# Moon gravity and tidal water.?

Hey. So I just had a thought. The event of tidal water is because the gravital forces of the moon and the sun pulls the water in the direction of the celestial body. My question is, how come only the water (maybe wind too?) is affected by the gravital pull? Why aren't objects lifting from the ground (lol) when the force apparently is enough to move million tons of mass?

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• 2 months ago

Truth is, everything with mass is affected by lunar gravity, just that the amount is tiny compared to earth gravity. It is measurable. Proven with mass and strain gauges.

Even you think you are standing perpendicular to earth, but the moon off to your left or right is also tugging you a bit, just not enough to notice.

• 2 months ago

There ate sold earth tides as well, but the acceleration of gravity from the Moon and the Sun is not great enough to make anything with a lot of mass start to float because Earth is so much closer. Inverse square of the distance law. Gravity gets weaker with the square of the distance. The Sun is 400 time the distance from Earth that he Moon is, but despite that making the Sun:s gravity160,000 times weaker, the Su 's mass is so much greater that up to 47% of the tides ard because of the Sun's mass.  In addition, the gravity from the Moon and the Sun is acting in DIFFERENT directions most of the time except around new and full Moon, so the pulls partially cancel each other.

https://en.m.wikipedia.org/wiki/Earth_tide

There is a tendency for earthquakes to occur AROUND the new and full Moon dates (the October, 1989 earthquake in San Francisco was near a new Moon, and the March 11, 2011eartgquake in Japan was near super full  Moon), but the data gathered so far is inconclusive. If I had gone on to get a Ph.D. in planetary geology, this probably would have been my research and dissertation topic . I would not have been the first graduate student to do this type of research.

All those thumb downs just confirm. I am correct.

• 2 months ago

In actuality, land can raise and fall by about a foot in some places due to tidal forces. While it affects *us* in a similar manner, Individual objects - like you, me, the book on your shelf - can also be affected, but the difference in gravitational pull from one end of a book to the other is so minor, as to be negligible.

• 2 months ago

The moon's gravity pulls on the ground just as much as on objects on it. The rocks and soil that make up Earth aren't immune from gravity.

Tidal forces are caused because the side of Earth nearest to the moon is pulled slightly harder than the side furthest away. This stress tries to pull the Earth apart. But atomic bonds hold the rocks together. Water is liquid and free to flow, so it gathers where the moon's gravity is strongest.

• oyubir
Lv 6
2 months ago

Because your conception of tidal forces is false.

If it were true, btw, the gravity from the Sun is 178 times bigger than the one from the Moon. Yet, you know that Moon is the principal responsible for tidal forces.

And the difference come precisely from the fact that gravity from this tow bodies affect both water and ground. And everything. Gravity from the Sun is the reason why Earth orbit the Sun. Earth, including everything that is on Earth. Including water. We are all in a vehicle that is affected by the gravity.

So, in that vehicle referential, we don't feel it. Exactly like the astronauts in ISS are in a vehicle whose movement comes from Earth gravity. So they don't feel it.

Another common misconception: it is not because they are in space, far from Earth, outside atmosphere, etc. that astronauts of the ISS feel 0 gravity. Gravity were they are is almost the same as ours, on the ground. It is because they are in a vehicle that is constantly free falling in this gravity field. They float in a 0g environment exactly as you would to, for a brief moment, in a falling elevator.

So, we (and the water), on Earth, are in the exact same situation: we are submitted to Moon and Sun gravity. But we don't feel it, because we are in a vehicle (The Earth) free falling in those gravity fields.

So, no, tides are not the water lifted by gravity.

What create tides is the slight difference between my analogies (you in a falling elevator, astronaut in the ISS) and the case of water on the Earth surface: the size of the vehicle.

The falling elevator, or the ISS are small enough so that you can consider the gravity identical in the whole vehicle. Theoretically, the top of the elevator is further from the center of the Earth than the bottom of the elevator. So gravity is not the exact same 9.81 m/s² in the whole elevator. But practically, the difference is ridiculously small.

It is not the same for Earth. Center of the Earth is not at the same distance from the Moon than the surface of the Earth. And that difference (6700 kms, if the Moon is at the zenith were you live) is small, but not negligible, compared to the distance of the Moon (384400 kms)

And Earth movement in space is the result of the application of gravity (from Sun and Moon) at Earth center of gravity. So, when you are at the surface of the Earth, there is a slight difference between the gravity that applies on you, and the gravity that applies onto your vehicle (the Earth). That is that slight difference that you can feel.

This explain 3 things that you could not explain with the popular misconception "sea is attracted upward by the Moon".

1) Why tides are more influenced by Moon than by Sun.

Because gravity from

Msun*G / Dearth-sun² = 1.989×10³⁰ *  6.67408×10¹¹ / (1.496×10⁹)² =

0.00593147909577054

And gravity from Moon is

Mmoon*G / Dearth-moon² = 3.3187616751541113 × 10⁻⁵

Obviously gravity from the sun is bigger

But if you look at the difference between gravity from the Sun at the center, and the one at the surface (h being earth diameter), it is at most

Msun*G/Dearth-sun² - Msun*G/(Dearth-sun+h)² =

Msun*G/Dearth-sun² * (1 - 1/(1+h/Dearth-sun)²) =

Msun*G/Dearth-sun² * (1 - (1-2h/Dearth-sun)) [first order dev] =

Msun*G/Dearth-sun³ * 2h/Dearth-sun =

2 Msun*G*h / Dearth-sun³ = 5.05×10⁻⁷

While the same computation with Moon leads to

2*Mmoon*G*h/Dearth-moon³ = 1.10×10⁻⁶

So, while gravity from the moon is smaller than the one from the Sun, the difference between gravity from the moon at the center of the earth and gravity from the moon at the surface of the earth, if bigger than the difference between gravity from the sun at the center of the earth and gravity from the sun at the surface of the earth (yes, this big sentence make sense, and is not even hard to understand if you try ;))

Which makes sense: sun is so far from the earth (150 millions kms) that 6700 kms more of less doesn't change a lot its gravity.

