

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