Why do things float?

[chris_uk_83]

Here's one for my fellow physicists: why on Earth do things float?

This can be either an object in a liquid (such as a boat on water), a gas in a liquid (air in water), or two gasses (hot and cold air for example).

I'm probably going to get the answer "Less dense things rise above more dense things" or "if the volume of water displaced by an object has a greater mass than the object, then the object will float", which is basically the same thing. This is true, but it doesn't address why.

Why should something less dense sit on top of something more dense? I've got a theory but I want to see what other people think before saying any more.

I'm off to see what Wikipedia has to say on the topic...

EDIT: And "because bouyancy keeps it up" is not an acceptable answer, unless you can explain why bouyancy does keep it up.

[Gary88]

meh

when something sits in a fluid it creates a pressure difference between the fluid level (ie the surface) and the fluid under the object.

the imbalance results in the upward force of buoyancy.


thats probably utter shite but its all i remember



edit,

that of course only explains the upwards force but not why some things float and others dont. the upward force is proportional to the mass of fluid displaced so only objects a sufficiently light mass will float given their own volume

[Wild Bill Kelso]

meh

when something sits in a fluid it creates a pressure difference between the fluid level (ie the surface) and the fluid under the object.

It also happens in solids- http://en.wikipedia.org/wiki/Salt_dome


As far as I know- density is derived by how compacted matter is within a solid. Just as a guess (maybe a chemist can anwer)- I imagine something like uranium has a very tight shell of orbiting electrons relative to it's size This would increase a substances density as heavier atoms would be occupying the same amouint of space as a substance made up of lighter atoms.

I think it's because if two things don't mix, and have different densities, the lighter one which is attracted less by gravity, will float on top of the heavier one, which is attracted more.

I don't think that is correct- If you dropped a cannonball and a feather in a perfect vacuum they would strike the ground at exactly the same time. Hence why the accelration of gravity on earth is 9.81m/s regardless of the weight of the object.

[Imperator Tim]

When an object is placed in a liquid it will displace a volume of water equal in mass to the objects mass.
This is because the liquid that is pushed out the way will be pushed back by the weight of liquid around it, as the liquid attempts to remain in the lowest energy state, ie as close to the centre of gravity as possible, so it will form a flat surface at the bottom of its container (ignoring miniscus, earths curvature etc etc).
The less dense object will therefore be pushed upward by the force of the liquid trying to get back, when floating the objects weight and the force of the water are in equilibrium.

The density is determined by a couple of factors, but it is basically mass/volume. How dense an object is depends on the mass of the atoms it consists of and how densly they are packed, a face centered cubic arrangement ie cube with atoms at corners
will be less dense than a hexagonal arrangement of the same mass atoms as they are packed closer together. The atomic radii does not have a direct effect as shells can overlap or have large gaps between them.

Thats all from A-level physics/chemistry but im pretty sure ive got it down right.
Imperator Tim

[Spurius]

I think it's because if two things don't mix, and have different densities, the lighter one which is attracted less by gravity, will float on top of the heavier one, which is attracted more.

There is no buoyancy without gravity, I think. It's the fact they don't mix. Two fluids would just do exactly that, and the heavier of the two lie on the bottom, eventually.

[Ummagumma]

To echo what the others have said, it is about the relative density of an object and about air-pressure/gravity. The things that have more energy in being forced down displace the things which have less energy, but as everything has the same amount of air pressure and gravity the only variable is the density of the object.
Essentially, it is not that the less dense (wooden) object floats, but more that the more dense (water) object has enough energy to displace the other object in it's attraction to the centre of the earth - it pushes the wood out of the way on it's way to the focus of the forces attracting it, and as it is downwards, it pushes the wood out of the way.

Post count +1. Yes I am bored and inebriated - this was answered more than adequately beforehand...

I don't think that is correct- If you dropped a cannonball and a feather in a perfect vacuum they would strike the ground at exactly the same time. Hence why the acceleration of gravity on earth is 9.81m/s regardless of the weight of the object.

In perfect vacuum. On earth density effects the air pressure - more or less weight for a given surface area.... "What he said was true, from a certain point of view"

I should also be a Jedi.

(Of course, everything floats down here - in which case it is because the clown in thrall of a dodgy giant spider says so.)

[ajm317]

Here's a good explanation:

Bouyancy

Basically the way it works is this:

So the pressure in a liquid increases as you increase the depth. As a result, you have a greater pressure on the "bottom" of a submerged object than you do on the top of the object. This leads to a force imbalance, which pushes a submerged object up.

Now how do we know the magnitude of the net bouyant force?

Imagine that you have a pool of water, and in that pool of water you place a cube of water (imagine it has a very thin plastic wall, although this is not necessary, you may just picture any arbitrary cube section of the pool). Obviously, the cube of water is going to have a net force on it of 0. Water doesn't just rise or fall in a pool. So you know that the bouyant force on your cube of water has to be equal to it's weight.

The force is caused by the same thing for a cube of water as it is for a cube of wood or aluminum, a difference in pressure that depends only on depth, so the bouyancy force for the two cases must be the same: the weight of an equivalently sized cube of water. Hence Archimedes principal.

[Spurius]

Essentially it's because fermions are antisocial, and gravity does the rest.

[ajm317]

Essentially it's because fermions are antisocial, and gravity does the rest.

Bouyancy would still happen with bosons.

