Re: Is there a future in space travel?

In article <42FBDB89.881C3DF9@xxxxxxxxxxx>,
Rafe Zetter <qwerty@xxxxxxxxxxx> wrote:
>> > [...] Why would some not be
>> >crushed when they were on the surface of Saturn? RH
>> You've more-or-less answered your own question. In round
>> numbers, the gravity is 100 times more because Saturn is more massive,
>> and 10x10 = 100 times less because you are 10 times as far away
>> from that mass when you are on the surface, so the effects cancel.
>10 times further away relative to... what exactly. Are you suggesting
>that the
>gravitational field is only produced at the centre of the planet.

No; though Newton showed that the gravitational force exerted
by the Earth on people or on the Moon is, to a good approximation, the
same *as if* the same mass were concentrated at the centre. In this
case, it is irrelevant -- if you make something ten times as big [and
change nothing else], then every part of it is ten times as far away
from someone standing on its surface, so every contribution to the
gravitational field is reduced by a factor 100, and so is the total.

>Unfortunately you also seem to be making basic error in assuming that
>the gravitational
>field strentgh is in direct proportion to the distance from the source
>mass. [...]

No, it is an inverse-square law.

>> [...] the Sun too
>> would rotate in a few hours were it not that most of the momentum is
>> carried by the outer planets. Draw your own conclusions.
>No this is wrong. The mass of the planets (relative to the sun) is way
>to small
>to have such a significant effect as you suggest.

No it isn't. For Jupiter, Saturn, Uranus and Neptune, their
relatively small masses are compensated by their huge distances from
the Sun and also by their orbital velocities. I can only suggest that
you work out the numbers.

>It may be that over a long period of time the rotation of the sun has
>slowed
>due to gravitational drag from the planets, but this is entirely
>different.

Any such cause is much more likely to be magnetic than
gravitational, but the mechanism barely matters. The total angular
momentum of the solar system is entirely normal, and is very similar
to that of fast rotating stars [that we see "out there"]; but it
mostly resides in the planets. It is much more natural to suppose
that other slow stars are like the Sun, and belong to systems in
which, somehow, the angular momentum belongs to bodies rather like
Jupiter or Saturn than to suppose that all other slow stars belong,
miraculously, to large but randomly situated parts of the galaxy
that happen not to be participating in its general rotation.

--
Andy Walker, School of MathSci., Univ. of Nott'm, UK.
anw@xxxxxxxxxxxxxxxx
.

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