# Earth's movement

## Main Question or Discussion Point

The earth is moving at a great speed about its own axis. Something inside a moving sphere also moves. Why don't we move with the earth? plz explain me.

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I'm not sure what you intended to ask. We are moving with the Earth.

sorry for not making my question precise. yes we are also moving with the earth. actually i want to know how can we stand inside the earth. How can we walk , do everything so comfortably inside a moving sphere (i.e. earth) as if it(earth) is stationary

russ_watters
Mentor
We can stand on the outside (not the inside) of the earth because we are moving with it. Newton's first law says that an object in motion stays in motion unless acted upon by an outside force. We move with the earth unless a force (say, a rocket engine) propells us in a different direction.

There is one caveat, of course: with rotation, there is a continuous change in direction (an acceleration). For us, it is caused by the same gravitational force that makes your feet hurt if you stand up for to long.

Exactly. If the Earth suddenly stopped moving it would send us flying into space. The same way if you hit the breaks on your car really hard it will send you into the front windshield. We walk on Earth the same way we walk on a moving train.

The earth travels 2.5 millions kilometers every day. Thats more than a hundred thousand km/hr. Now, if you were on the equator, your speed, relative to the center of the Earth (not counting the hundred thousand miles we're already going at) is 1670km/hr. You can imagine, then, that, disregarding the speed of the solar system and the speed we gain caused by the accelerating universe is 2.5 million +/- 1670 km/hr. Thats fast, so why aren't we flying off? my_wan and russ_watters are right: the linear momentum is conserved. As our solar system formed, we (or the atoms we are made of) slowly gained speed from gravity to where we are now. You can imagine conservation of linear momentum through http://upload.wikimedia.org/wikipedia/commons/d/d3/Newtons_cradle_animation_book_2.gif/200px-Newtons_cradle_animation_book_2.gif" [Broken]. The momentum is transfered in the collision of the balls. The same rules keep us from flying off the globe.

Now, you might be imagining a tennis ball with sand on it. If you sped the ball up, the sand would fly off, no matter how slowly you sped it up. There are two reasons this did not happen to us. First, there is air resistance in space, but there is if you imagine throwing the ball on Earth. If you sped up the tennis ball and the sand in space, it would work fine, unless you started rotating it too fast. The reason has to do with gravity. The tennis ball to sand particle mass ratio is a lot smaller than the Earth to human ratio, since the tennis ball is hollow. Since it does not have as much mass, it does not have as much gravity keeping the sand from flying off.

Think of gravity as a rubber band and ball. The rubber band is really strong, but stretches as you swing it around your head. If we were spinning fast enough around the earth, the rubber band of gravity would stretch, and we would be flying high above the atmosphere, but still going around the Earth (unless we sped up too fast, and achieved escape velocity, in which case we would go floating off forever, without coming back to earth.) (note: this is assuming the earth is sped up on its own accord, not through some external force) This concept is known as the conservation of angular momentum, and has a lot of parallels with linear momentum.

If you have any more questions, or are confused, feel free to say so!

Alexandre Colavin

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now i understood thatfact, thanks.. well i have another question. what about the water of the ocean? Why can't this water flow and drop outside the earth? my friend replied me due to gravitation it can't drop outside the earth. is he correct?

You have heard of the tides, thats the gravitational effect of the sun & moon on the ocean's water to create waves & changes in water levels.

For example the water level in a well is much lower in a moonless sky, but a lot higher when moon is present...

Thats all the effect which can be caused by the external heavenly bodies to attract things in order to move outside earth.

There is also something called escape velocity. As long as your speed doesn't reach the escape velocity limit you cannot escape earth's gravitational force...

I hope I've been a help...

For example the water level in a well is much lower in a moonless sky, but a lot higher when moon is present...
That means you want to say that in a well, the moon will rise the level of the water as it does to the ocean water.
Now, If i place a full glass of water in a moonlit night, then according to you the moon will attract the glass of water and increase its level in the glass and cause the overflow of water. Is it correct?

