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Author Topic: Why do we have two high tides a day?  (Read 17369 times)

Offline PmbPhy

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Re: Why do we have two high tides a day?
« Reply #25 on: 29/05/2015 23:59:08 »
Quote from: rmolnav
In pdf linked in reply #23, I can see that Schutz also mentions only the different gravitatory pulls from the Moon …
Did you read that PDF file carefully? Did you see an error in it?

Why do you believe that centrifugal force plays such an important role in ocean tides?

Quote from: rmolnav
That cannot be due to anything similar to what stated by Schutz relative to tides: CENTRIFUGAL force is the cause.
Let me get this right. You actually believe that tides are caused entirely by centrifugal forces? If so then that's clearly wrong. It can't be correct because the existence of tides corresponds to the position of the moon and centrifugal forces can't account for that. It also can't account for the periodicity of tides either.
 

Offline PmbPhy

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Re: Why do we have two high tides a day?
« Reply #26 on: 30/05/2015 00:33:40 »
Quote from: rmolnav
That cannot be due to anything similar to what stated by Schutz relative to tides: CENTRIFUGAL force is the cause.
You appear to think that tidal forces are caused by centrifugal forces and that's simply not true. That would mean that there'd be no tides since the centrifugal force on matter at any place on earth doesn't change with the time of day like ocean tides do.
 

Offline PmbPhy

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Re: Why do we have two high tides a day?
« Reply #27 on: 30/05/2015 00:35:18 »
Quote from: rmolnav
That cannot be due to anything similar to what stated by Schutz relative to tides: CENTRIFUGAL force is the cause.
You appear to think that tidal forces are caused by centrifugal forces and that's simply not true. That would mean that there'd be no tides since the centrifugal force on matter at any place on earth doesn't change with the time of day like ocean tides do.
 

Offline rmolnav

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Re: Why do we have two high tides a day?
« Reply #28 on: 31/05/2015 11:41:39 »
I know centrifugal force can be considered as an inertial, actually kind of ficticious force … But not only that way.
Imagine a solid spherical object rotating as artificial satellite around Earth. As all we know, the hole object is globally subjected only to one gravitatory centripetal force, which is producing an acceleration. Being that acceleration perpendicular to the object velocity, it doesn´t change its linear velocity, only its direction, in such a way that the object rotates. To simplify we can consider the orbit is circular.
That is possible only with suitable values  of angular speed and distance: those values have to match. The gravitatory force divided by object´s mass has to be equal to the square of the angular speed multiplied by the radius (distance from Earth).
For a given angular speed, centripetal acceleration of that satellite is proportional to the distance, but gravitatory acceleration is inversely proportional to the square of the distance.
If we made the simplification of considering the hole mass of the satellite concentrated in its center of gravity, for a given angular speed there would be a distance suitable to get the object moving as a satellite.
In that case, no centrifugal force would actually be affecting the object. That force, in the fashion I´m  using this concept, derives from action/reaction principle: if Earth is producing a gravitatory, centripetal force on the object, this is also producing another equal opposite force on the Earth. But the object does´t suffer that reaction force.
But the object has a size. Different parts of it are at different distances from Earth. Subsequently gravitational forces are different. But the hole object is kind of obliged to rotate at the same angular speed … New internal forces appear affecting different parts of the object!
Let us imagine the object divided into many thin cylindrical slices (with radius equal to distance to Earth). All parts of each slice rotate with same radius. If there were no internal forces, only a central slice could continue rotating with the given global angular speed, but the rest could not.
All parts of the object interact with contiguous ones, but we can consider central parts of the object “ run the show”: they are rotating at the “ correct” speed, and forcing the rest to keep their pace. 
Each slice experiences the gravitatory force at that distance, and additional internal forces from contiguous slices. The sum of all those forces has to produce the centripetal acceleration which makes it rotate at the given angular speed.
Going from center slice to further side, slices are being forced by contiguous closer ones to rotate with a centripetal acceleration bigger than what only gravity would produce on each slice. That is acheived by inner direct tensile forces, from closer slices to further contiguous ones. And that fills the centripetal acceleration “gaps”. Due to action/reaction principle, further slices produces an opposite and equal force on closer ones. Those forces  are CENTRIFUGAL, not ficticious but real, and affect all those slices of the object.
Something similar happens going from center slice to closer side. In this case gravitatory forces get bigger and bigger than what required to produce the centripetal force at the given angular speed. Further slices force contiguous closer ones to rotate with a  centripetal acceleration smaller than what produced by gravity there. That is achieved by direct internal tensile forces from further slices on closer ones, which is also a CENTRIFUGAL, real force.
Those “ internal” imbalances are the cause of the tendency of any celestial object rotating around another (actually around a common axis of rotation) to get an ovoid shape, causing many interesting phenomena, some of them mentioned by Schutz previously linked work as a pdf. And, as far as I can see, they are also the actual cause of the two Earth´s high tides.
I have other arguments that further “ prove” my (?) theory, related to tide cycle due to the other “couple” Sun/Earth, but this post is already too long …
By the way, Schutz (or whoever has written “Investigation 5.1: Tides and eclipses“ at mentioned work), make a bizarre calculation when comparing Sun and Moon gravitatory effects, that I also find wrong ...
     
