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Re: [Phys-l] Lagrange points



My point was that, in the rotating frame and in the modern view, that third pseudoforce is just another gravitational force as are all inertial forces. Since nothing but frame-dependent "gravitational forces" act on the object and since it is "at rest" in that frame, then there must be a "gravitational balance." More properly we would simply say that since the object is in free-fall it is subject to no gravitational force in its own frame.

(Note also that, in the modern view, an object can only be subject to *one* gravitational force and it is determined entirely by the acceleration of the object relative to local free-falling objects. The only way there can be "gravitational balance" is for their to be NO gravitational force, period.)

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

Moreover, I am quite skeptical that many members of the general public would appreciate the nuance that the Earth must slightly balance the force from the Sun in order to keep the satellite in a synchronized orbit. I suspect that any attempt to convey that nuance in a news article would merely serve to further confuse the issue for most people.

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.

John Mallinckrodt
Cal Poly Pomona

On Dec 3, 2009, at 8:58 AM, John M Clement wrote:

Ah, but in a rotating frame there is a third "pseudoforce" and the
Earth and the Sun do not balance each other. The big problem here is
that the writing is for the general public, and one must present the
situation in a proper Newtonian fashion. Considering that most
readers have a very hazy idea of physics, the explantion needs to be
at least that the gravitational force of the Earth slightly balances
that of the Sun so that the satellite be in an orbit which is
sycronized with the Earth's orbit. But stating the gravity balances
is playing into a major misconception, which the author may even have.
We MUST be hard on major misconceptions when they appear in popular
publications. Presumably they should never appear in scientific
reports and journals.

John M. Clement
Houston, TX


John Mallinckrodt <ajm@csupomona.edu> wrote:
I wouldn't be too hard on them. It's not at all unreasonable to
work
in a rotating frame when thinking about things like Lagrange points.

Moreover, notice that they didn't even talk about gravitational
*forces*. They simply said, "the gravity of the Earth and sun
balance out." That's really not all that far from a completely
reasonable general relativistic statement.

John Mallinckrodt
Cal Poly Pomona

On Dec 2, 2009, at 6:39 PM, John Clement wrote:

I recently read an article about scientific innovations in the
recent Time
magazine while waiting in the optometrists orbit. One innovation
was an
infrared observatory which is stationed at a Lagrange point where
the
"Earth's gravitational force balances the sun's".

So I checked online and came up with a hit on the Brittanica

http://www.britannica.com/EBchecked/topic/524909/satellite- observatory
" the Solar and Heliospheric Observatory (SOHO; 1995) was
maneuvered to the
vicinity of a gravitational balance point (L1, one of the Sun-Earth
Lagrangian points)"

The Wikipedia explains the Lagrangian points correctly!

Where are the scientific proofreaders? The writers would fail the
FCI
because they don't seem to know that when the forces balance the
object goes
in a straight line and will NOT orbit.

John M. Clement
Houston, TX

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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