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Re: [Phys-l] Lagrange points

John C. - thanks for bringing up this fascinating topic and this example of 'popularization' of physics.
and thank you John M. for your reply.

I would see this as a 'teachable moment' for a physics class but NOT as a condemnation of the Time article. We need MORE Time articles such as this.

In HS (or earlier even) I would often be attracted to adult 'arguments' on physics type stuff.
One such topic was the origin of the TWO tidal lobes. 'Centrifugal Force' seemed to be the convincing idea.

Lots of handwaving by graduate chem engineers. No General Relativity arguments.
'Balancing' gravity doesn't have to necessarily require equal and opposite forces.

I'd ask a student or interested adult what the result of TOTALLY balancing the forces could be expected to produce in an orbital situation.

In general - I'm of the opinion that 'Puristic Fundamentalism' doesn't help pedagogy.

At 10:58 AM -0600 12/3/09, 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