"Scout" wrote in message
...
"Jeff Morris" wrote
[snip]
So take your pick, either explanation works, and I'm sure there are others. However,
I hope you can appreciate that explanations like "gravity creates the inner bulge,
centrifugal force creates the outer bulge" makes physicists wince!

Yes, I can, as I've watched my physics teaching partner wince quite a bit this week as we
discussed this thread. He was quick to cover our whiteboard with formulas and drawings. It's an
interesting thread though, and notwithstanding my oversimplified analogies, I've learned a lot
from it. By the way, I saw that same wince from a black history professor when I suggested that
the Civil War was fought to free the slaves, and then again when I suggested to an ancient lit
professor that The Odyssey has all the earmarks of an Arnold Schwarzenegger movie. Probably
explains why I like a good fart joke.

Sort of like the wince I got from an African-American/Cherokee friend when I asked him how he was
going to celebrate Columbus Day?

jeff

ps So how does your partner rate my explanations?

"Jeff Morris" wrote
[snip] Sort of like the wince I got from an African-American/Cherokee
friend when I asked him how he was going to celebrate Columbus Day?
Ouch!

ps So how does your partner rate my explanations?
The great mediator saw truth in both models. He was a bit more forgiving of
my "impellor in a great centrifugal pump" analogy, but slapped my wrist for
saying this about centrifugal force: if it feels real, mustn't it be real?
By the time he was done, my head was spinning and yet somehow my brain
seemed to be bulging (quite appropriately) out of both sides of my head. In
the end he called me an English teacher, which is his way of slandering me,
and told me my paltry general science achievements were no match for his
superior physics and math skills. I told him he could forget about me taking
him sailing again and he took it all back.
He's never done the newsgroups though, and was quite impressed with the ease
with which like minded folks could debate a worthy topic.
Scout

Well just to confuse things a bit mo
Even if we only focus on the tide generating potential, there is a
cupple of things that we haven't discussed yet, and one of them has to
do with rotation: "The Coriolis freqency". The other thing one could
include is the "parallax".
I mention this just to make clear that the two models discussed above
both are incomplete.

Peter S/Y Anicula

"Jeff Morris" skrev i en meddelelse
...
"Scout" wrote in message
...
Jeff,
Remember that I first posted that very same sentiment, and even
provided a
graphic. I still believe that to be true, but have modified my
internal
model, giving allowance for the centrifugal force. I'm not a
physicist, but
the way I'm seeing it, there is a middle ground in this
discussion. I'm
curious to know if you're discounting centrifugal force as a
contributor to
the far bulge.
Scout

I've always said that Centrifugal Force can be used as part of the
explanation, as
long as you end up with the same answer. There are several
different ways of looking
at this, all valid. (I hope I can get through this without mangling
the terms too
badly ...)

The problem with Centrifugal Force is that it is a "fictional
force." It is only
needed if you work in a non-inertial, or accelerating reference
frame. If you are in
a car going around a curve, your reference frame is accelerating
towards the center of
the curve, and thus you feel a Centrifugal Force in the opposite
direction. To an
outside observer, the CF doesn't exist, the only force is the car
pulling the
passenger around the turn. The outside observer can analyze the
situation completely
without invoking CF. (The passenger feels CF push him outward, the
observer sees the
car pull the passenger inward.)

In the Earth-Moon system there is gravity pulling both the Earth and
Moon around
curves. Because the gravity acts on all objects, we don't notice
ourselves being
pulled around. The magnitude of the Centrifugal force is to small to
notice, but in
that reference frame it exists. To the outside observer, we're
just in freefall,
being pulled inward by gravity.

The problem with CF arises when you look carefully at the math. One
pitfall Nav fell
into was trying to calculate CF as a function that varies with the
distance to the
barycenter. However, all points on the Earth do not rotate around
the barycenter,
only the center does. Other points describe the same circle around
nearby points, so
that all points on Earth feel the same Centrifugal Force. (This is
a tough concept to
explain in words; its easier to do it graphically. Consider a plate
wobbling around a
point but with no rotation - each point on the plate describes the
same circle.)

BTW, Nav provided two commonly used formulas, one for gravity and
the other for CF.
Although they look quite different, you should appreciate that they
are the same,
since the angular velocity is determined by the gravitational force.
The CF will be
the same (with the opposite sign) as the gravitational pull at the
Earth's center.

Since the CF is a constant force, it can't describe the two bulges
in opposite
directions. It is gravity itself that varies with distance. The
differential force
can be derived either by subtracting the average gravitational force
which causes the
freefall at the center of the Earth, or it can be derived by adding
the centrifugal
force. Since the two are the same, except for the sign, the math is
identical.

So take your pick, either explanation works, and I'm sure there are
others. However,
I hope you can appreciate that explanations like "gravity creates
the inner bulge,
centrifugal force creates the outer bulge" makes physicists wince!

mission accomplished! lol
Scout

"Peter S/Y Anicula" wrote
Well just to confuse things a bit mo
Even if we only focus on the tide generating potential, there is a
cupple of things that we haven't discussed yet, and one of them has to
do with rotation: "The Coriolis freqency". The other thing one could
include is the "parallax".
I mention this just to make clear that the two models discussed above
both are incomplete.

Scout,

I was determined not to re-enter this discussion, except to needle Donal
about the 1/4 point.

Your post about your physic teacher gave me enough enjoyment to venture
another post.

