"Nav" wrote in message
...


Peter S/Y Anicula wrote:

The gravitational force acts only toward the center of mass of the

system. This cannot by itself produce two bulges.

When you say that, you are mixing two explanations. That doesn't work.

We can certainly look at the gravitational force from the moon and the
gravitational force of the earth separately, and then ad the two, to
have a look at the combined forces. If you do not include part of the
rotation element, it works just fine.

If you only look at the gravitational forces, you can explain the two
bulges!


Well you keep saying that but it is not so. Unless you unclude the fact
that the system is rotating you cannot make two bulges on opposite
sides. Jeff posted a URL, have a read and then you will see the problem
-I hope.

There is no problem. The "differential" explanation starts by subtracting out
the total, net gravitational force and looking at just the differences at
various places on the Earth. What is the effect of this net component? It
accelerates the Earth towards the center of the Earth-Moon system. Thus, when
looking at the left over differences, you're already accounting for the rotation
of the Earth in this way.

As I mentioned in my other post, the net gravitational force subtracted out is
simply the opposite of the centrifugal force you've mentioned. To my mind,
neither of these causes the bulges, its when you subtract (or add its negative)
and looking at the differences around the Earth that you get the answer.

Jeff Morris wrote:
"Nav" wrote in message
...


Jeff Morris wrote:


You might make a case that the centrifugal explanation is easier for some

people

to understand, but claiming that gravity doesn't cause the tides is just

plain

bogus!

What are you talking about? I never said that gravity was not a part of
the equation.


Let me repost:
"The gravitational force acts only toward the center of mass of the
system. This cannot by itself produce two bulges."

Note the "by itself".


That's exactly the point - gravity is the only force at work here. Gravity does
cause the bulges. The centrifugal forces are a "fiction" caused by the
accelerating reference frame. Why is it accelerating? Because of gravity!



I'll repeat myself, the key is to understanding the _two tides_ problem
is that the system is rotating and "centrifugal" forces are balanced
only at the centers of the masses by gravity.



That is a simplified way to look at it. If it helps your understanding, fine.
Your problem, however, is that you're insisting that this is the *only* way to
understand the problem. The are numerous correct ways to look at this. You
don't have to use centrifugal force to explain the far bulge.

Frankly, for me, it doesn't help at all, because the centrifugal force is
constant throughout the Earth. If it produces the bulge on the far side, how
can it also produce a bulge in the opposite direction on the near side? The
answer, of course, is that you have to add the centrifugal force to
gravitational force. which is different throughout the Earth. The resulting
force is exactly the same as the differential gravity from the other model. Why
is this? Because the centrifugal force is a "fiction" - it is simply the
opposite of the net gravitational force that causes the Earth to rotate around
the Earth-Moon system. In the differential model you subtract this out, in the
centrifugal model you add it.

So I have trouble thinking of centrifugal force as pushing out the far bulge;
for me the bulge is caused because the far side receives less pull from the Moon
than the rest of the Earth. However, arguing that one model is more correct
than the other is like arguing whether A+B=C or A=C-B.


Good points. Well A=C-B ;-)

But, lets open this can of worms a bit further. I take and largely agree
with most of your view, but it is the kinetic energy in the system that
is powering the tides. If you locked the moon to the earth with a big
pole you would not have two tides would you? The mass is the same and so
is it's center... Gravity still works... but, just one tidal bulge.


Cheers

If the center of mass was the only factor involved, wouldn't the bulge be on
one side of the earth only?
Scout

"Nav" wrote in message
...
Yes, you can. Where is the center of mass of the earth moon system?

Cheers

Peter S/Y Anicula wrote:

We can certainly look at the gravitational force from the moon and the
gravitational force of the earth seperatly, and then ad the two, to
have a look at the combined forces.

Peter S/Y Anicula


"Nav" skrev i en meddelelse
...