 

Offline rmolnav

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Re: Why do we have two high tides a day?
« Reply #29 on: 31/05/2015 11:48:20 »
Sorry. An "n" is missing at "But the object does**´t suffer that reaction force". It is clear, isn´t it?
 

Offline rmolnav

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Re: Why do we have two high tides a day?
« Reply #30 on: 01/06/2015 09:02:21 »
After my last posts I´ve found my point can be put in a simpler way:
Each slice is affected by its own gravity, and internal stresses from both nearer and further contiguous slices. The net result of all forces has to produce the centripetal acceleration to make the slice rotate at the given angular speed.
The closer the slice, the bigger the gravitatory force (it increases inversely proportionally to the square of the distance), but the smaller the centripetal force necessary to produce the given angular speed (proportionally to the radius or distance). The dynamic equilibrium of satellite rotation can only be achieved if the net force produced on each slice by contiguous ones (either direct or reaction forces) is in an outward direction: a CENTRIFUGAL force, which affects all slices bar central one, where net force from contiguous slices is null (gravitatory force equal to what necessary to produce the given angular speed).
 

Offline rmolnav

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Re: Why do we have two high tides a day?
« Reply #31 on: 01/06/2015 15:24:02 »
I HAD NOT SEEN Reply #       :
"You appear to think that tidal forces are caused by centrifugal forces and that's simply not true. That would mean that there'd be no tides since the centrifugal force on matter at any place on earth doesn't change with the time of day like ocean tides doI".

That is utterly erroneous. In any low latitude area, when Moon is almost just over there, even solid earth get deformed, in this case mainly due gravitatory pull from the Moon. And it happens the same at antipodes, there mainly due to centrifugal force. Logically, the deformation is smaller than at open oceans, where "fluidity" of water makes deformation much easier ... Even we ourselves weigh a little less, similar to what happens at the equator, compared to our weight at poles.
Something similar, but smaller, happens at noon and midnight, in relation to Sun/Earth effects. 
 

Offline PmbPhy

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Re: Why do we have two high tides a day?
« Reply #32 on: 02/06/2015 03:15:25 »
Quote from: rmolnav
That is utterly erroneous.
And you are utterly wrong. You're claiming that tidal forces, i.e. ocean tides, are due solely do to centrifugal forces which means that the moon has nothing to do with it because centrifugal forces are not caused by the moon.

You may not like it and you may not understand it but the descriptions in those PDF files are the correct description for tidal forces. There is no doubt about it whatsoever. If you think that those derivations are wrong then you are seriously mistaken.

 You have the derivations right in front of you and they've been there for days now and you've made no attempt to state what's in those derivations that's wrong. I.e. you haven't proved that the derivations and therefore the conclusions are wrong.
Keep it civil - Mod
« Last Edit: 05/06/2015 13:15:35 by evan_au »
 

Offline rmolnav

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Re: Why do we have two high tides a day?
« Reply #33 on: 02/06/2015 07:04:07 »
Reply #31:
"... You're claiming that tidal forces, i.e. ocean tides, are due solely do to centrifugal forces which means that the moon has nothing to do with it because centrifugal forces are not caused by the moon".
Sorry, but i have NOT claimed what you say. What in several ways I have said is that ocean tides are mainly due to an imbalance between gravitatory pull from the Moon (the closer, the higher the pull), and centrifugal forces due to the fact that it is not 100% right that Moon is rotating around Earth: both are rotating around a common barycentral axis, and the further from that axis, the bigger the centrifugal force.
And I´m not promoting my own idea and "not inviting critical debate about it ...". F.e., twice I have invited anybody to answer the question:
"If those three zones of the Earth experience only those gravitatory accelerations from the Moon, why ALL of them don´t cause a movement of the Earth towards the Moon? “
To be honest, I admit that perhaps I"ve been too assertive. and I beg your pardon. It would have been due to the fact that I feel pretty sure about the issue. Logically I know I could be wrong. If any of you think it is so, please kindly show it to me with facts and arguments, not with sweeping, not accurate generalizations. 