Your experience brought back memoirs of mine own days in High School
with my own Physic Teacher about Tides and a Commercial Clammer who help
me learn sailing. By the time I got finished, I had more faith in the
Clammer. He lived in a shed on a float in the middle of the harbor. He
was fond of the booze an when he know enough about any subject to
satisfy himself, then he was satisfied.

His explanation of the Tide, given to me willingly. was; and I quote.
The Moon causes the Tides. It cause high tide because of its pull on
earth, which screws up the pull between the earth and the Sun. Since the
Tides are made up of water, the higher water, under the Moon creates
less water on the other side of the Earth or low Tide. Since the Tide
are water and the pull is less the farther away from the Moon they are
not as high on the side of the earth facing the moon but higher than the
water on the side of the Earth facing away away from the Moon. People
call this difference 1/2 tide. The height of High Tide and Low Tide
along with 1/2 Tides are affected by the Phase of the Moon.

The difference of the location and of the time of tides are caused by
the differences of the rotation of the Earth and the time of revolution
and direction of the Moon

This is explanation has served me well for over 60 years.

Ole Thom

Scout,

I forgot to mention my old friend. Frank, also told me he had Tide
Tables and Almanics for the heigth and times of the tides and the
position on the Moon.

Ole Thom

You're absolutely correct. There are numerous effects we're not considering. We've
only attempted to understand the primary cause of two tides a day. Even then, the
math is a bit more complex than the simple formulas we've used.

The parallax effects are certainly significant - they are caused non-circular orbits.
And then there's Lunar declination to fold in.

Of course, spring and neap must be considered - does everyone know when Syzygy is?

And these are just the global effect - there's a whole litany of local effects to
consider. Or you can keep a copy of Eldridge or Reed's handy.



"Peter S/Y Anicula" wrote in message
...
Well just to confuse things a bit mo
Even if we only focus on the tide generating potential, there is a
cupple of things that we haven't discussed yet, and one of them has to
do with rotation: "The Coriolis freqency". The other thing one could
include is the "parallax".
I mention this just to make clear that the two models discussed above
both are incomplete.

Peter S/Y Anicula

"Jeff Morris" skrev i en meddelelse
...
"Scout" wrote in message
...
Jeff,
Remember that I first posted that very same sentiment, and even
provided a
graphic. I still believe that to be true, but have modified my
internal
model, giving allowance for the centrifugal force. I'm not a
physicist, but
the way I'm seeing it, there is a middle ground in this
discussion. I'm
curious to know if you're discounting centrifugal force as a
contributor to
the far bulge.
Scout

I've always said that Centrifugal Force can be used as part of the
explanation, as
long as you end up with the same answer. There are several
different ways of looking
at this, all valid. (I hope I can get through this without mangling
the terms too
badly ...)

The problem with Centrifugal Force is that it is a "fictional
force." It is only
needed if you work in a non-inertial, or accelerating reference
frame. If you are in
a car going around a curve, your reference frame is accelerating
towards the center of
the curve, and thus you feel a Centrifugal Force in the opposite
direction. To an
outside observer, the CF doesn't exist, the only force is the car
pulling the
passenger around the turn. The outside observer can analyze the
situation completely
without invoking CF. (The passenger feels CF push him outward, the
observer sees the
car pull the passenger inward.)

In the Earth-Moon system there is gravity pulling both the Earth and
Moon around
curves. Because the gravity acts on all objects, we don't notice
ourselves being
pulled around. The magnitude of the Centrifugal force is to small to
notice, but in
that reference frame it exists. To the outside observer, we're
just in freefall,
being pulled inward by gravity.

The problem with CF arises when you look carefully at the math. One
pitfall Nav fell
into was trying to calculate CF as a function that varies with the
distance to the
barycenter. However, all points on the Earth do not rotate around
the barycenter,
only the center does. Other points describe the same circle around
nearby points, so
that all points on Earth feel the same Centrifugal Force. (This is
a tough concept to
explain in words; its easier to do it graphically. Consider a plate
wobbling around a
point but with no rotation - each point on the plate describes the
same circle.)

BTW, Nav provided two commonly used formulas, one for gravity and
the other for CF.
Although they look quite different, you should appreciate that they
are the same,
since the angular velocity is determined by the gravitational force.
The CF will be
the same (with the opposite sign) as the gravitational pull at the
Earth's center.

Since the CF is a constant force, it can't describe the two bulges
in opposite
directions. It is gravity itself that varies with distance. The
differential force
can be derived either by subtracting the average gravitational force
which causes the
freefall at the center of the Earth, or it can be derived by adding
the centrifugal
force. Since the two are the same, except for the sign, the math is
identical.

So take your pick, either explanation works, and I'm sure there are
others. However,
I hope you can appreciate that explanations like "gravity creates
the inner bulge,
centrifugal force creates the outer bulge" makes physicists wince!

Thanks Thom,
thank God for those ol' salty guys ~ they always manage to keep it real!
Scout

"Thom Stewart" wrote
[snip]
Your post about your physic teacher gave me enough enjoyment to venture
another post.

Your experience brought back memoirs of mine own days in High School
with my own Physic Teacher about Tides and a Commercial Clammer who help
me learn sailing. By the time I got finished, I had more faith in the
Clammer. He lived in a shed on a float in the middle of the harbor. He
was fond of the booze an when he know enough about any subject to
satisfy himself, then he was satisfied.

"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.

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.

"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
--