Well Peter, I have to disagree there. The gravitational force acts

only

toward the center of mass of the system. This cannot by itself

produce

two bulges. To clarify this, try imagining the forces of gravity in

2D

on a piece of paper. In all cases, water would be pulled toward the
center of the Earth-Moon pair. This would lead to less water on the

far

side and more water as you move toward the moon... -two bulges would

not

be present.

Cheers

"Nav" wrote in message
...

[deleted stuff where we unfortunately seem to agree]


Good points. Well A=C-B ;-)

But, lets open this can of worms a bit further. I take and largely agree
with most of your view, but it is the kinetic energy in the system that
is powering the tides. If you locked the moon to the earth with a big
pole you would not have two tides would you? The mass is the same and so
is it's center... Gravity still works... but, just one tidal bulge.


I don't think its fair to do this - you can mathematically eliminate effects by
shifting the reference frame, but "locking" objects together is changing the
problem at a more fundamental level. In this case, how to you "lock" the Earth?
In fact, the crux of this problem is that different parts of the Earth are
actually acting somewhat independently.

However, this brings up an interesting point. At some point in the distant
future the tides will be eliminated. How will this happen? Because the tides
lag the Moon the high tide is not directly under the Moon, but offset. This
creates soon torque that is transferring energy from the Earth to the Moon. The
result is that the Earth is slowing down, and the Moon's orbit is increasing.
This will continue (some say) until the Earth's rotation slows down to match the
Moon, and the bulge stays under the Moon. The Earth and Moon will at that point
be locked together. Because the Moon is smaller, it has already assumed this
orientation WRT the Earth.

If we work this backwards we find the in the distant past the Moon's orbit was
much closer to the Earth, and the Earth's day much shorter. Exactly how much
depends on what other theory you're trying to support or disprove. However, we
do know the effect is real - the measurement using equipment left behind by the
astronauts shows the distance increasing about 4 cm a year, and the Earth's day
lengthening by 1.5 milliseconds a century.

well then, you'd have to use a telescoping pole! ; )
Scout

"Jeff Morris" wrote in message
...
"Nav" wrote in message
...

[deleted stuff where we unfortunately seem to agree]


Good points. Well A=C-B ;-)

But, lets open this can of worms a bit further. I take and largely agree
with most of your view, but it is the kinetic energy in the system that
is powering the tides. If you locked the moon to the earth with a big
pole you would not have two tides would you? The mass is the same and so
is it's center... Gravity still works... but, just one tidal bulge.


I don't think its fair to do this - you can mathematically eliminate
effects by
shifting the reference frame, but "locking" objects together is changing
the
problem at a more fundamental level. In this case, how to you "lock" the
Earth?
In fact, the crux of this problem is that different parts of the Earth are
actually acting somewhat independently.

However, this brings up an interesting point. At some point in the
distant
future the tides will be eliminated. How will this happen? Because the
tides
lag the Moon the high tide is not directly under the Moon, but offset.
This
creates soon torque that is transferring energy from the Earth to the
Moon. The
result is that the Earth is slowing down, and the Moon's orbit is
increasing.
This will continue (some say) until the Earth's rotation slows down to
match the
Moon, and the bulge stays under the Moon. The Earth and Moon will at that
point
be locked together. Because the Moon is smaller, it has already assumed
this
orientation WRT the Earth.

If we work this backwards we find the in the distant past the Moon's orbit
was
much closer to the Earth, and the Earth's day much shorter. Exactly how
much
depends on what other theory you're trying to support or disprove.
However, we
do know the effect is real - the measurement using equipment left behind
by the
astronauts shows the distance increasing about 4 cm a year, and the
Earth's day
lengthening by 1.5 milliseconds a century.

Yes, so...

Cheers


Scout wrote:

If the center of mass was the only factor involved, wouldn't the bulge be on
one side of the earth only?
Scout

"Nav" wrote in message
...

Yes, you can. Where is the center of mass of the earth moon system?