 

Offline rmolnav

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Re: Why do we have two high tides a day?
« Reply #34 on: 02/06/2015 10:11:44 »
If anybody think I was utterly wrong at #30, as said at #31, please kindly google "tidal earth crust deformation" ...
There are plenty of studies considering a fact that deformation.
 

Offline rmolnav

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Re: Why do we have two high tides a day?
« Reply #35 on: 04/06/2015 07:54:58 »
No reply in two days ... I do hope somebody could be considering my point, without seeing clearly wether I´m right or wrong.
Please kindly compare #29 with what follows. People with some knowledge on calculation of internal stresses of structure materials know that the way I dealt with the issue there is basic on that field.
Another analogy to make it clearer for people not familiar with the matter:
Imaging just a steel cylindrical bar hanging from one of its ends. If we made a horizontal cut, the lower part would fell down. Why it didn´t fall before? Because of internal tensile stresses: upper side of the section was pulling lower one, exactly with a force equal to the weight of lower part of the bar.
If we had made the cut a little lower, we could say the same. In this case the weight of lower part would be a little less: just the weight of the slice between the two cuts.
As the slice is not experiencing any acceleration, the sum of all forces applied to it is null. The sum of internal stresses it suffers from contiguous material, plus its own weight, has to be null.
If we produced any upward acceleration to the hanging point, internal stresses would increase in such a way that the sum of weight of the slice plus stresses from contiguous material would give a net force that divided by slice mass would be equal to the acceleration.
Very similar is what happens in the case of two objects rotating around a common axis. In this case each slice of one of the objects is subjected to gravitational attraction from the other object, and that force plus net forces from contiguos slices, divided by the slice mass, has to be equal to the square of angular speed multiplied by the radius (or distance), actual centripetal acceleration at rotatory movements.
Please kindly go now back to #29 ... As far as I can see, it should be clear for most people. 
   
 

Offline PmbPhy

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Re: Why do we have two high tides a day?
« Reply #36 on: 04/06/2015 16:12:04 »
Quote from: rmolnav
No reply in two days ...
That's because when we see that someone can't understand an argument and/or correctly back up there's then we usually give up. In this case you still haven't done what's required of you and that's to find an error in the derivations that are in those PDF files that I posted which are from various textbooks.
 

Offline rmolnav

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Re: Why do we have two high tides a day?
« Reply #37 on: 05/06/2015 07:36:16 »
Sorry, but what said in #35 is NOT true. F. e., THREE times I have previously asked:
"If those three zones of the Earth experience ONLY those gravitatory accelerations from the Moon, why ALL of them don´t cause a movement of the Earth towards the Moon? “
It´s you, and rest of readers, who haven´t reply that question. Do you think that question is absurd and doesn´t deserve to be answered?
Within the theory in mentioned PDF, that question could be answered saying that those forces are producing the centripetal accelerations of the rotating movement. But, as I´ve already shown in different ways, in different earth areas gravitational pull from the moon and rotatory centripetal force are NOT equal to each other ... Forces from internal stresses have to be considered.
ANOTHER error, as far as I can see:
In mentioned PDF, tidal forces are considered to be derived  from just the variation of pull from the moon with the distance. That´s why they conclude that although gravitational force decrease to the square of the distance, tidal force decrease to the third power of the distance.
Beeing honest, I have to say I can´t refute their mathematical explanation ... But I insist: stuff different from just gravitatory pull and distance have to be considered to explain tides. Such us internal stresses as I´ve already said, some of them centrifugal. And also what follows.
Even considering a bellow the moon ocean bulge, that could be cause just by pull from the moon, we could call it high tide only in comparison to what happens 90 degrees away, where low tide. But the reason is not just a difference in what they call tidal forces (smaller pull where further from the moon). Even if the pull were the same at both places, tides would happen.
Why? Because main cause of ocean surface shape is OTHER force which has also to be considered: gravitatory one from earth itself. That alone would produce a spherical shape. Adding moon pull bellow it (now I´m "forgetting" inner stresses), the result is kind of less weight of the water there. BUT 90 DEGREES AWAY from there moon´s pull doesn´t actually affect water weight, and we have low tide.
Tides are kind of a dynamic equilibrium that results from MORE forces than just pull from the moon and its variation with the distance ...
 
