[Physics] Does gravity cause Archimedes’ principle and how

buoyancydensityfluid-staticsnewtonian-gravity

Why do lighter objects float and denser sink? I understand this from the perspective that if the object can displace the equal mass of water it will float, but I wonder from the perspective of gravity! How does gravity cause Archimedes' principle? It must be gravity, right, because in space Archimedes' principle doesn't work!

The ultimate question I need to answer is how all this force interplay causes density stratification. For example, how does gravity cause Earth to have a density gradient: the densest elements in the core, and the lightest in the crust?

Here's what I got from the comments so far. I though it should be a good starting point if someone wants to write an answer. It also can be totally wrong.

There is a pressure gradient inside a body of liquid along the depth gradient. It is caused by the fact that the distance between two objects is squared and inversely proportional to the gravitational pull (see the image below). The deeper water thus is attracted to the Earth stronger, and where there is a pressure difference, there is a force, the buoyant force in this case.

However, now I have even more questions than I used to:

  1. If there is a pressure gradient, why there's no flow in water along the depth gradient?
  2. What mechanism (or what part of the gravity equation) makes denser objects sink despite the buoyant force? Is it the mass? What about Galileo's experiment then? Doesn't it show that the effect of mass is negligible?

enter image description here

Best Answer

The deeper you are under water the higher pressure. This is because the deeper you are the more water is above you. And water is pushing downwards due to gravity, thus more water above you means more push and hence more pressure.

Please note that deep under water the pressure is not caused by higher gravity. The gravity at depth is in fact lower than on the surface. The only cause of water pressure is the weight of the water above.

Objects under water occupy certain space and they have certain non-zero height. There is a pressure difference between the top and the bottom part of the object. This is causing a difference in the pressure force pushing on the object from above and from below: the force from below is bigger because of the weight of the water above. The difference is enough to overcome the weight of the object, if the object is less dense than water. In other words, if the object weights less than water of the same volume.

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