"Jeff Morris" wrote in message
...
"Donal" wrote in message
...
It doesn't "know" anything. Because the Earth and Moon are an
"orbiting
pair," as you
say, they are falling towards each other. Because the gravitational
field
varies, the
near side falls faster than the middle; and the far side falls slower.

Very neat! However, your view seems to be a little bit simplistic.

It is simple. That's because there really isn't that much going on (at
this level).
Just the pull of gravity, which varies with distance. Everything else is
red herring.



Why should a solid fall more slowly than a fluid in a gravitational
field?

Why is there any difference? They both feel the same force. And, the
land surfaces
are distorted by the tides, roughly a meter, IIRC. To be honest, I could
never figure
out if the water is distorted more for some reason (its lighter?), or is
in simply
free to move, and thus get involved in the local shoreline effects. (That
is, is the
tide in the middle of the ocean the same as in the middle of a continent?)

If your theory was correct, then there wouldn't be any tide at all.

No, the force distorts both the land and the water. These distortions are
the two
bulges. In fact, because there is a difference in force, there must be
some
distortion - how much is a detail for the engineers!


You seem to be ignoring momentum.

Nope. If you use the "free fall" approach, momentum isn't really a factor
in
computing the force, though I suppose it gets involved when you calculate
the actual
motion. You sort of consider momentum with the centrifugal force
approach, but you
don't calculate it because the CF gets cancelled out.

Consider an astronaut space walking outside a space station. They both
float
together, feeling no force, although they are both in freefall in their
orbit. If the
astronaut moves to a lower orbit, he will feel a stronger pull and be
drawn in, unless
he speeds up to compensate. If the astronaut moves to a higher orbit, the
force is
reduced. As I said, the force can be calculated without consideration of
momentum.

I don't understant this. In orbit, momentum is the force that balances the
effect of gravity. Without momentum, your astronaut wouldn't "float" - he
would crash straight into the Earth.

In fact, I think that your use of the word "float" reveals that you don't
understand the situation at all. Your astronaut wouldn't feel any
difference between a free fall orbit and a headlong race into deepest
space, - would he? Furthermore, if he slowed down, then he would still feel
like he was floating -- apart from the temperature, and perhaps the braking
effect of the atmosphere.

This makes me think that the orbiting "free-fall" astronaut doesn't feel
that he is floating at all. He must feel a constant force as his direction
of travel changes. I wonder if this has been documented on the Internet?
What does your physics friend say about this?



A purist might say momentum is considered because the mass and velocity of
the every
object in the system is folded together. And, the pure way force is
defined is by how
it changes momentum. But I don't think this is what you're talking about.


I'm not sure. I'm certainly *not* a purist.


Regards

Donal
--