Floating On Water

[Jahee]

Say theres a solid, fairly heavy object e.g. a piece of wood floating on the surface of water, does this mean the object is less dense than the water, or is it due to the fact that the object has air trapped in it, or does that fact that there is air trapped in it make it less dense?

 

[bobsmith1492]

It's all about the displacement; if it can displace more water than its own mass, it will float. Basically, if it's more dense than water, it's the air 'trapped' in it like you said - think of a bowl floating in dishwater (or if you've never done dishes, try it!) It will sink down just enough so that the mass of the volume of water it takes up below the surface of the water is equal to its mass. Put something else in the bowl to increase its mass and it will sink further.

Wood (of most kinds) is indeed less dense than water, so it floats regardless of its total mass - total mass has nothing to do with it, only the density.

 

[Jahee]

oh right i see..

Wood (of most kinds) is indeed less dense than water, so it floats regardless of its total mass - total mass has nothing to do with it, only the density.

yea sorry that was a silly mistake on my part, was thinkin from my schooldays whereas if somethings dense its automatically heavy lol.

 

[CycloWizard]

The upward force (towards the surface of the water) is a buoyancy force, which depends on the shape and surface area of the object, as well as its density relative to water. In the opposing direction (downward into the water), you have the force of gravity, which depends on the object density. The former force is a traction, acting on the surface of the object, while the latter is a body force, acting on the volume of the object. There are other forces involved, but for most basic examples these are the most important.

 

[BrownTown]

also, things can float that are more sense than water if there surface area is large enough to have the surface tension of water lift them up. For example aluminum foil will easily float even though aluminum is much denser than water.

 

[CycloWizard]

Originally posted by: BrownTown
also, things can float that are more sense than water if there surface area is large enough to have the surface tension of water lift them up. For example aluminum foil will easily float even though aluminum is much denser than water.

A sheet of aluminum foil will float, but a ball of it will not. Similarly, there is an annual civil engineering contest to make a concrete canoe of a given weight that floats the highest out of water. It's all about the shape.

 

[imported_Tick]

Something else fun to try is playing with the density of water. Get a bowl, one that is heat safe, and place it in a pot of cold water. Then weigh it down with stuff until it floats with just a mm or two above the water line. Then heat the water. At some point, the bowl will sink, because the water will become less dense.

 

[bobsmith1492]

Wouldn't that take a while? I didn't think water's density changes much with temperature... I'd have to break out the thermo book if I knew where it was...

 

[imported_Tick]

Originally posted by: bobsmith1492
Wouldn't that take a while? I didn't think water's density changes much with temperature... I'd have to break out the thermo book if I knew where it was...

That all depends on how close the water line is to the top of the bowl, and how large the bowl is. All I know is it works.

Edit: I believe it drops about 35 kg/m^3 from stp to near boiling, which should be enough to accomplish this experiment.

 

[BrownTown]

Originally posted by: CycloWizard

Originally posted by: BrownTown
also, things can float that are more sense than water if there surface area is large enough to have the surface tension of water lift them up. For example aluminum foil will easily float even though aluminum is much denser than water.

A sheet of aluminum foil will float, but a ball of it will not. Similarly, there is an annual civil engineering contest to make a concrete canoe of a given weight that floats the highest out of water. It's all about the shape.

No, the concrete canoe is different because it is the air inside the canoe that is keeping it afloat, not the shape of the canoe. The ball of aluminum foil will obviously sink because you have the same surface area touching the water but alot more mass.

 

[imported_Seer]

Originally posted by: BrownTown

Originally posted by: CycloWizard

Originally posted by: BrownTown
also, things can float that are more sense than water if there surface area is large enough to have the surface tension of water lift them up. For example aluminum foil will easily float even though aluminum is much denser than water.

A sheet of aluminum foil will float, but a ball of it will not. Similarly, there is an annual civil engineering contest to make a concrete canoe of a given weight that floats the highest out of water. It's all about the shape.

No, the concrete canoe is different because it is the air inside the canoe that is keeping it afloat, not the shape of the canoe. The ball of aluminum foil will obviously sink because you have the same surface area touching the water but alot more mass.

Riiight.............

So, if we took that same concrete canoe, and reformed it into, lets say, a solid block, it would still float because its shape has nothing to do with it, right? Oh wait, maybe its the shape of the canoe that causes air to be inside it in the first place, hmmm?

