Why do things float?

Mario95 · 01-09-2014 1:19am #1

So I have a seemingly simple question that got me thinking for the past few minutes. Its almost half past one in the morning, so I am probably missing out something obvious.

Why exactly does the 'floatability' of an object depend on the ratio of its mass to the space occupied by such object?

If you can, please refer to the fundamental forces, because I find the term 'buoyancy' meaningless.

citrus burst · 01-09-2014 3:16am

Heavier materials have "more" momentum (p = mv) than lighter materials.

Iron has a mass of ~26 water has a mass of ~18. When these two particles collide head on the momentum transfer results in the iron continuing on in the same direction, and the water being turned around and sent back the way it came, it "sinks." If we do the same with Carbon (mass ~12) the opposite happens, the water continues on in the same direction and the carbon turns around and "floats."

If we double the number of water particles, we double the density. The iron particle that was slowed down by the first water particle gets a 2nd kick by the 2nd water particle, which turns it around and causes it to "float".

The fundamental forces at play are primarily electrostatics, and gravitational, but any external force will do.

Gru · 01-09-2014 10:14am

Here are two videos that might be of interest. Watch the first one then go on to the second for the explanation. I think it's a pretty nice explanation/demonstration of buoyancy.

https://www.youtube.com/watch?v=C_covjcIcZ4

https://www.youtube.com/watch?v=CTnkoIWZZPI

dlouth15 · 01-09-2014 1:11pm

OP, if you fully submerge an object in water, a volume equal to the submerged object is forced upwards - the level of water in the container rises. But gravity still acts on this displaced volume of water trying to bring it down again forcing the object, if it is light enough, upwards.

So the fundamental force is simply gravity. Another concept is density (mass per unit volume). If the object is the same density as water, then the displaced water will weigh the same as the object itself. The force of gravity on the object and the force of gravity on the displaced water will balance out. There will be no tendency for the object to move up or down. This is called neutral boyancy.

If the object is of lower density than water then the displaced water will weigh more than the object. The water will win out in the gravitational battle and the object will rise and the displaced water fall. This is called positive boyancy.

If the object is of higher desnity than water then the submerged object will weigh more than the displaced water and will sink. This is negative boyancy.

It is like two weights on a see-saw. The heavier object falls and the ligher object rises. But it is simply gravity acting on both objects to different degrees.

Morbert · 01-09-2014 1:57pm

citrus burst wrote: »

Heavier materials have "more" momentum (p = mv) than lighter materials.

Iron has a mass of ~26 water has a mass of ~18. When these two particles collide head on the momentum transfer results in the iron continuing on in the same direction, and the water being turned around and sent back the way it came, it "sinks." If we do the same with Carbon (mass ~12) the opposite happens, the water continues on in the same direction and the carbon turns around and "floats."

If we double the number of water particles, we double the density. The iron particle that was slowed down by the first water particle gets a 2nd kick by the 2nd water particle, which turns it around and causes it to "float".

The fundamental forces at play are primarily electrostatics, and gravitational, but any external force will do.

Though a sufficiently large, hollow iron ball will also float.

Mario95 · 01-09-2014 3:45pm

Thank you for your answers guys, but I still don't understand how the volume of displaced water is causing the upward force.

Morbert · 01-09-2014 6:35pm

Mario95 wrote: »

Thank you for your answers guys, but I still don't understand how the volume of displaced water is causing the upward force.

It's not the volume of displaced water that is pushing the object upwards. It is the water underneath the object. The water pushes the object upwards with a force that's larger than the object's gravitational force.

More fundamentally, the water molecules bombard the bottom of the object, pushing it upwards. If the object is fully submerged, the molecules will bombard the object from all sides, but there will be more bombardment underneath the object, as the water will be more dense below the object, and so the object will still rise. An object will sink if its gravitational force is large enough to overcome the bombardment.

dlouth15 · 01-09-2014 8:55pm

Morbert wrote: »

as the water will be more dense below the object, and so the object will still rise.

I think "at higher pressure" rather than "more dense" is probably a better way of putting it as a completely incompressible fluid would still cause objects to float if the object is less dense on average than the fluid. The pressure on the lower side of the object would still be higher than the top side.

Morbert · 02-09-2014 2:21pm

dlouth15 wrote: »

I think "at higher pressure" rather than "more dense" is probably a better way of putting it as a completely incompressible fluid would still cause objects to float if the object is less dense on average than the fluid. The pressure on the lower side of the object would still be higher than the top side.

Yeah I shouldn't have referenced density. It would be more generally correct to say the increase in the bombardment is due to the weight of the surrounding fluid being greater at the bottom of the object.

Why do things float?

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