Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] Centrifugal redux



Anthony is doing what answer B asks. That doesn't make A incorrect -- it's
just an approach to the problem in a different frame of reference. But I
think beginners choosing A are not thinking about it correctly at all.
Doing A the right way is not "beginner stuff."

Saying "there's nothing more to add" will not lead to understanding. I have
a feeling the idea of "balancing" centrifugal forces and gravity gives
people the idea that it's a sort of "delicate" balance, and that if things
are not exactly right, the spacecraft will "fall out of orbit."

But that doesn't happen. You can fire engines and give the craft a boost or
a little braking and it just goes into a differently shaped orbit. Maybe
answer A could be expanded upon by looking at elliptical orbits. Then even
in a rotating frame the centrifugal and gravity forces are not always
balanced, because the spacecraft will move inward and outward as it
oscillates between perigee and apogee.

So, to me the dangerous word here is "balance," not so much "centrifugal."

Steve Highland
Duluth MN



Circular motion concepts are some of the most difficult for students to
understand as they are applications of Newton's laws.

I say answer C. Since the satellite's velocity is changing, it must be
accelerating (a = v2/r). It's moving in a circle, so the net force is
toward the center (F = ma). This is gravity (satellite's "true" weight)
acting downward (toward the Earth), and is called the centripetal force.
Turn off gravity, and the satellite will move in a straight line (N1L).

Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
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.

_______________________________________________
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
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
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l