As I said, I was leaving this as an exercise for the reader.

I have heard tides on the far side of the Earth described in terms of the
centrifugal force caused by the Earth's rotation around Earth-Moon system.
Although this is a consistent way of describing it, I've never liked using
"fictional" forces. Here's a site that uses that approach:

http://co-ops.nos.noaa.gov/restles3.html


"Nav" wrote in message
...
Jeff, you really cannot explain two tides a day unless you also include
the centripetal forces of the earth moon pair -this is the key that is
seems repeatedly lost.

Cheers


Jeff Morris wrote:

"DSK" wrote in message
...

Peter, thanks for your educational posts.

Peter S/Y Anicula wrote:

On both sides the change in gravitational pull from the moon reduces
or counteracts the gravitational force of the earth on the
water-molecule(making it lighter, so to speak).
This should explain why there is to tides a day, one when the moon is
culminating and one when it is on the other side.

I would think that when the moon is on the opposite side, it's
gravitation effect would be cumulative, acting to depress the water
level. But it would be far less than when it's overhead, and the water
has been put in motion. My (relatively vague) understanding of the
science behind tides is that it's partly gravity and partly harmonics.


Gravity is the force that drives it, harmonics determines the timing.

Here's a site that describes the Differential Gravity in a fairly simple
way:
http://burro.astr.cwru.edu/Academics...ity/tides.html

Now you're actually claiming you did it purpose to be sarcastic?

While its true that you mentioned once that you had lived in Baltimore, you
ignored that fact that I (and others) mentioned Boston as the home of the
Constitution a number of times. And we said a number of times that you seemed
to be describing a different ship from what we all knew. Any reasonable person
would have stopped to consider if they were making a mistake. But not you,
Navvie!

And it wasn't that you made a simple mistake, it was the insisted that you knew
more about the Constitution than everyone else! Classic!



"Nav" wrote in message
...
And your point is what, that I was describing a different ship with a
very similar name? Big deal. It's hilarious that you still don't see the
sarcasm in my refence to her "motor". But if it makes you feel good I'll
say it again, I _was_ talking about the Constellation in _BALTIMORE_ and
not the Constitution -although why you seemed to continually miss the
fact that I was talking about a ship in Baltimore is beyond me. I guess
you only see what you want. At least I can acknowlege when I'm wrong.

Now why don't you tell us again about how great an engineer you are?

Cheers

DSK the wiper wrote:


http://groups.google.com/groups?hl=e...com%26rnum%3D2


Or since long URLs can be troublesome

http://tinyurl.com/3uc5a

The discussion was originally about "Old Ironsides" and went on for a
dozen or more posts about the "USS Constitution" ver specifically.

Amazing what just a few seconds in the archives can reveal.



Nav wrote:

Constellation actually, little man.




Are you sure?


Nav wrote:

Yes, and with each snipe you get smaller.


And with each denial and/or backpedal, you get....

DSK

OzOne wrote:

On Fri, 01 Oct 2004 09:57:01 +1200, Nav
scribbled thusly:


The key to understanding resides in where the center of mass of the
earth-moon system resides.

Cheers


Nav, you need to go sailing....without the textbooks OK?


Hey, I learn't that long ago. The amizing thing is the number of
explanations out there that try to come up with the right answer just
using a gravity argument -even in school textbooks!

Cheers

Hey, we were talking at crossed prurposes. I mentioned Baltimore several
times -whether (or not) the ship might have gone to Boston would have
not helped let me see we were talking about different ships with very
similar names would it? I think it's much more revealing how certain
people crow on about an innocent mistaken identity don't you?

Cheers


Jeff Morris wrote:

Now you're actually claiming you did it purpose to be sarcastic?

While its true that you mentioned once that you had lived in Baltimore, you
ignored that fact that I (and others) mentioned Boston as the home of the
Constitution a number of times. And we said a number of times that you seemed
to be describing a different ship from what we all knew. Any reasonable person
would have stopped to consider if they were making a mistake. But not you,
Navvie!

And it wasn't that you made a simple mistake, it was the insisted that you knew
more about the Constitution than everyone else! Classic!



"Nav" wrote in message
...

And your point is what, that I was describing a different ship with a
very similar name? Big deal. It's hilarious that you still don't see the
sarcasm in my refence to her "motor". But if it makes you feel good I'll
say it again, I _was_ talking about the Constellation in _BALTIMORE_ and
not the Constitution -although why you seemed to continually miss the
fact that I was talking about a ship in Baltimore is beyond me. I guess
you only see what you want. At least I can acknowlege when I'm wrong.

