Re: Gravity and the Sun

From: RP (no_mail_no_spam_at_yahoo.com)
Date: 08/13/04 

Date: Fri, 13 Aug 2004 11:58:40 -0500

Greg Neill wrote:
> "RP" <no_mail_no_spam@yahoo.com> wrote in message news:2o3if5F6b05eU1@uni-berlin.de...
>
>
>>LOL. His explanation is misinformed, but his initial comment remains
>>correct: The distance between the Earth and the Sun won't necessarily
>>increase. OTOH, just for fun: If Gravity is switched off all bets are
>>off, no need to assume that angular momentum will be conserved under
>>those circumstances. The objects would have to become massless, thus the
>>system would have zero angular momentum :)
>
>
> If you are extending the hypothetical situation to include
> the banishment of all physics during the interval, then yes,
> all bets are off. But I see no reason to presume that
> inertial mass must be "turned off" if gravity is.

Because of the equivalence principle.

>
> In fact, one could imagine a way to mimic the situation by placing
> a net electrical charge upon the Earth and Sun, the mutual
> repulsion of which would exactly counter the gravitational force.
> After the duration of the "switch-off" period, the charges are
> removed.

And one would get the same result that: Moving from aphelion to
perihelion a suddenly nongravitating planet in a formally elliptical
orbit of nonzero eccentricity will move closer to the Sun up through a
distance of sqrt(r^2 + ds^2), ds r being the initial displacement
between the Sun and planet, and ds being the radius from the Sun's
center that intersects at right angles the line of motion of the planet.

>
> The energy of the orbit will increase, so the major axis
> of the orbit will increase: a = -u/(2E), where E is the
> total mechanical energy of the orbit, u is the gravitational
> parameter equal to G*(M+m), and a is the semimajor axis length.

That is only a temporary truth. When such a planet concludes its orbit
it may or may not suffer a decrease in distance from the Sun at
perihelion. In fact, without a precision adjustment to the planet's
trajectory it will be launched into an unstable orbit that could plunge
it into the Suns surface.

Richard Perry

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