Orbital mechanics question (moon)

The moon literally moves oceans by creating tides which have a lot of energy
(and in fact some of it is converted to electricity with tidal power dams).

Does the gravitational pull of moon against the moving oceans result in the
moon actualy losing kinetic energy (which would mean that its orbit would
decay over time ?)

If the moon's orbit does not in any way lose any energy even though it moves
oceans, where does the energy that creates the tides come from ?

Or is this an example of a perpetual motion machine that never loses any
energy ?

"John Doe" wrote in message ...
The moon literally moves oceans by creating tides which have a lot of
energy
(and in fact some of it is converted to electricity with tidal power
dams).

Does the gravitational pull of moon against the moving oceans result in
the
moon actualy losing kinetic energy (which would mean that its orbit would
decay over time ?)


Correct. It does lose kinetic energy. (though I think that means its orbit
in fact gets larger over time.)


If the moon's orbit does not in any way lose any energy even though it
moves
oceans, where does the energy that creates the tides come from ?

Or is this an example of a perpetual motion machine that never loses any
energy ?

Nope. No such thing.

Correct. It does lose kinetic energy. (though I think that means its orbit
in fact gets larger over time.)


is this measurable with the aser reflectors left on the moon?
..
..
End the dangerous wasteful shuttle now before it kills any more astronauts....

(bob haller) writes:


Correct. It does lose kinetic energy. (though I think that means its orbit
in fact gets larger over time.)


is this measurable with the aser reflectors left on the moon?

Yes, it's about 3.8 cm/year - nothing to panic about.

http://www.nasa.gov/vision/space/fea...21jul_llr.html

Lou Scheffer

"Greg D. Moore \(Strider\)" wrote in message ...
"John Doe" wrote in message ...
The moon literally moves oceans by creating tides which have a lot of
energy
(and in fact some of it is converted to electricity with tidal power
dams).

Does the gravitational pull of moon against the moving oceans result in
the
moon actualy losing kinetic energy (which would mean that its orbit would
decay over time ?)


Correct. It does lose kinetic energy. (though I think that means its orbit
in fact gets larger over time.)

Half right. The moon actually GAINS energy from the tides, which is why its
orbit gets larger over time. Remember the Earth is rotating nearly 30 times
faster than the moon is orbiting. So the pull of the oceans on the moon
tends to speed the moon up a bit, as it slows the Earth's rotation down
a bit. Tides slowly transfer a wee bit of angular momentum from the
Earth to the moon.

Louis Scheffer wrote:
Yes, it's about 3.8 cm/year - nothing to panic about.

http://www.nasa.gov/vision/space/fea...21jul_llr.html

Thanks for the pointer. But I do not undersdtand how removing energy from the
moon would result in a higher orbit. Shouldn't the moon get closer to the
earth as it loses energy to the tides ?

"John Doe" wrote in message ...
The moon literally moves oceans by creating tides which have a lot of
energy
(and in fact some of it is converted to electricity with tidal power
dams).

Does the gravitational pull of moon against the moving oceans result in
the
moon actually losing kinetic energy (which would mean that its orbit would
decay over time ?)

If the moon's orbit does not in any way lose any energy even though it
moves
oceans, where does the energy that creates the tides come from ?

Or is this an example of a perpetual motion machine that never loses any
energy ?
Well, my education may not have been to University standards, but I seem to
recall that the Moon is gradually getting further away from us, which is a
result of the interaction of its mass with other masses, in your case, our
oceans, and to some extent, the land as well. Its not exactly friction, but
it is obvious that some loss of energy has to occur. Higher orbits are
slower orbits of course, and presumably, as the moon recedes, the tidal
effect will too. The reason the effect on the Moon is so small is that, in
effect, the gearing of the work it is doing is huge, in that a lot of work
is done, but very slowly.

Remember, that the Earth is also rotating, and I'd expect this motion to
also be altered as the work gets done.

Of course, the Sun has an effect as well.

No figures from my addled brain, but I'm sure there is a tidal anorak about
with them at their fingertips.

What I want to know though is this.

If you take energy by moving mass from the moon, and it gets further away,
and you take energy out of a spacecraft in LEO by friction and it re enters,
where is the point where it, in effect crosses over?

:-)

Brian

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"John Doe" wrote in message ...

Thanks for the pointer. But I do not undersdtand how removing energy from
the
moon would result in a higher orbit. Shouldn't the moon get closer to the
earth as it loses energy to the tides ?

nope, in orbital mechanics lower orbit = faster, and vice-versa

--
Terrell Miller


" A strong conviction that something must be done is the parent of many
bad measures."
-- Daniel Webster

"Brian Gaff" wrote in message . uk...
You lose energy, and the capture of the moon is lessened, is one way to look
at it. Remember, gravity has less effect at greater distances, and ignoring
the fact that their is a force of repulsion with a different law to Gravity,
if you take energy away, you will lessen the grip, so to speak.

It may be counter intuitive, but that is the way it works.
Brian

Nope, as has been pointed out, the moon GAINS energy from the tides.
In order to increase the size of the orbit, it needs to increase
its potential energy. Keplers laws say that, as it gains potential
energy, it loses a bit of kinetic energy, because the higher orbit
is slower, but the net result is an overall gain of energy.
It ALWAYS takes more energy to put any satellite into a higher
orbit, even though the higher orbit is slower. The earth is
what loses energy. The tides cause a transfer of angular
momentum from the earth to the moon. Simultaneously, energy
is transferred from the Earth to the moon, and some energy is
lost in tidal friction.

See
http://www.astronomynotes.com/gravappl/s10.htm

http://info.citruscollege.com/FF/RCa...sofMotion.html

http://zebu.uoregon.edu/%7Esoper/Moon/tidalresults.html

http://www-astronomy.mps.ohio-state....t161/oct11.pdf

--Rich