 

Offline rmolnav

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Re: Why do we have two high tides a day?
« Reply #38 on: 05/06/2015 10:21:03 »
A couple of things more.
Firstly, I beg your pardon for my many English errors. Perhaps they make more tiring to read my posts. It takes me long to write them, and when written I am not sufficiently patient to check them carefully before sending ...
I´m afraid PmbPhy didn´t completely read #34.

But what said there is BASIC Physics. Standard Finite Element Methods to calculate structures are based on what said there: ALL internal and external forces affecting each small finite element of any object have to be considered. If not in balance, deformations and/or movements (with acceleration) happens ...
By the way: in #34, for people not familiar with the subject, I considered the hanging bar initially without any acceleration (still, but it could also be in movement at constant speed). Afterwards with an upward acceleration produced by an artificial force applied to the hanging point: internal stresses would increase.
Needless (?) to say that if we let the bar fall down freely, internal stresses would disappear. The unique force which would be affecting each slice would be its weight, with the result of "g" acceleration there ...
The sum of ALL forces affecting ANY part of the object, and the resulting acceleration, has to match. 

 
« Last Edit: 09/06/2015 12:09:46 by evan_au »
 

Offline rmolnav

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Re: Why do we have two high tides a day?
« Reply #39 on: 07/06/2015 12:20:58 »
Two days more and … I´m going to post a “divertimento” on the issue , hoping it may be interesting to some folks.
Nature is far more complex than what seems at first sight. Ocean surface shape depends on not just a few physical phenomena.
I´ll try and list what, as far as I know, affects it, in a decreasing order of importance. I hope not to forget anything. If any disagreement, please let me know, and we can discuss the issue.
1) Gravity from earth itself. If our planet is considered alone, without any movement, without any other celestial object gravitationally affecting it, etc, sea surface “should" be spherical …
2) I say “should” because that is only in theory: gravity is not uniform over sea surface, because amount and density of inside material varies …
That produces sea local level differences much much bigger than tidal differences.
3) Let us “allow” earth daily rotation … Due to centrifugal forces, equator diameter gets bigger than distance from pole to pole.
4) If we now include moon/earth rotation, then we have main part of tides, which we have been discussing. We shouldn´t forget that local conditions (mainly size and shape of continents) produce changes on “theoretical” tides, because water is not completely free to “obey” the forces we have been discussing.
5) Now earth translation (actually rotation around the sun): it produces tides similar to moon related ones, but smaller. These don´t change with the clock. For similar reasons to lunar tides, a high tide happens “bellow” the sun, at noon, and another at antipodes, at midnight. Also with a certain gap due to the fact that the water bulges cannot catch up with their theoretical position …
Needless to say that when lunar tides syncronize with sun ones, we have the strongest actual tides. This happens when full and new moons.
6) Meteorological conditions also affect: atmospheric pressure, winds, etc.
7) I haven´t mentioned water temperature, as if it did´t change with space and time … That isn´t actually so, and the result is that some additional, but relatively small changes happen.
An additional curiosity in this respect. Local increase in temperatures of ocean upper part (f.e., at El Niño zone) are beeing detected thanks to “gravitational” Physics … The temperature increase produces there tiny bulges that are detected with sophisticated satellites (f.e., GRACE, a couple of satellites), thanks to very very tiny changes in the distance to each other, derived from gravitational changes.
The software they use must be “unbelievable”: how can they tell apart the many different gravitatory existing factors?
 

Offline PmbPhy

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Re: Why do we have two high tides a day?
« Reply #40 on: 08/06/2015 00:45:06 »
rmolnav - Who are you talking to? I lost interest in this thread myself.

I'm curious. If you're just posting to everyone then why don't you modify one thread and put everything in that thread instead of creating multiple posts?
 

