"Donal" wrote in message
...
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.

Momentum is not a force. You're right that the astronaut has momentum,
and that the
force of gravity alters his momentum. In fact, Force is defined by how
much it
changes momentum. (Many people learn F=ma; in physics that is normally
written as
F=dp/dt, or Force equals the rate of change of momentum with respect to
time.)

Ok. What force opposes gravity so that a body may remain in orbit?

Nothing. The body remains in orbit because it has enough forward velocity (momentum
also, but its the velocity that counts) so that while it "falls" into the Earth, the
forward velocity keeps the body from hitting the Earth. For a low orbit, we normally
thing of circular orbits, but for high orbits they can be quite eccentic. If the
Earth were a "point source" it would only take a small velocity to stay in orbit. Of
course, there are many ways to calculate this - somtimes its done in terms of
"energy," other times as "delta V" so if you want to say the momentum of the body
keeps it from falling into the Earth, that's OK.




My point is that you can determine the force on the astronaut without
considering his
momentum.

In which case, there must be a "force" that is counteracting the effect of
gravity. After all, gravity is trying to pull the orbiting Astronaut
straight towards Earth. There must be another force that is opposing
gravity.


See above. If the body were motionless (WRT Earth), it would fall directly in. But
if it has any velocity, its has a chance of missing it. The reason why I say velocity
is important, and not momentum, is that two bodies of different mass (and hence,
different momentum) will float together in orbit. Of course, if you want to calculate
the amount of fuel to burn, momentum becomes important.

On Earth, we always have air resistance, and other forms of friction, so momentum is
more significant.




To figure out how the force would alter his orbit, you would probably take
momentum into account.

Remember, I'm not trying to calculate the tides, only to show how gravity
can cause
two equal size bulges on the Earth.

Do you think that centrifugal force plays any part? If so, what do you
think the ratio is between the centrifugal and differential gravity forces?


It is possible to look at the problem without considering Centrifugal Force. However,
even if you use CF, it is a constant, and equal to the net gravitation force. So if
it helps to expain how there can be a force away from the moon, that's OK, but you
must remember that it is the gravitation force that varies, so that's where the
interesting math comes from.





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?

So tell us, what is the difference?

Acceleration.

No, the acceleration is the same, more or less. (Not counting the difference in
distance from the Earth, or air resistance, etc.) The only real difference is that
the astronaut has enough velocity (hopefully) to miss the Earth as he falls.



Floating in a space station is call "free fall"
because it feels the same as jumping off a cliff.

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.

Yes, but that's not the point. The point is, if he is in a lower orbit,
he
experiences more gravity; in a higher orbit, less gravity. If his speed
is not
adjusted to compensate, he will drift further away from the space station.
Just like
the tides.


This makes me think that the orbiting "free-fall" astronaut doesn't feel
that he is floating at all.

Haven't you ever seen astronauts floating?

Yes.... but they are constantly changing direction.... and therefor they
should be aware of the effects of acceleration.


I must admit this subtlety has perplexed me - clearly the don't feel the G force,
since it the same as a car in a tight turn. But I keep thinking it should be
detectable, if only because the path is curving.



He must feel a constant force as his direction
of travel changes. I wonder if this has been documented on the
Internet?

http://science.howstuffworks.com/weightlessness1.htm

That is a very simplistic explanation. It refers to the fact that the
astronauts will feel the acceleration at take-off, and yet it doesn't seem
to understand that a change of direction is also acceleration.

We humans can detect acceleration. If you sit in an automobile with your
eyes closed, then you can feel an increase or decrease in speed .... or a
change of direction!! As the astronauts are subjected to a constant change
of direction, I suspect that they might not feel that they are completely
free-floating.


Of course, from a General Relativity, Gravity Well point of view, the obital path is a
straight line in curved space. I should know the answer here - let me cogitate ...



What does your physics friend say about this?
He would probably deplore the lack of education in your country.

Ask him anyway!

Actually, it was Scout's friend. However, you should remember I majored in physics
and worked for NASA doing spacecraft navigation. I may be rusty now, but 25 years ago
I really knew this stuf!



Perhaps, if you allowed him to read the thread, he might be amazed at your
lack of reading ability. After all, I've already explained that I gave up
Physics at an early stage.

I haven't forgotton that. It was just a little dig since usually you Brits complain
about our sorry education.