Now why don't you tell us again about how great an engineer you are?

Cheers

DSK the wiper wrote:


http://groups.google.com/groups?hl=e...com%26rnum%3D2


Or since long URLs can be troublesome

http://tinyurl.com/3uc5a

The discussion was originally about "Old Ironsides" and went on for a
dozen or more posts about the "USS Constitution" ver specifically.

Amazing what just a few seconds in the archives can reveal.



Nav wrote:


Constellation actually, little man.




Are you sure?


Nav wrote:


Yes, and with each snipe you get smaller.


And with each denial and/or backpedal, you get....

DSK

Yep it's spot on. I like the pointed quote:

"1. The Effect of Centrifugal Force. It is this little known aspect of
the moon's orbital motion which is responsible for one of the two force
components creating the tides."

The question is how many sites that try to explain the two tide problem
ignore this? Answer -almost all!!!! Even the one from your astronomy
Professor! That the tidal problem can be repeated incorrectly so many
times really annoys to me. I'd say it is not beyond the ability of most
children to understand the correct answer is it?

Cheers

Jeff Morris wrote:

As I said, I was leaving this as an exercise for the reader.

I have heard tides on the far side of the Earth described in terms of the
centrifugal force caused by the Earth's rotation around Earth-Moon system.
Although this is a consistent way of describing it, I've never liked using
"fictional" forces. Here's a site that uses that approach:

http://co-ops.nos.noaa.gov/restles3.html


"Nav" wrote in message
...

Jeff, you really cannot explain two tides a day unless you also include
the centripetal forces of the earth moon pair -this is the key that is
seems repeatedly lost.

Cheers


Jeff Morris wrote:


"DSK" wrote in message
. net...


Peter, thanks for your educational posts.

Peter S/Y Anicula wrote:


On both sides the change in gravitational pull from the moon reduces
or counteracts the gravitational force of the earth on the
water-molecule(making it lighter, so to speak).
This should explain why there is to tides a day, one when the moon is
culminating and one when it is on the other side.

I would think that when the moon is on the opposite side, it's
gravitation effect would be cumulative, acting to depress the water
level. But it would be far less than when it's overhead, and the water
has been put in motion. My (relatively vague) understanding of the
science behind tides is that it's partly gravity and partly harmonics.


Gravity is the force that drives it, harmonics determines the timing.

Here's a site that describes the Differential Gravity in a fairly simple

way:

http://burro.astr.cwru.edu/Academics...ity/tides.html

Jeff,

I think the term centrifuigal is appropriate in this context. It is a
term in the system that can be appreciated without needing to consider
Newtonian forces. To understand centripetal forces is a lot harder than
just demonstrating the effect. similarly gravity can be demonstrated
without maths. Thus the explanation becomes really simple e.g.:

The earth-moon body rotates around a common point and water tries to
move away from the center (water in a bucket swung on a rope analogy).
The moon exerts gravity which is stronger on the side of the moon. Thus
water forms two bulges on opposite sides and makes two tides as the
earth rotates...


Cheers


Jeff Morris wrote:

As I said, I was leaving this as an exercise for the reader.

I have heard tides on the far side of the Earth described in terms of the
centrifugal force caused by the Earth's rotation around Earth-Moon system.
Although this is a consistent way of describing it, I've never liked using
"fictional" forces.

A "standard" tide prediction uses (at least) 32 factors. The most
important being the difference in graviton from the moon over the
earth surface. That leaves out at least 31 factors of practical
importance. One of these is the difference in centrifugal forces due
to the rotation around the mass-centre of the moon-earth system. There
are still 30 left. I was not trying to give the full explanation of
the tide generating forces.

I was answering a question as to why the influence of the sun on the
tides was smaller than the influence of the moon. I think that the
fact that while the gravitation of the sun is larger than that of the
moon, the difference of the gravitation over the earth surface is
smaller, answers that question fairly well.

Peter S/Y Anicula

"Nav" skrev i en meddelelse
...


DSK wrote:
Peter, thanks for your educational posts.

Peter S/Y Anicula wrote:

On both sides the change in gravitational pull from the moon
reduces
or counteracts the gravitational force of the earth on the
water-molecule(making it lighter, so to speak).
This should explain why there is to tides a day, one when the
moon is
culminating and one when it is on the other side.


