Hey, Scepti, if gravity is atmospheric pressure, then why do metal balls sink in water but foam ones float?

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Objects that are less dense than water will float. It's not about gravity at all. It's about the pressure of the water around the object which is technically it's atmosphere.

But it

*is* about gravity. Before I continue, a setup by Shmeggley:

There is more force on the air at the surface because of the weight of the air above it. Naturally the air pushes back with equal force. Therefore there is greater air pressure at the surface, and less the higher up you go.

Let us assume this is true of water as well (the same arguments can be applied to water, after all). So we place a foam ball, say 20 cm diameter, completely underwater. The water above it pushes down with a certain force F, and the water beneath pushes back with a force F + the weight of the 20 cm of water between. Thus the ball is pushed upwards until the water pushing up equals the air (when it's on the surface) pushing down.

If we put a steel ball 20 cm diameter in the same place, the exact same forces are acting on it by the water with a net force of a bit upward. But this one goes against that force and drops down through the water.

Thus, it can be shown that something other than water is affecting the steel balls.

Nautical science tells us that something will float if it weighs less than the amount of water it displaces. So a 1 cm

^{3} object will float if and only if it weighs less than 1 gram. This means that if it weighs more than 1 gram, this other unknown force will counteract that of the water's upward push. This also implies the net upward force is equal to the weight of the water displaced by the object.

Since the upward force of water only comes because of an imbalance in the forces above and below the object, what if we removed all the water above something? For instance, ice, at around .9 grams per cubic centimeter, if placed just under the water, will experience an upward force of water

_{below} - (water

_{above} + air

_{above}). But when it suddenly breaks the surface, there's no more water above it, completely removing that portion of the equation. Thus the upward force should be suddenly much greater than it was when it was just beneath surface.

But the ice floats only just atop the water, such that only 10% is not submerged. This means it's

*still* displacing its own weight in water. What could be keeping it that low if not this unknown force?

This unknown force seems to act on the mass, not size, of an object since the 20 cm foam ball floated whilst the 20 cm steel ball sank when put in the same conditions. So, we've got a downward force that's dependent on the mass of objects.

Why don't we call this force 'gravity'? That would certainly explain why helium balloons float whilst carbon-dioxide balloons don't--helium is lighter than air, and thus this downward force based on its mass can't counteract the upward force of the air. Carbon-dioxide, on the other hand, is heavier, and thus this downward based on its mass does counteract the upward force of the air.