Offline rmolnav

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Re: Why do we have two high tides a day?
« Reply #41 on: 08/06/2015 13:13:50 »
PmbPhy:
I´m posting ideas which intend to answer the asked question: why are there two tides a day?.  Or at least related to the issue, which could be interesting to somebody.

COLMIK (especially for you, but possibly interesting to some other folks):
In relation to something you said in #18, I´ll put another curious physical detail.
You said that so called “spring tides” (strongest ones) do not happen the day when full or new moon, but actually a few days later.
That´s certainly so, and something I had not previously found any explanation to, consistent with rest of “my” theory …
Last night I started to ruminate it in bed. I found following possible explanation.
In some previous posts I´ve mentioned the fact that moon related high tide doesn´t happen when moon is exactly over our meridian, but with some delay. That´s because the top of the tidal bulge can´t catch up with below moon meridian, due to the high linear velocity of earth rotation (40,000 km/24h at equator).
Although main component of attraction between moon and main part of the tidal bulge is vertical, a relatively tiny tangential component does actually occur. Naturally, in both senses (action and reaction principle)
I´m not going to expose now interesting consequences that fact has been having for the moon, which I even discussed some time ago with Neil F. Comins, after seeing a very interesting tv show where he appeared (relative to moon and earth early stages).
But that tangential component of mentioned attraction tries to decrease the delay. Ocean surface quickly moves eastward due to earth rotation, so tidal bulge is east to moon, but always “moving” westwards trying to reach moon meridian.
Something similar happens when considering only sun related part of tides. At noon the sun is over our meridian, and high sun related tide happens. Actually some time later, for reasons similar to what said for moon tides. But now, being the sun much far away, the tangential component of the sun pull on the main part of the bulge must be almost negligible. Subsequently, the angular gap between bulge and sun must be bigger than moon related one.
A couple of days after new moon, the two bulges are on same meridian. Sun is then west to the moon, but the higher delay of its related bulge puts it  same place than moon related bulge. They fully add up and tide coefficient reaches then a maximum.
Any comments would be wellcomed.
« Last Edit: 09/06/2015 12:06:50 by evan_au »
 

Offline rmolnav

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Re: Why do we have two high tides a day?
« Reply #42 on: 13/06/2015 10:34:48 »
Looking for some information relative to local solid earth effects on ocean tides, I´ve seen in wikipedia an article titled "Tide".
There the term "centrifugal force" is also kind of forbidden ... But there is a paragraph which has "open my eyes", in a way consistent with "my" theory.
As far as I can see, denial of the existence of any real centrifugal force is a physical error (as I´ve previously explained), which requires a mathematical "trick" to reach an explanation of tides.
Mentioned paragraph says:
"The tidal force produced by a massive object (Moon, hereafter) on a small particle located on or in an extensive body (Earth, hereafter) is the vector difference between the gravitational force exerted by the Moon on the particle, and the gravitational force that would be exerted on the particle if it were located at the Earth's center of mass".
A DIFFERENCE OF A REAL VECTOR (MOON ATRACTION ON A PARTICLE), AND THAT ATRACTION "IF" THE PARTICLE WERE LOCATED SOMEWHERE ELSE IS A PHYSICAL NONSENSE. Sorry. For me it is just a mathematical trick.
That mention of the "Earth´s center of mass" reminds me what I said that on a slice there located actual Moon attraction and rotational movement centripetal force are in balance. But not in the rest, and internal stresses appear. Each slice is supporting its own weight, Moon attraction on it, and internal forces (stresses multiplied by surfaces) from contiguos slices, many of them CENTRIFUGAL.
A dynamic equilibrium is reached on EACH particle if the result of adding up all those vectors divided by the mass of the particle is the actual rotational centripetal acceleration where considered particle is located. Any imbalance would produce internal stress changes and/or additional movement of the particle. 

