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Re: [Phys-l] Centrifugal redux



Before I talk about centrifugal forces, my students play with some Interactive Physics demonstrations: they watch what happens to a pumpkin that was riding on the roof of a car until the driver steps on the brakes. A nice feature of Interactive Physics is that you can watch the same events from different reference frames. So first we watch the pumpkin flying forward from the (apparently) stationary car, thrown forward by a "forwardial" force. Then we watch the pumpkin continue at constant velocity as the car slows down from beneath it. This time, no force acts on the pumpkin. So before my students discuss circular motion, they have already wrestled with the idea that observers in accelerating reference frames observe forces that are not observed (or needed!) by the non-accelerating observer. These discussions take up a couple of class periods but I don't see how to explain centrifugal forces to students who have not been through the linear discussion first. Then we look at what happens to a pumpkin when you go around a curve...again from both RF's.

BTW if anyone would like copies of the IP files, please email me and I'll send them over.

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Marty Weiss
Sent: Wednesday, April 01, 2009 11:22 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Centrifugal redux

But, then, if you explain it in terms of *B* as you did in the
explanation for choosing *B*, well, you'd better be prepared to
elucidate that: clean up that explanation so a typical beginning
high school student would understand it and not be totally lost in
doing so. Not an easy task! (And one that may not be the best way to
approach the subject for the first time student, right or wrong.)
Look at it this way... if the student said *A* then he/she was
taught to understand it that way. They just didn't come up with that
on their own and since they are in your physics class, I would guess
that's what they got from the lessons and labs on the concept plus
from the study of Newton's Laws. If they are in physics for the first
time it's going to be rather difficult to get anyone to understand
what you mean by explanation *B* and then if you do try to correct
them you stand a good chance of losing them altogether. I can just
see the eyes glaze over and the the mind shut off with a perfunctory,
"whatever".
This is not an easy subject for most kids!

Marty


On Apr 1, 2009, at 10:53 AM, Philip Keller wrote:

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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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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