Also, the spread out Al and crumpled Al have exactly the same mass...so, how this will "obviously" make the ball sink, I'm not quite sure. Once again, it has everything to do with the shape and how much water is displaced WRT the mass of the object. If the object can be shaped in such a way that more grams of water are displaced than are grams of object, then it floats. Otherwise, it sinks.

 

[CycloWizard]

Originally posted by: BrownTown
No, the concrete canoe is different because it is the air inside the canoe that is keeping it afloat, not the shape of the canoe. The ball of aluminum foil will obviously sink because you have the same surface area touching the water but alot more mass.

Not entirely true. The canoe should be shaped to maximize the magnitude of the upward resultant (i.e. the net upward buoyancy force), which is a pretty interesting problem that is not trivial to solve. I'm not sure where the mass of a ball of aluminum foil will have more mass or the same surface area as a sheet of foil. In fact, I'm sure that neither is true, as we are only concerned with the external surface area. So, if you take a sheet of aluminum and ball it up into an assumed closed, nonporous sphere, the mass is the same but the surface area is minimized and it will sink.

 

[bobsmith1492]

Originally posted by: Seer

Originally posted by: BrownTown

Originally posted by: CycloWizard

Originally posted by: BrownTown
also, things can float that are more sense than water if there surface area is large enough to have the surface tension of water lift them up. For example aluminum foil will easily float even though aluminum is much denser than water.

A sheet of aluminum foil will float, but a ball of it will not. Similarly, there is an annual civil engineering contest to make a concrete canoe of a given weight that floats the highest out of water. It's all about the shape.

No, the concrete canoe is different because it is the air inside the canoe that is keeping it afloat, not the shape of the canoe. The ball of aluminum foil will obviously sink because you have the same surface area touching the water but alot more mass.

Riiight.............

So, if we took that same concrete canoe, and reformed it into, lets say, a solid block, it would still float because its shape has nothing to do with it, right? Oh wait, maybe its the shape of the canoe that causes air to be inside it in the first place, hmmm?

Also, the spread out Al and crumpled Al have exactly the same mass...so, how this will "obviously" make the ball sink, I'm not quite sure. Once again, it has everything to do with the shape and how much water is displaced WRT the mass of the object. If the object can be shaped in such a way that more grams of water are displaced than are grams of object, then it floats. Otherwise, it sinks.

Um, you missed the fact that surface tension is what he posits will make the aluminum foil float - an entirely different subject than that of density and displacement.

 

[Gibsons]

Balls of aluminum foil float pretty well.

 

[BrownTown]

Originally posted by: Seer

Originally posted by: BrownTown

Originally posted by: CycloWizard

Originally posted by: BrownTown
also, things can float that are more sense than water if there surface area is large enough to have the surface tension of water lift them up. For example aluminum foil will easily float even though aluminum is much denser than water.

A sheet of aluminum foil will float, but a ball of it will not. Similarly, there is an annual civil engineering contest to make a concrete canoe of a given weight that floats the highest out of water. It's all about the shape.

No, the concrete canoe is different because it is the air inside the canoe that is keeping it afloat, not the shape of the canoe. The ball of aluminum foil will obviously sink because you have the same surface area touching the water but alot more mass.

Riiight.............

So, if we took that same concrete canoe, and reformed it into, lets say, a solid block, it would still float because its shape has nothing to do with it, right? Oh wait, maybe its the shape of the canoe that causes air to be inside it in the first place, hmmm?

Also, the spread out Al and crumpled Al have exactly the same mass...so, how this will "obviously" make the ball sink, I'm not quite sure. Once again, it has everything to do with the shape and how much water is displaced WRT the mass of the object. If the object can be shaped in such a way that more grams of water are displaced than are grams of object, then it floats. Otherwise, it sinks.

If you had read what I wrote you would see that there are 2 forces at play here, the effect of surface tension, and the effect of buoyancy/displacement. The reason that the concrete canoe example is different is because the driving force in that example is that amount of water displaced by the canoe. The shape is obviously of the highest importance, but this is in order to maximize the amount of water displaced by the fixed mass, NOT to maximize the effect of surface tension. So, its a completely different situation.

As for the ball of aluminum example, you are again contributing the effect of floating only to water displacement. The reason a sheet of aluminum floats is due to surface tension, it displaces a smaller mass of water than its mass, and yet it still floats. A ball of aluminum of the same mass would have a smaller area being acted on by the surface tension, and at some point the fact that it is denser than water will cause it to sink. However, no matter what the shape is (unless you are creating sealed pores that water cant enter), the aluminum will displace a SMALLER mass of water than the mass of the aluminum.

