Thread: Water

The tidal force comes from the difference in gravity from one part of a body being close to the source of gravity and the other part being farther away. It just so happens the the Earth is large enough that the force of the moon's gravity on the near side is noticeably stronger than on the far side (actually it's more complicated but that is the general idea).

But you're only about 5 or 6 feet tall, so the gravity at your feet is about the same as the gravity at your head. In fact, if anything, the tidal force of the Earth would be the one affecting you, not the moon. Now, if you were 1000 km tall, you would feel something.

Expecially if its a full moon?

Is the moon closest to the earth during full moon?

Um... No. It is just all illuminated by the sun. Duh.

So then the gravity would be strongest when it's closest, not necessarily when it's full. This may be an indication of two effects of the moon.

The effect of gravity and the effect of the sun's reflected light off the moon.

Maybe position of the Earth, Moon and Sun relative to each other all factor into the effect of lunar light. I'm pretty sure the position of the Earth to the Sun affects the effect of solar light and heat.

Which makes me wonder about effect the heat collected by the moon's surface may have. I hypothesize its effect is insignificant.

But the effect on water, I can't say. I think water may have some undiscovered properties. It's such a unique substance.

The light from the moon doesn't affect the tides.

Maybe it has other effects. Iunno.

Sure. But we're talking about the tides.

Compared to the light from the sun, the light of a full moon is tiny.

Light does actually exert a force but it has nowhere near the effect of gravitation.

Actually, during a full moon the sun, Earth and moon are roughly lined up, and that does increase the tidal force very slightly.

Thanks every one for their very helpful input. Im really glad I got some replies. I liked all your explanations and coments.
Thanks again.

Thanks everyone!

The moon effects the whole planet, not just water. Tidal forces effect the shape of the planet, its just that those effects are most visible over water because it is a fluid. The shape of the earth is actually ellipsoidal, not spherical, and this is because of the tidal force.

I thought it was because of the greater centripetal force required at the equator.

Originally Posted by Xaqaria

The shape of the earth is actually ellipsoidal, not spherical, and this is because of the tidal force.

Wrong. The Earth is ellipsoidal because it rotates.

Originally Posted by drewmandan

Wrong. The Earth is ellipsoidal because it rotates.

I'm pretty sure that the ellipsoidal shape are due to the tidal forces and the moon's gravity.
The earth is Rotating in a weightless environment so with out the moon the gravitaional pull would probalby not be enough to make the earth eliptical would it?

Originally Posted by Howie

I'm pretty sure that the ellipsoidal shape are due to the tidal forces and the moon's gravity.
The earth is Rotating in a weightless environment so with out the moon the gravitaional pull would probalby not be enough to make the earth eliptical would it?

There's so much fail in this post that I'm not going to bother addressing the numerous misunderstandings of basic physics.
http://en.wikipedia.org/wiki/Earth#Shape

I'm pretty sure that the ellipsoidal shape are due to the tidal forces and the moon's gravity.
The earth is Rotating in a weightless environment so with out the moon the gravitaional pull would probalby not be enough to make the earth eliptical would it?

If there was a significantly stronger pull on one side of the Earth than the other, you'd expect an egg shape really (and catastrophic tectonic activity as a result of the rotation).

I'm not sure what you mean by weightless environment.

Originally Posted by Xei

If there was a significantly stronger pull on one side of the Earth than the other, you'd expect an egg shape really (and catastrophic tectonic activity as a result of the rotation).

I'm not sure what you mean by weightless environment.

Actually, the tidal force causes oceans (and everything else) to bulge on both sides. Why? Because on the near side, the oceans feel stronger gravity than the ocean floor, so they bulge towards the moon (up). And on the far side, the ocean floor feels stronger gravity than the oceans, so the floor pulls away (down), leaving the ocean itself bulged up.

Ohhh okay. I could never find a good explanation of why there's two tides.

But yeah, the Earth is quite ellipsoid due to its rotation; however it is uniform. If the moon caused it, then the shape would be elliptical from a birdseye view... I'm thinking the thermal energy generated by the constant changing of the Earth's shape with its rotation would melt it. Isn't one of Jupiter's moon's similar?

The net result of this is that the Earth gets deformed into a slightly squashed, ellipsoidal shape due to these tidal forces. This happens because if we resolve the tidal forces at each point on the Earth into a local vertical and horizontal component, the horizontal components are not zero, and are directed towards the two points along the line connecting the Earth and the Moon's centers. These horizontal forces cause rock and water to feel a gravitational force which results in the flow of rock and water into the 'tidal bulges'. There will be exactly two of these bulges. At exactly the positions of the tidal bulges where the Moon is at the zenith and at the nadir positions, there are no horizontal tidal forces and the flow stops. The water gets piled up, and the only effect is to slightly lower the weight of the water along the vertical direction.

Another way of thinking about this is that the gravitational force of the Moon causes the Earth to accelerate slightly towards the Moon causing the water to get pulled towards the Moon faster than the solid rock on the side nearest the Moon. On the far side, the solid Earth 'leaves behind' some of the water which is not as strongly accelerated towards the Moon as the Earth is. This produces the bulge on the 'back side' of the Earth.
http://www.astronomycafe.net/qadir/q380.html

Xei, What I meant was isn't the earth and moon in weightless space? I realize that there are many other forces of gravity that are existant, but I did not think it was a relevant eough amount to make the earth oblong.
But not all of us have the basic understading of physics down as drewmandan apparently does.

I'm not sure what you could mean by weightless space... we normally completely ignore space when talking about gravitation. Weight is a force of attraction between two masses. Space doesn't have mass. We're only considering the force of attraction between the Earth and the Moon.

Originally Posted by Xei

I'm not sure what you could mean by weightless space... we normally completely ignore space when talking about gravitation. Weight is a force of attraction between two masses. Space doesn't have mass. We're only considering the force of attraction between the Earth and the Moon.

Then if that is the case just the rotation of the earth would make the earth ellipsoidal because of it's mass?

Wrong. The Earth is ellipsoidal because it rotates.

I guess what I'm envisioning is the weightlessness that accompanies…. let’s say an astronaut in space. Their mass and weight do not have the same effects as it does in our atmosphere.
Does that have any relevance?

The explanation for rotation causing a bulging is that that if you add up the various force vectors at the surface of a rotating body near the equator, you will see that they don't add up properly when you add in the centrifugal force. The only way to balance them is to shift the normal force, meaning the shape of the body changes to a more ellipsoidal shape.

From the outside perspective, the only way to get a cetripetal force pointing in is to have g and N not parallel, and this forces a bulged shape.

Originally Posted by drewmandan

The explanation for rotation causing a bulging is that that if you add up the various force vectors at the surface of a rotating body near the equator, you will see that they don't add up properly when you add in the centrifugal force. The only way to balance them is to shift the normal force, meaning the shape of the body changes to a more ellipsoidal shape.

From the outside perspective, the only way to get a cetripetal force pointing in is to have g and N not parallel, and this forces a bulged shape.

Ok thanks!