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PLANET1 WATER PLANET2 ?

 

Well planet 1 would attract the water and planet 2, the water would attract both planets and planet 2 would attract the water and planet 1. So everything would move towards the middle, but there would also be some friction from the water...

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I could imagine the question was going into the direction of "why does <something that swims> swim on water rather than sinking in due to gravitational attraction?".

 

In that case what he wants to know about is buoyancy. An object in a fluid will weigh less by an amount equal to the weight of the fluid it displaces.

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To explain a bit more, the reason for that is because the water weighs more per unit volume (has a higher density) than what is floating in it, so the gravity is pulling on the water more than the object, and the object gets pushed out of the way and up. This is called "positive buoyancy." If you have the exact same density as water, you have "neutral buoyancy," and you don't get pushed either up or down, almost as if you were "weightless," even though you aren't.

 

In fact, human beings have a density very close to water, which is why you can barely float if you fill your lungs with air, and barely sink if you empty them. The expanded lungs increase your volume without increasing your weight much, so your overall density decreases. (A submarine does basically the same thing by filling and emptying tanks of water). Also, this can vary from person to person. Muscle is more dense than fat, so fat people can float with much less effort than thin people.

 

Other facts:

 

Things with positive buoyancy (a boat, for example) will end up floating on the surface, where it displaces (pushes out of the way) exactly as much water as the boat weighs. In other words, the amount of water it would take to fill the "hole in the water" that the boat makes would weigh the same as the boat.

 

Buoyancy works in all mediums, not just water. A helium balloon, for example, still has gravity pulling it down, but it is less dense than the air around it, and so that air pushes it up out of the way, and the balloon floats. If there were no air, the balloon would fall just as fast as a rock.

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