Re: [Phys-l] Centrifugal redux

[Philip Keller <PKeller@holmdelschools.org>]

Well, let me ask again (though I am tempted to let this thread go...)

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?

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Bill Nettles
Sent: Tuesday, March 31, 2009 10:53 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Centrifugal redux

Thanks, Joe.  You said it much better than I would have. My response was "so let's just tell them Newton's 1st law isn't that important."  Of course, we ALL have to apologize to the general relativists because we keep talking about those fictional gravitational forces instead of exposing them to the complex beauty of space-time curvature and its effect on basketballs. :)

Bill

> > > Joseph Bellina <jbellina@saintmarys.edu> 3/18/2009 11:59 am >>>
It seems to me that if you accept the student's answer you have just
lost a marvelous opportunity to reinforce the deeply counterintuitive
notion that constant velocity not at rest is the natural state of
things.  Why would you waste such a gem of an opportunity to revisit
an important idea in a new framework...a key strategy in supporting
conceptual change.

cheers,

joe

Joseph J. Bellina, Jr. Ph.D.
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556

On Mar 18, 2009, at 12:53 PM, Philip Keller wrote:

> I agree.  If a student says that a satellite remains in orbit
> because the gravitational force balances the centripetal force, I
> can't say "No, you are wrong."  If I insist on banishing
> centrifugal forces from my class, the more honest response is: "No,
> I don't want you to think about it that way."  Maybe better to open
> the door...time spent explaining why these forces do not really
> exist could just as easily be spent explaining when it's OK to use
> them.
>
> ***********
>
> So what is the point of forcing students to go through an intermediate
> phase where they are taught centrifugal fields don't exist?  For all
> of their life before class, after class, and north/south/east/west of
> class, centrifugal fields exist.
>
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