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Balloons and lifting gases

I understand that density of an object dictates whether or not it will float, but what I don't get and can't get any satisfactory explanation... What causes an object of lower density to float?

Can someone clarify? Differences in pressure ( a little more on that) ?

Thus far, I've rationalized it in this way ( I sense it's wrong, but still):

Object with a lower density has a spread out mass distribution along the volume and therefore, the contact surface. Since water's density is 1 g/cm$^3$ ( our definition of mass says that 1 g is equal exactly to a 1 cm$^3$ volume of water)... When you have an object with a lower than water density, it exerts a force on the water due to gravity, but because at every specific point the weight of the object is always less than the one of water, it is unable to clear its volume of water out.

I know, doesn't make any sense, please help! Every explanation on the Web just says it's dependent on the density of the object and that's that.

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marked as duplicate by David Z Sep 6 '11 at 1:17

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

I think this is covered by the various questions that have been asked here about buoyancy, including the answer I posted to the linked (duplicate) question explaining how buoyant force arises from pressure differences in the fluid. If that doesn't satisfy you, though, you can edit this question to explain what you're still confused about and I'll reopen it. – David Z Sep 6 '11 at 1:19

The answer hinges on Archimedes' principle wiki. If the object is denser than water it is more massive than the water that it displaces. This means that the object experiences greater gravitational force than the water and so sinks.

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I agree with hrandjet but I think the best way to "get a feel" for what's going on is to think of the floating object and the medium in which it floats in a competition. Both the object and the liquid "want" to get to the lowest point possible (the location with the lowest potential energy).

If you think of a microscopic object less dense than water, it is easy to imagine that a small drop of water is more dense and thus "feels" a greater force per unit surface area. If our microscopic object were placed in water, successive drops of water would slip around the object and the object would rise to the surface and float. When an object like a boat floats, the same argument holds except that each individual little microscopic pieces of the hull float and they are all connected together. The argument doesn't strictly apply since the microscopic pieces of the hull are individually more dense than water, but the pseudo-density, if you will, is calculated from the density of the entire ship.

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