Moon is closer (only 384000 kms) that 6700 kms more or less shows more.

2) Why there are two tides a day, not one.

If what count were directly the gravity, tide should be high were the moon is at zenith, and lowest at the opposite point of the Earth. So, roughly, there should be only one high tide a day, when the moon is at is highest point. Tho, if you live at the beach, you know that there are two high tides a day. One when the moon is high, another some 13 hours after.

Why: because, again, it is not gravity that count. But difference between gravity that the Earth undergoes (the one at its center of gravity, roughly) and gravity that the water undergoes (the one were you are). Under the moon, gravity from the moon is higher (since you are closer to the moon. And is upward.

At the opposite point, gravity is downward (moon is under your feet) but also smaller than the one undergone by Earth, since you are further from the Moon than the Earth (its center). So less downward gravity means water is "less heavy that it should be".

Hence a high tide here too.

So there are two areas where tide is at its highest point, on Earth: the area that is right under the Moon (the place on earth where Moon is just above your head), and the opposite one (the place where Moon is just under your feet).

Hence two high tides a day, not 1.

3) Your very question: why the beach is not lifted with the sea, and so, why can we see tides.

Because the ground is part of a solid object, and that solid object is submitted to the gravity that applies to its center of mass. While the water is liquid.

Note that water is not really lifted by gravity. The gravity water is submitted to is still downward, even when the moon is just over your head. And roughly the same 9.81 m/s².

It is not like gravity from the moon were pulling water upward. Water is, whatever, pulled downward by earth gravity.

It is the variations, of the gravity that change the shape of water.

You can try this at home: find a rotating plate and put a bowl ow water on it: when it rotates, water surface of the bowl is no longer plane, but parabolic (surface is higher on the side of the bowl, and lower at the center -  it is even a method to build parabolic miror).

Yet nothing explains that water is lifted on the sides. There is no reason for that.

What happens when the bowl is rotating, is that, in addition to the downward gravity (9.81 m/s²) an horizontal centrifugal force appears. It is null at the center of the bown, and highest at the side. But it is not at all upward. So it can't explain that water is "lifted" at the side of the bowl. If anything, apparent gravity is higher, not lower, at the sides of the bowl.

What explains why water is apparently lifted at the sides of the bowl is not the magnitude of the gravity, but the change of direction: the surface of a liquid has do be perpendicular to the direction of the gravity. The surface that does that in the case of a bowl submitted to a vertical gravity + centrifugal forces from its center, is a paraboloid.

So the water in the bowl has to follow this shape. And since water is not compressible, there is no choice: for water to follow this shape, water level has to be under average at the center, and over average on the side. Average being the flat water level when bowl is not rotating.

Hence the reason why rotating the bowl, while not at all diminishing gravity on the sides of the bown, "lift" water there.

tl;dr: Earth is submitted to the same gravity than the water on it. So it is not that gravity that count (exactly like gravity that an astronaut undergoes when in ISS doesn't count, because ISS is submitted to the same gravity). What count is the slight difference between the gravity at a given point of the surface of the Earth, and the gravity at the center of the Earth.

And even once that said, it is not the magnitude of this gravity difference that counts, but its direction: water surface has do be normal to the gravity field. And because of this slide difference, this gravity field normal is not exactly a sphere.

Hence the reason why

1) Tides are more influenced by Moon than by Sun (gravity from the Sun is bigger. But gravity gradient from the Moon is bigger)

2) There are 2 tides a day (there are two points were that difference leads to a vertical "upper than average" gravity)

3) Your question: water is deformable and its surface is normal to the gravity field. Earth is not deformable (or less so) so what count for earth is gravity at its center.

• Retief
Lv 7
2 months ago

No, tides are because the water on the near and the farther parts of the Earth from the moon are affected by differing amounts by gravity.

• Anonymous
2 months ago

it affects everything, solid,liquid or gas

the actual force applied is very small, but, it acts each individual molecule considered separately

with solid objects like your car or a goldfish, the individual molecules are bound up together.  tidal forces generally can't overcome the forces binding those molecules to each other, and there's no noticeable, cumulative effect on their movements

but with water, the molecules are only loosely connected, and easily slide past, around each other, and wander around freely in general.

so tidal forces, acting on each water molecule individually, can influence their movements enough so that, cumulatively, over vast areas of ocean, they can respond to that tidal force by piling up a little bit in the direction of force (and also in opposite direction, on the other side of the world of course...)

• Jonatan2 months agoReport

Good explanation. Thanks

• roger
Lv 7
2 months ago

Because the  pull from the sun and moon pulls objects sideways  (not up and down).  Only the water is free to move.  The  tides are caused by the  sideways resonant  sloshing  of the water in  harbors and bays.  There may be  some effect on the wind  but that would be hard to  measure  and the  wind  does not  "slosh".   Think  about it. If the    sun and  moon  only pulled up and  down then the tide would be the same everywhere....