When I got home from my morning hike, CJ was sitting at the kitchen table, flicking their fingernail against a vase of cut flowers.
“Whatcha doing?” I asked
“Tapping the vase in different places,” they said. “It sounds different if I tap above or below the waterline.”
My eyes lit up. The kid was showing interest in science! “What do you think will happen,” I asked, “if we add more water?”
“Don’t add water, Dad,” they said. “You’ll drown the flowers.”
I added water. “What sound will it make it you tap it now?”
“The same,” they said, “maybe a little softer.”
“Try it.” I resisted the temptation to tell them that the pitch would be higher. I wanted them to discover that for themself.
They tapped the vase. It sounded about the same. Maybe a little softer.
“Well, that’s not right,” I said. “We didn’t put in enough water.” I started filling a pitcher.
“Dad,” they said, “what are you doing?”
I poured the pitcher into the vase. “What if you flick it now?” I asked. “Will the pitch be lower or higher?”
“Lower,” they said.
I smiled. “Try it.”
They flicked their fingernail against the vase. “See,” they said, “lower.”
“Not higher?” I asked.
“No, lower.”
“Hm.” Frankly, I wasn’t sure. I’d forgotten the original sound while I was filling the pitcher.
“It should be higher,” I said. “You see: the sound comes from the column of air above the water. The more water you put in the glass, the shorter the column of air. And shorter wavelengths make higher sounds.”
CJ shrugged. “But that’s not how it is.”
(If you’ve seen the Oppenheimer picture, you might remember Robert Oppenheimer calculating that it wasn’t possible to split the atom. And Ernest Lawrence pointing to Luis Alvarez, who was splitting the atom at that very moment. “Theory only gets you so far,” Lawrence explained.)
“We need to do this faster,” I said, “so we don’t forget the sound.”
I put a tall glass in the sink and tapped it with a spoon. Then I turned on the faucet and ran just a little water into the bottom of the glass.
I turned the faucet off and tapped the glass again. It sounded about the same.
I filled the glass halfway and tapped it again. The pitch was different now. But not higher. It was lower.
I kept adding water, and every time I tapped the glass the pitch got lower.
“CJ!” I shouted, because of course they’d left some time ago. “Check this out! You were right!”
CJ came back and I repeated the experiment. “The sound gets lower when you add more water,” I said.
“I know,” they said.
“But why?” I asked.
They shrugged.
“We come from a long line of physicists!” I said. “We should be able to figure this out.”
And I did what great physicists have done for generations. I looked it up in Scientific American.
According to SA, the water adds inertia to the glass, so that it doesn’t vibrate as fast when you tap it. And slower vibrations—lower frequency vibrations—make lower sounds.
And that’s it, I guess. They didn’t even mention the height of the air column.
Theory only gets you so far.
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