 

[Calin]

Originally posted by: BrownTown

Originally posted by: CycloWizard

Originally posted by: BrownTown
also, things can float that are more sense than water if there surface area is large enough to have the surface tension of water lift them up. For example aluminum foil will easily float even though aluminum is much denser than water.

A sheet of aluminum foil will float, but a ball of it will not. Similarly, there is an annual civil engineering contest to make a concrete canoe of a given weight that floats the highest out of water. It's all about the shape.

No, the concrete canoe is different because it is the air inside the canoe that is keeping it afloat, not the shape of the canoe. The ball of aluminum foil will obviously sink because you have the same surface area touching the water but alot more mass.

You could create a "concrete" with a density lower than the water's density. This way, a ball of it will float IF water is prevented from infiltrate into the empty spaces inside (something like a foam of cement). Useful for thermo-isolation (I've heard they made it using cement, water and CaC4 instead of CaOH).

 

[gsellis]

I thought anything that weighed the same as a duck would float...

And surface tension makes a big deal. That is how ducks do it. Add something to break the surface tension and ducks sink. As a group of firefighters who attended a conference at the Memphis Peabody might have demonstrated. Seems that a bunch of drunk firefighters who happen to have a 5 gal container of Light Water are not to be trusted. If said Light Water were added to, say the fountain in the Peabody lobby where the ducks swim, it might change things. Ducks sink, but alert hotel staff can rescue ducks.

 

[CycloWizard]

Originally posted by: BrownTown
If you had read what I wrote you would see that there are 2 forces at play here, the effect of surface tension, and the effect of buoyancy/displacement. The reason that the concrete canoe example is different is because the driving force in that example is that amount of water displaced by the canoe. The shape is obviously of the highest importance, but this is in order to maximize the amount of water displaced by the fixed mass, NOT to maximize the effect of surface tension. So, its a completely different situation.

As for the ball of aluminum example, you are again contributing the effect of floating only to water displacement. The reason a sheet of aluminum floats is due to surface tension, it displaces a smaller mass of water than its mass, and yet it still floats. A ball of aluminum of the same mass would have a smaller area being acted on by the surface tension, and at some point the fact that it is denser than water will cause it to sink. However, no matter what the shape is (unless you are creating sealed pores that water cant enter), the aluminum will displace a SMALLER mass of water than the mass of the aluminum.

Exactly. Surface tension was one of my 'other forces' that is only sometimes important and need not be considered for the case of wood on water, since buoyancy is the dominant force.

 

[Googer]

Originally posted by: Jahee
oh right i see..

Wood (of most kinds) is indeed less dense than water, so it floats regardless of its total mass - total mass has nothing to do with it, only the density.

yea sorry that was a silly mistake on my part, was thinkin from my schooldays whereas if somethings dense its automatically heavy lol.

Think of an Iron Ship.

 

[jagec]

Originally posted by: gsellis
I thought anything that weighed the same as a duck would float...

And surface tension makes a big deal. That is how ducks do it. Add something to break the surface tension and ducks sink. As a group of firefighters who attended a conference at the Memphis Peabody might have demonstrated. Seems that a bunch of drunk firefighters who happen to have a 5 gal container of Light Water are not to be trusted. If said Light Water were added to, say the fountain in the Peabody lobby where the ducks swim, it might change things. Ducks sink, but alert hotel staff can rescue ducks.

That's begging for a vid clip

I thought that ducks trapped enough air under their feathers to float without depending on surface tension, but I must be wrong.

 

[gsellis]

Originally posted by: jagec

Originally posted by: gsellis
I thought anything that weighed the same as a duck would float...

And surface tension makes a big deal. That is how ducks do it. Add something to break the surface tension and ducks sink. As a group of firefighters who attended a conference at the Memphis Peabody might have demonstrated. Seems that a bunch of drunk firefighters who happen to have a 5 gal container of Light Water are not to be trusted. If said Light Water were added to, say the fountain in the Peabody lobby where the ducks swim, it might change things. Ducks sink, but alert hotel staff can rescue ducks.

That's begging for a vid clip

I thought that ducks trapped enough air under their feathers to float without depending on surface tension, but I must be wrong.

The oil on the feathers creates the whole thing. The cormmorant and another bird I am spelling challenged on this early (aka the water turkey) are examples. They must dry off and swim around with their heads out of water.