Yes but your glass has to be almost very full to overflow, due to the total amount of water which is quite a little compared to the well... Its difficult to observe this effect with a glass full of water but yet not impossible.

Yes but your glass has to be almost very full to overflow, due to the total amount of water which is quite a little compared to the well... Its difficult to observe this effect with a glass full of water but yet not impossible.
Actually, I think this is not a true statement. The force of gravity from the moon felt by the center of mass of the cup, with the average force felt by every molecule in the cup, would negate each other. You could argue that water tension could allow for water to be closer to the moon, but strictly speaking, water would not gain volume out of the cup and leave vacuum at the bottom of the cup.

The tides are actually caused because the water is a non-rigid attachment to the rotating Earth. Imagine a cup of water with a rotating cylinder down its length in the center inside. If the cylinder has rotational W, then eventually through friction between the water and cylinder, the water will gain rotational speed to W too. Now this is what is happening with the Earth-water system. If you add the moon to the system, you have to consider gravity.

The reason your cup of water wont flow over is because it is all on one side of the Earth. All molecules in your cup feel the same gravity from the moon, and the cup as well, so it will all get pulled the same. If your cup stretched around the world, then you would see a difference in elevation from end to end.

You might have heard that the earth bulges at its center because of the gravity from the moon rotating about it. This makes the earth not quite sphere like - the term is 'oblate spheroid'. The same thing happens with the water on earth - except there is one big difference between the Earth and the water - the water is not rigid! So water closer to the moon will move more quickly to the moon, and water on the other side of the Earth will feel less attraction to the moon. This is why we have two tides every 24 hours, instead of one - half of the water is moving faster than the Earth, and half of it is moving slower, and after 12 hours, the opposite happens.

An interesting note: Because the water is rotating at a slower rate than the Earth because of the pull from gravity causing the tides, the rotation of the Earth slows down. This change is very very slow, but it is interesting to think about. The phenomenon is called tidal friction, and is also the reason that the moon is always facing towards us, and that it is slowly going away from us.

Alexandre Colavin

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think about you and your friend is in a train.. the train is moving.. are you moving?? or is your friend moving??

First of all I would like to welcome you to PF community!

But I don't really believe in some of your statements, maybe just a misunderstanding... but further comments of yours will help :)

Actually, I think this is not a true statement. The force of gravity from the moon felt by the center of mass of the cup, with the average force felt by every molecule in the cup, would negate each other.
Solids & liquids are not the same... I think you mean the force acting on the cup & water inside the cup will cause the same effect on both objects. if so, as far as I know you are a little wrong... The force between solid molecules is much much much stronger compared to the liquids, hence liquid will overflow but no specific effect on the solid.

but strictly speaking, water would not gain volume out of the cup and leave vacuum at the bottom of the cup.
There wont be any sort of vacuum at the bottom of the glass, as the bottom of the well is not vacuum...

The tides are actually caused because the water is a non-rigid attachment to the rotating Earth. Imagine a cup of water with a rotating cylinder down its length in the center inside. If the cylinder has rotational W, then eventually through friction between the water and cylinder, the water will gain rotational speed to W too. Now this is what is happening with the Earth-water system. If you add the moon to the system, you have to consider gravity.
About non-rigid attachment, so is the water in the cup...

If your statements are true I think there will be one unsolved question; Why does the water level in the well rise even higher during an eclipse?

All the best.

russ_watters
Mentor
If your statements are true I think there will be one unsolved question; Why does the water level in the well rise even higher during an eclipse?
The water level in a well rises and falls with the tides because underwater aquifers are essentially flowing bodies of water.

Regarding the water in the glass, tidal forces would affect it just like it would affect the earth, but since the glass is small, the tidal force in it is also very small. It should, however, cause a bulge at the top of the glass, in the center - which wouldn't cause the glass to overflow, since it would lower the water level at the sides of the glass.