 

Offline rmolnav

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Re: Why do we have two high tides a day?
« Reply #43 on: 14/06/2015 22:08:06 »
Perhaps yesterday I was kind of too assertive when saying what quoted from wikipedia was a “physical nonsense”. I´ll try to explain why I said so.
I do understand a comparison between real location of an earth particle and “if” it were located at earth center of gravity can be made. And a term as “tidal force” or something similar could be defined in relation to that comparison …
After all, that´s similar to what I said that if all mass of a rotating object were at its center of gravity the gravitatory pull from the other object would be equal to rotation centripetal acceleration multiplied by its mass, the centripetal force. And in that case there wouldn´t be any centrifugal force (neither a tide whatsoever). BUT objects do have a size, and then …
Following the idea of the quote, I would say something such as:
“Tidal effect (on a given particle) is the difference between net forces supported by the particle in two scenarios: the real one, and another fictitious with the particle situated at the center of gravity of the earth”.
Considering just differences between moon pull vectors is NOT sufficient. How could the particle physically “feel” that difference? How could it “know” it is in a location away from center of gravity?
The two scenarios differ more than just that:
- The particle at earth center of gravity has a rotational movement in balance: net internal force affecting the particle would be null. But at its actual location that balance doesn´t exist, and the sum of forces affecting the particle, due to internal stresses (centripetal, centrifugal and others) is not null, in general.
- Particle own weight is real at its actual location, but at earth center of gravity would be null.     
In real scenario, the particle does “feel” moon gravitatory pull, also its own weight, and stresses from contiguous particles, and does “know” how to react: with actual centripetal rotational acceleration, which has to be the same across the earth. And, if not in that dynamic equilibrium, with additional movement and/or deformation (which would affect back internal stresses …).
That´s why tides happen, included the usual two high tides a day.
Another important detail. In some posts i have said something like “radius or distance”, in relation to the rotation of a solid object around earth, as artificial satellite. That could also be correct if considering earth rotation around the sun. But in moon related tides, as both moon and earth rotate around an axis distant from earth center less than earth radius, the radius of that earth rotation is much much smaller than the distance to the other object of the couple, the moon. Pull from the moon is inversely proportional to the square of that distance (for different parts of the earth), but centripetal force required for the rotation, for actual angular speed (360º/plus 27 days), is proportional to the rotation radius (for each earth particle). Differences between different parts of the earth are relatively much much higher in centripetal accelerations (causing earth rotation around barycentral axis), than in moon pulls.
That produces higher imbalances, and higher internal stresses, included centrifugal ones, than if we were considering just distance to the moon differences.
Curious enough, earth part closest to moon is at same side of rotation axis than moon: moon pull (causing that earth rotation) is there in the sense opposite to centripetal acceleration …
That produces centrifugal forces similar to what happens with earth daily normal rotation. But in an asymmetrical way, and much much slower.
That´s why for me the second moon related high tide, at opposite side to moon, is mainly due to centrifugal force. 
 

Offline rmolnav

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Re: Why do we have two high tides a day?
« Reply #44 on: 17/06/2015 10:10:57 »
Just a short post to put another argument/evidence supporting "my" theory ... as far as I can see.
It is well known that Io, one of the satellites of Jupiter, is internally really hot, even with strong vulcanism that most recent, sophisticated studies say it is due to internal tide friction.
Could all that energy come just from differences across Io in the gravitational pull from Jupiter, being Io´s diameter only 3,643.2 km? For me, NO.
It is true that Jupiter´s pull is really big, but the radius of Io´s rotation around Jupiter is similar to our Moon´s. Relative gravitational differences cannot be big enough to produce such friction.
It is close to Jupiter, and that´s why it needs to rotate along its orbit really quickly, in less than a couple of our days.
Such a high angular speed produces really high imbalances between centripetal acceleration and gravitational pull at different locations. That produces really high internal stresses, and deformation and movements happen quickly, with the result of really high friction.
In the universe there have been satellites which even broke/exploded due to really high internal, tidal stresses. It would be unthinkable that were due just to differences in gravitational pull, without the existence of the imbalances and the internal forces (centrifugal ones included) I´ve been referring to. 
 

Offline evan_au

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Re: Why do we have two high tides a day?
« Reply #45 on: 18/06/2015 11:22:23 »
Quote from: rmolnav
In the universe there have been satellites which even broke/exploded due to really high internal, tidal stresses.
The Roche limit is where the gravitational tug of a massive body will disrupt a smaller body that is held together by gravitational forces. Effectively, the tidal forces tear the smaller body apart.

This applies to comets which are "rubble piles" held together only by gravity. However, bodies which are made of solid rock or solid iron will survive these tidal forces at a closer radius than the Roche limit.
 