[Spurius]

There's even buoyancy exchange in oceans. Doesn't float your boat, though

[ajm317]

There's even buoyancy exchange in oceans. Doesn't float your boat, though

Well, your boat would still float in a sea of bosons. If it was at normal densities and temperatures anyway.

Of course, if you have low temperature or high density, then things start behaving funny.

[Ummagumma]

Overcomplications.....

Things with the highest weight in the smallest areas will literally push things out of the way in it's need to reach the centre of the earth.
He is after understanding Gravity as a concept, not about arguing the physical source of the effect.
Unless I misunderstood the question, In which case I apologise....

[chris_uk_83]

To all those talking about buoyancy and explaining that the pressure difference pushes the less dense object up, great. But why up? What happens when you take away gravity?

What I'm led to believe is that the more dense a material is the greater the force due to gravity per unit volume (note I said force not acceleration; the force is greater). Therefore the denser substance is being pulled down harder and the less one just gets shoved out of the way: up.

As far as I know - density is derived by how compacted matter is within a solid. Just as a guess (maybe a chemist can anwer)- I imagine something like uranium has a very tight shell of orbiting electrons relative to it's size This would increase a substances density as heavier atoms would be occupying the same amount of space as a substance made up of lighter atoms.

Yes and no. Density is determined by how compacted matter is, but electron shells don't really come into it. It's more to do with the size of the nucleus. Bigger nuclei = more dense substances. You also have the lattice (arrangement of atoms within a solid) to worry about, and the temperature, which determines average distance between the molecules, but then it gets complicated

[ajm317]

To all those talking about buoyancy and explaining that the pressure difference pushes the less dense object up, great. But why up? What happens when you take away gravity?

There would be no bouyancy.

Here's an explanation of the pressure difference:

http://en.wikipedia.org/wiki/Pascal%27s_law

Here's my very rough, very hand waving explanation:

Consider the atmosphere. Cut it up into tiny slices, let's say they're the height of your object.

TEXT DIAGRAM ACTION!

Upper slice
___________________________________
Middle slice []<object
___________________________________
Bottom slice

Now, at the boundary between the middle slice and the upper slice, you've got all the weight of the upper slice, and all the slices above it, pushing down on you. This is going to affect the pressure at the top of the object.

At the boundary between the bottom slice and the middle slice (the bottom of the object) you've got all the weight of the top slice, all the slices above the top slice PLUS the middle slice pushing down. By Newton's 3rd law that means the bottom slice is pushing up by that same amount. This affects the pressure at the bottom of the object.

This is where you get your pressure difference. This leads to Pascal's law, which in turn leads to Archimedes principle by the reasoning I posted earlier.

[Simetrical]

I admit I felt a little contemptuous when I first read the question, but having thought about it, it's not nearly so obvious.

Now, at the boundary between the middle slice and the upper slice, you've got all the weight of the upper slice, and all the slices above it, pushing down on you. This is going to affect the pressure at the top of the object.

But why? In the molecular-kinetic model, pressure is due to particles bouncing around. Higher pressure must correspond to either proportionally more particles, or proportionally higher average force per particle. Which is it, and why? Clearly there are not proportionally more particles; there will be slightly more but not much, at least not in a liquid. So the lower particles must be exerting a greater force than the higher ones. Are they actually moving faster? In gases we know they aren't, the speed follows the Maxwell-Boltzmann distribution, which I believe neglects gravity entirely. I would expect a bias toward downward movement, in fact, due to gravity, so that pressure downward would be a bit greater than pressure upward at any given point.

My first-semester physics textbook just postulates pressure as obvious, it doesn't seem to discuss physical mechanisms for it before introducing Pascal's law and so on. I guess once you have pressure altogether, you can derive all this more or less axiomatically, but I think Chris is looking for an intuitive answer, and I can't think of one.

[ajm317]

But why? In the molecular-kinetic model, pressure is due to particles bouncing around. Higher pressure must correspond to either proportionally more particles, or proportionally higher average force per particle. Which is it, and why?

Well presumably it's a little bit of both. If we look at the case of the atmosphere, we know that as you descend in alititude both the density and temperature of the air increase. Assuming that the atmosphere is in equilibrium, and there is no net flow of air either up or down, the kind of reasoning that leads to Pascal's law and Archimedes principle can tell you what the magnitude of that pressure change must be.

In gases we know they aren't, the speed follows the Maxwell-Boltzmann distribution, which I believe neglects gravity entirely.

Maxwelll-Boltzmann statistics do not ignore gravity, or any other source of potential energy in the system (and neither do Fermi-Dirac or Bose-Einstein statistics) although you can construct approximations based on Maxwell-Boltzmann statistics which do. If you go on to take a senior level course in Statistical Mechanics, you will probably have to solve problems like "determine the density as a function of altitude". Certainly you will in grad school.

[Spurius]

Yes, and the lighter stuff is forced up, because that's the only place it can go. But essentially it's like I said, they don't mix. Essentially there is no 'antigravity' force or anything. Or buoyancy 'particles':

http://www.hypeskeptic.com/Mattias/D...yParticles.asp

[Muizer]

I'm no physicist by any means. In highschool I always prefered to use the energy approach in these matters. IIRC the energy state due to gravity is expressed as mass*height*gravitational constant. So, the situation with the denser object closer to the center of the earth than the lighter represents a lower energy state than the reverse.

[Spurius]

Like I said, no buoyancy without gravity. There apparently *is* something like 'quantum buoyancy', but wiki has not found out about that, yet.

[Rush Limbaugh]

Simple

She's a witch!