"Jeff Morris" wrote in message
...

"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.)

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


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.



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?




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.

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.



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.



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

Ask him anyway!

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.


Regards


Donal
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