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This question already has an answer here:

Why do objects float in liquids denser than themselves? I know that a balloon floats on water because it has air in it, but why?

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marked as duplicate by John Rennie homework-and-exercises Apr 23 '18 at 7:40

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    $\begingroup$ mass falls down. What configuration has the most mass as far down as possible? $\endgroup$ – JEB Apr 23 '18 at 0:56
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Objects sitting in a fluid (liquid or gas) experience a pressure on every surface, equal to the pressure of the fluid.

Horizontally these pressures cancel out - the pressure pushing on the left hand side of an object will be the same as on the right, and the object has no net sideways force.

Vertically however is a different story. The pressure of a fluid increases as you move lower (deeper) into it. Going back to our object, its bottom is deeper into the fluid than its top is, so the pressure on the bottom is larger than the pressure on the top. This difference results in a net force upwards.

We call this net upwards force buoyancy, and if you work through the maths it turns out that the upward force is equal to the weight of the amount of fluid that would fit in the same space.

If the object is less dense than the fluid, its own weight is therefore smaller than the buoyancy force upwards, and it floats.

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If an object is completely immersed in a liquid denser than it then the resulting buoyant force will exceed the weight of the object because the weight of the liquid displaced by the object is greater than weight of the object.As a result, the object cannot remain completely submerged and this causes the object to float.

Search up more about buoyant force to learn more about this

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