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Re: [Phys-l] Centrifugal redux



On 04/01/2009 07:53 AM, Philip Keller wrote:

Student says: A satellite remains in orbit because the gravitational
force balances the centrifugal force.

Here are some possible responses for the teacher:

A. Yes, that is correct and I have nothing more that I can add.

B. Yes, that is correct, but I would also like you to be able to
explain matters from the point of view of a non-rotating observer, in
which case you may not refer to a "centrifugal force". The reason I
want you to be able to do this is that I suspect that your preference
for the use of the rotating frame at this point in your education
masks a fundamental misunderstanding of Newton's laws. But once you
can convince me that you can do this, then please see choice A,
above.

C. No, that is incorrect because....

As a teacher, I can see that depending on who I am teaching I might
choose A, but most of the time would choose B. Can anyone finish
choice C and then tell me why I should choose it?

In this context, choice B strikes me as exceeeedingly reasonable.

Taking a step back, I tell students that in the real world they
are allowed to solve problems by whatever means they choose, and
with occasional exceptions the "real world" rules apply in my class.
In the exceptional cases where I want to see a particular method, I
will make this explicit and I will try to explain the pedagogical
purpose of the restriction.

As for choice A, depending on circumstances one might want to flesh
it out with some "show your work" questions to defend against some
common misconceptions.

======

The proverb says, "If you don't have anything nice to say, don't
say anything". So I'm not going to comment on choice C.