I would think that when the moon is on the opposite side, it's
gravitation effect would be cumulative, acting to depress the
water
level. But it would be far less than when it's overhead, and the
water
has been put in motion. My (relatively vague) understanding of the
science behind tides is that it's partly gravity and partly
harmonics.


The key to understanding resides in where the center of mass of the
earth-moon system resides.

Cheers

OK. I think you mean that as the gravitational field is flatter for the
Earth Sun pair compared to the Earth Moon pair the differential effect
of sun position is smaller?

Cheers

Peter S/Y Anicula wrote:

A "standard" tide prediction uses (at least) 32 factors. The most
important being the difference in graviton from the moon over the
earth surface. That leaves out at least 31 factors of practical
importance. One of these is the difference in centrifugal forces due
to the rotation around the mass-centre of the moon-earth system. There
are still 30 left. I was not trying to give the full explanation of
the tide generating forces.

I was answering a question as to why the influence of the sun on the
tides was smaller than the influence of the moon. I think that the
fact that while the gravitation of the sun is larger than that of the
moon, the difference of the gravitation over the earth surface is
smaller, answers that question fairly well.

Peter S/Y Anicula

"Nav" skrev i en meddelelse
...


DSK wrote:

Peter, thanks for your educational posts.

Peter S/Y Anicula wrote:


On both sides the change in gravitational pull from the moon

reduces

or counteracts the gravitational force of the earth on the
water-molecule(making it lighter, so to speak).
This should explain why there is to tides a day, one when the

moon is

culminating and one when it is on the other side.


I would think that when the moon is on the opposite side, it's
gravitation effect would be cumulative, acting to depress the

water

level. But it would be far less than when it's overhead, and the

water

has been put in motion. My (relatively vague) understanding of the
science behind tides is that it's partly gravity and partly

harmonics.

The key to understanding resides in where the center of mass of the
earth-moon system resides.

Cheers

You may be right, certainly proponents of this explanation use "centrifugal
force."

However, differential gravity can be explained a number of ways. For example,
the Moon's pull causes the Earth to accelerate towards the Moon. That portion
of the Earth closer feels more force, and thus falls faster; that portion on the
far side feels less force, and thus falls slower. These differences cause the
bulges on the near and far sides.

Remember that Centrifugal Force may be a handy explanation, but it is a
"fictional force" that only appears real to an observer in an accelerating frame
of reference. Therefore, whenever it is used to explain something, there must
be another explanation that works in a non-accelerating frame.



"Nav" wrote in message
...
Jeff,

I think the term centrifuigal is appropriate in this context. It is a
term in the system that can be appreciated without needing to consider
Newtonian forces. To understand centripetal forces is a lot harder than
just demonstrating the effect. similarly gravity can be demonstrated
without maths. Thus the explanation becomes really simple e.g.:

The earth-moon body rotates around a common point and water tries to
move away from the center (water in a bucket swung on a rope analogy).
The moon exerts gravity which is stronger on the side of the moon. Thus
water forms two bulges on opposite sides and makes two tides as the
earth rotates...


Cheers


Jeff Morris wrote:

As I said, I was leaving this as an exercise for the reader.

I have heard tides on the far side of the Earth described in terms of the
centrifugal force caused by the Earth's rotation around Earth-Moon system.
Although this is a consistent way of describing it, I've never liked using
"fictional" forces.

Jeff Morris wrote:

You may be right, certainly proponents of this explanation use "centrifugal
force."

However, differential gravity can be explained a number of ways. For example,
the Moon's pull causes the Earth to accelerate towards the Moon. That portion
of the Earth closer feels more force, and thus falls faster; that portion on the
far side feels less force, and thus falls slower. These differences cause the
bulges on the near and far sides.


I like it! That's a way of putting it I've not heard before and it is
quite elegant (provided the listener can accept that the Earth is
falling toward the Moon!)


Remember that Centrifugal Force may be a handy explanation, but it is a
"fictional force" that only appears real to an observer in an accelerating frame
of reference. Therefore, whenever it is used to explain something, there must
be another explanation that works in a non-accelerating frame.


I could be devious and say we are all in an accelerating frame! But you
are quite right about the artifice of a virtual force. Nevertheless,
children want to know about tides and for them centripetal force can be
experienced more easily than the idea they are on an earth that is
falling...

Cheers