Re: Gravity and the Sun

From: Greg Neill (gneillREM_at_OVE.THIS.netcom.ca)
Date: 08/13/04 

Date: Fri, 13 Aug 2004 18:16:44 -0400

"RP" <no_mail_no_spam@yahoo.com> wrote in message news:2o4a7fF6miu5U1@uni-berlin.de...
>
>
> 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.

Well, I don't know if equality can be safely assumed
to imply identity. Further, turning off the central
force doesn't imply affecting the physical properties
of the orbiting body... Suppose only the Sun were
rendered massless for the few moments.

>
> >
> > 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.

It may move closer to the Sun, however, the new position
will lie *outside* the old orbit; the new radius vector
will cross the old orbit, even if in fact the body is
then closer to the Sun than it was when the gravity was
stwitched off.

>
> >
> > 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.

The body carries its kinetic energy out to the new position,
outside of the old orbit while the Sun's gravity is absent.
The total energy of the orbit is increased, increasing its
major axis. I'm pretty sure that the semi-latus rectum will
be increased, as well as the perihelion distance.

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