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Re: [Phys-l] The "why" questions



At 11:43 -0500 11/29/2010, LaMontagne, Bob wrote:

A physics instructor places a toy car on a table and pushes it from behind. He/she then asks the class "What just happened?"

Student: The car just accelrated because you pushed it.

Teacher: We don't use "because" when we talk about force and acceleration.

Student: But I just saw you push it!

Teacher: Who are you going to believe, me or your own lying eyes?

Both the teacher and the student seem to have missed the point of this question. The student added something that wasn't asked, and the teacher didn't correct what the student did wrong.

The teacher asked "What just happened?" but the student added an interpretation of what happened to the question. That's what the teacher should have corrected, so the correct response from the teacher should have been, "But I didn't ask you why what you saw happened, I only asked you what you saw. If I want to know why it happened, I will ask that question. And besides, your response even to the question itself, was incomplete. Exactly when did the car accelerate--while I was pushing it, after I was pushing it, or both (or neither), or is it possible to say more that it accelerated? If I had placed the car at the top of a ramp and released it, what would you have said?"

If the student then answers, "Gravity pulled the car down the ramp," the teacher needs to stop and make sure the student understands that the question was "what happened?" not "why did it happen?'

Students need to be vary careful that they don't answer more than what is asked, because in so doing they are making implicit assumptions that may well be wrong. This, I think, is the important point of Newton's proscriptions about hypotheses. If you are asked to describe what happens, don't do more than that, because to do so assumes that you know something of the "whys" involved, and that assumption is usually unjustified. Note that Kepler's laws and the laws of linear friction make no such assumptions--the merely state rules by which one can calculate what is going on in certain circumstances. Whether Newton's laws say more than that is perhaps a matter of debate.

Hugh
--

Hugh Haskell
mailto:hugh@ieer.org
mailto:haskellh@verizon.net

It isn't easy being green.

--Kermit Lagrenouille