Offline rmolnav

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Re: Why do we have two high tides a day?
« Reply #46 on: 18/06/2015 12:05:33 »
Thank you, evan_au. I was just drafting something more, but relative to what I said about Io ...
Being honest, I have to say something DOESN´T MATCH.
In relation to what I said about Io, after sending my last post I remembered several times I had previously thought:
"How odd that huge energy dissipation from Io! If tidal friction is so high, why Io has´t got synchronized with Jupiter yet, as our Moon is with Earth? Io must be very young …"
Because tidal friction happens within Earth thanks to its daily rotation. The two normal bulges are continuosly changing location, internal stresses produce deformation and additional movements, and friction occurs. But there is no tidal friction in the Moon ... Its shape has a couple of relatively small bulges, but at fixed locations. It was precisely tidal friction, happening there long long ago, which slowly synchronized it with Earth (in the sense that it is still rotating around its axis, but exactly 360º in same period of its rotation around Earth).
I said to myself: let us google Io ... And in wikipedia I found that Io is already synchronized with Jupiter !!!
Then, as far as I can see, Io huge friction cannot come from tides, not in “my” model (including internal centrifugal forces), let alone if we consider just local differences in gravitational pull ...
An explanation is given in the article, rather odd for me.
They say it is due to "Laplace resonance". The periods of rotation of Io (the closest) and the two other Jupiter´s satellites (Europa and Ganymede) happen to be proportional to 2,3 and 4 … Periodically they get in line with Jupiter … Well, anyone interested can have a look in the web.
But small celestial objects such as Europa and Ganymede, especially compared to Jupiter, “shoudn,t” affect Io that much. When any of them in line with Io and Jupiter, outer Io bulges must get a little bigger, some deformation happens, and some heat is released … But, can that be sufficient to feed Io´s huge volcanic activity?
Really strange for me!
Any comments would be appreciated.   

 
 

Offline rmolnav

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Re: Why do we have two high tides a day?
« Reply #47 on: 18/06/2015 12:17:14 »
Sorry. I said "outer Io bulges must get a little bigger ...". I mean both bulges: if an (or two) outer satellite pull outward, Jupiter has to compensate increasing its inward pull ... Otherwise Io would get out of orbit ... Well, that is kind of simplification. Those phenomena must be really complex.
 

Offline alancalverd

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Re: Why do we have two high tides a day?
« Reply #48 on: 18/06/2015 16:13:01 »
Who "we"? Southampton has four high tides per day, some Mediterranean ports have almost no tidal range at all.
 

Offline rmolnav

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Re: Why do we have two high tides a day?
« Reply #49 on: 20/06/2015 12:12:23 »
alancalverd, #46
For me, your second question is easier to answer than the first. Firstly I´ll put my answer to the second, and make just a small comment/guess about the first afterwards.
For the sake of simplification, let us consider only the pull from the Moon, and suppose that Moon´s orbit and our equator´s plane were the same. And in open sea, not to have local influences (and not far from the equator)
If we have the Moon over us, we have high tide. Moon´s pull is in opposite direction to water weight, actual (or apparent) density of water decreases (though very, very slightly), and sea surface get deformed due to that. Low tides are 90º away from high tide location. There Moon pull vector and water weight are perpendicular to each other, and Moon´s pull don´t affect water density at all. In intermediate locations, something also intermediate happens.
From west to east ends of Mediterranean sea there are less than 4,000 km. When Moon is over one end, the above mentioned effect there and at the other end are not too different, and sea surface deformation is small.
Regarding second question, my guess is related to the fact that, you know, we all actually have FOUR high tides: two related to the Moon (below it and at antipodes), and two related to the Sun (at noon and midnight, with some delay I commented about in previous posts). But we only see the result of adding up Moon and Sun effects …
Perhaps in Southhampton, due to its latitude, some local conditions, and the fact that apparent orbits of Moon and Sun in our skies are different, at least in some periods of the year Sun related high tides are not “hidden" by Moon related tides …
That could match with four not uniformly distributed over the day high tides: two at fixed time (1 or 2 hours after noon and midnight), related to the Sun, and the two related to the Moon changing with its location in local skies … And not equally distinguishable over the year, due to changes in apparent orbits of Moon and Sun …
Has Southhampton annual cycle of tides such kind of characteristics?
If not, I can´t guess any other possible explanation ...
 

The Naked Scientists Forum

Re: Why do we have two high tides a day?
« Reply #49 on: 20/06/2015 12:12:23 »

 

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