Thursday, July 24, 2008

Trusting your senses....

Bernoulli's principle is one of those things even many (most) science teachers get wrong, or at least use the wrong examples. A curveball tossed by Josh Beckett does, in fact, break (or bend) as it approaches home plate, but Bernoulli's principle alone doesn't explain it. Airplane wings do get some lift, but not sufficient lift from the effect of Bernoulli's principle to lift up the plane. (There's a lovely reminiscence by Jef Raskin, who was sent to the principal's office for questioning his 6th grade teacher's version of Bernoulli and wondered how could planes fly upside down?)

But that is not the point of the story today.

One nice demonstration of Bernoulli's principle is simply holding two strips of paper side by side about an inch apart, then trying to blow them away from each other by blowing in between them.

One of my test questions was this:

If you blow between two sheets of paper, what is most likely to happen?

a. The sheets move apart.
b. The sheets move together.
c. It cannot be predict
The students were allowed to do it during the test, and I was willing to model during the test.

I had one student in my class Fernando, a peripheral member of the Wackadoodle Club, properly do the demo. The papers clung together. He did it again. I smile and nodded. He asked me if it was a trick question. No, Fernando, it's not. He did it again. And again. And again.

He was getting more and more agitated each time he did it. I finally told him to pick the answer that reflected what he saw.

He got it wrong.

I asked him the next day what happened? And he looked upset--"I know what I saw, but what I saw couldn't be right, so I picked "a"--I thought I was doing something wrong."

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