"Donal" wrote in message
...

"Jeff Morris" wrote in message

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.

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.)

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



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? 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?

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

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





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
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