Re: [Phys-l] Lagrange points

[John Denker <jsd@av8n.com>]

On 12/03/2009 10:56 AM, John Mallinckrodt wrote:

> As always, I'm not suggesting that we teach this to introductory  
> students.  I just think that we ought to understand the modern view  
> and, as much as possible in our teaching, avoid needlessly  
> constructing barriers to future enlightenment.  

Amen, brother.

> We should at least  
> spend a little time talking about the equivalence principle and  
> emphasizing it's incredibly deep physical significance.

Yes.

> The fact is that most people DO have a very well-developed gut level  
> (pun intended) understanding of the equivalence principle whether or  
> not they can express it properly.

Yes.

Some of this understanding comes from experience riding
in cars (and maybe airplanes), using a comoving reference
frame.  Students know from first-hand observation that
physics using such a frame is not much different from
physics in the lab frame.

If a teacher wants to say that rotating reference frames
are beyond the scope of the course, I don't have a 
problem with that.

In contrast, consider the following anecdote:  the HS
football coach is explaining something to his players.
He mentions centrifugal force as part of the explanation.
A couple of the students contradict him.  "There's no
such thing as centrifugal force.  We learned in physics
class that there's no such thing as centrifugal force."

Then I have to explain to the students that what the
football coach said was entirely correct, and what
they learned in physics class is a bunch of baloney.

Once, one of the kids said to me, "You're a physics
guy; why don't you stick up for the physics teacher?"
I said "I stick up for the guy who is right.  It
really offends me when the guy who is exactly right
gets contradicted."

========

The essential physics is easy to remember and easy 
to explain:  The centrifugal field exists in the 
rotating frame and not otherwise.

If you wish to use a non-rotating reference frame,
that's your choice ... but you must respect the
right of others to choose differently.

The centrifugal field exists in the rotating frame
and not otherwise.  What matters is the motion of
the frame, whether or not any particular object is
moving along with the frame.