Cheers

Peter S/Y Anicula wrote:


We can certainly look at the gravitational force from the moon and the
gravitational force of the earth seperatly, and then ad the two, to
have a look at the combined forces.

Peter S/Y Anicula


"Nav" skrev i en meddelelse
...


Well Peter, I have to disagree there. The gravitational force acts

only


toward the center of mass of the system. This cannot by itself

produce


two bulges. To clarify this, try imagining the forces of gravity in

2D


on a piece of paper. In all cases, water would be pulled toward the
center of the Earth-Moon pair. This would lead to less water on the

far


side and more water as you move toward the moon... -two bulges would

not


be present.

Cheers

Jeff Morris wrote:

"Nav" wrote in message
...

[deleted stuff where we unfortunately seem to agree]


Good points. Well A=C-B ;-)

But, lets open this can of worms a bit further. I take and largely agree
with most of your view, but it is the kinetic energy in the system that
is powering the tides. If you locked the moon to the earth with a big
pole you would not have two tides would you? The mass is the same and so
is it's center... Gravity still works... but, just one tidal bulge.



I don't think its fair to do this - you can mathematically eliminate effects by
shifting the reference frame, but "locking" objects together is changing the
problem at a more fundamental level.

I don't see it that way, the explanation for the two tides based on
differential gravity alone does not care whether the earth is "moon
locked" at (say) an L point -and that why it is not the correct
explanation in my opinion. Of course it all comes down to gravity and
the energy of the system but the simplest close answer should consider
the rotation as well.

In this case, how to you "lock" the Earth?
In fact, the crux of this problem is that different parts of the Earth are
actually acting somewhat independently.

However, this brings up an interesting point. At some point in the distant
future the tides will be eliminated.

(Well not really, unless you ignore the Sun). But I think this point
reinforces what I've been trying to get across, without considering the
rotation(s) about the center of mass you don't get a two tide situation.
Any description that does not explicitly consider the relative motion
will not generate two tides -do you agree?

How will this happen? Because the tides
lag the Moon the high tide is not directly under the Moon, but offset. This
creates soon torque that is transferring energy from the Earth to the Moon. The
result is that the Earth is slowing down, and the Moon's orbit is increasing.
This will continue (some say) until the Earth's rotation slows down to match the
Moon, and the bulge stays under the Moon. The Earth and Moon will at that point
be locked together. Because the Moon is smaller, it has already assumed this
orientation WRT the Earth.

If we work this backwards we find the in the distant past the Moon's orbit was
much closer to the Earth, and the Earth's day much shorter. Exactly how much
depends on what other theory you're trying to support or disprove. However, we
do know the effect is real - the measurement using equipment left behind by the
astronauts shows the distance increasing about 4 cm a year, and the Earth's day
lengthening by 1.5 milliseconds a century.


I never looked it up but would have guessed the rate of slow down would
be larger than that. From that number you can calculate the energy cost
of the tidal forces... Here's a thought, at current rate of energy
consumption growth how long before even this energy source would be
insufficient for our needs? :)

Cheers

I was hoping you could solve this riddle.
But I'll toss in my oversimplified guess: the moon's gravity attracts the
water closest to it resulting in high high tide on the moon side of earth,
and also pulls the earth away from the water on the far side, resulting in a
low high tide on the side farthest from the moon.
Scout


"Nav" wrote in message
...
Yes, so...

Cheers


Scout wrote:

If the center of mass was the only factor involved, wouldn't the bulge be
on one side of the earth only?
Scout

"Nav" wrote in message
...

Yes, you can. Where is the center of mass of the earth moon system?

Cheers

Peter S/Y Anicula wrote:


We can certainly look at the gravitational force from the moon and the
gravitational force of the earth seperatly, and then ad the two, to
have a look at the combined forces.

Peter S/Y Anicula


"Nav" skrev i en meddelelse
...


Well Peter, I have to disagree there. The gravitational force acts

only


toward the center of mass of the system. This cannot by itself

produce


two bulges. To clarify this, try imagining the forces of gravity in

2D


on a piece of paper. In all cases, water would be pulled toward the
center of the Earth-Moon pair. This would lead to less water on the

far


side and more water as you move toward the moon... -two bulges would

not


be present.

Cheers

I knew I saw this somewhere
Scout

The following diagram shows how the moon causes tides on Earth:




In this diagram, you can see that the moon's gravitational force pulls on water in the oceans so that there are "bulges" in the ocean on both sides of the planet. The moon pulls water toward it, and this causes the bulge toward the moon. The bulge on the side of the Earth opposite the moon is caused by the moon "pulling the Earth away" from the water on that side.

If you are on the coast and the moon is directly overhead, you should experience a high tide. If the moon is directly overhead on the opposite side of the planet, you should also experience a high tide.

During the day, the Earth rotates 180 degrees in 12 hours. The moon, meanwhile, rotates 6 degrees around the earth in 12 hours. The twin bulges and the moon's rotation mean that any given coastal city experiences a high tide every 12 hours and 25 minutes or so.





"Scout" wrote in message ...
I was hoping you could solve this riddle.
But I'll toss in my oversimplified guess: the moon's gravity attracts the
water closest to it resulting in high high tide on the moon side of earth,
and also pulls the earth away from the water on the far side, resulting in a
low high tide on the side farthest from the moon.
Scout


"Nav" wrote in message
...
Yes, so...

Cheers


Scout wrote:

If the center of mass was the only factor involved, wouldn't the bulge be
on one side of the earth only?
Scout

"Nav" wrote in message
...

Yes, you can. Where is the center of mass of the earth moon system?

Cheers

Peter S/Y Anicula wrote:


We can certainly look at the gravitational force from the moon and the
gravitational force of the earth seperatly, and then ad the two, to
have a look at the combined forces.

Peter S/Y Anicula


"Nav" skrev i en meddelelse
...


Well Peter, I have to disagree there. The gravitational force acts

only


toward the center of mass of the system. This cannot by itself

produce


two bulges. To clarify this, try imagining the forces of gravity in

2D


on a piece of paper. In all cases, water would be pulled toward the
center of the Earth-Moon pair. This would lead to less water on the

far


side and more water as you move toward the moon... -two bulges would

not


be present.

Cheers

Yup. That's about it. As I said a while back, the Earth is "falling" towards
the Moon as the two rotate around their common center. The near part of the
Earth falls a bit faster, the far part falls a bit slower. The result is the
two bulges.

Nav has been asking what happens if we prevent the Earth from "falling" but
somehow still had the Moon's gravity. Then we would have higher tide on the
near side, and low (but not as low as normal) tide on the far side.



"Scout" wrote in message
...
I was hoping you could solve this riddle.
But I'll toss in my oversimplified guess: the moon's gravity attracts the
water closest to it resulting in high high tide on the moon side of earth,
and also pulls the earth away from the water on the far side, resulting in a
low high tide on the side farthest from the moon.
Scout


"Nav" wrote in message
...
Yes, so...

Cheers


Scout wrote:

If the center of mass was the only factor involved, wouldn't the bulge be
on one side of the earth only?
Scout

"Nav" wrote in message
...

Yes, you can. Where is the center of mass of the earth moon system?

Cheers

Peter S/Y Anicula wrote:


We can certainly look at the gravitational force from the moon and the
gravitational force of the earth seperatly, and then ad the two, to
have a look at the combined forces.

Peter S/Y Anicula


"Nav" skrev i en meddelelse
...


Well Peter, I have to disagree there. The gravitational force acts

only


toward the center of mass of the system. This cannot by itself

produce


two bulges. To clarify this, try imagining the forces of gravity in

2D


on a piece of paper. In all cases, water would be pulled toward the
center of the Earth-Moon pair. This would lead to less water on the

far


side and more water as you move toward the moon... -two bulges would

not


be present.

Cheers