basic physics - why do things float


I was wondering why things float (leading up to buoyancy) from a physics point of view. My viewpoint is that something floats when it is less dense than water. But i was wondering how floating is explained from a forces point of view.

I have looked at Archimedes principle and this says 'Any floating object displaces its own weight of fluid.' I was not aware that floating objects displaced any fluid. I naively thought a floating object simply sat on top of the liquid. I do have more questions but I may as well wait until someone clears that misconception up for me :)

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phoffricConnect With a Mentor Commented:
Let rho1 = density of water
Let rho2 = density of a material 2 with property that rho2 < rho1

Consider a cylinder of water having a base of 1 m^2 and height h.
It's volume is V=h*1=h
What is the weight of the water in this cylinder? It is W1=rho1 * V = rho1*h

Pressure = force/area

What is the pressure of the water on the bottom of the cylinder? P=F/A=W/1 = rho1*h

The pressure at d meters below sea level is rho1*d

Take a cylinder made up of material having density rho2 (same base of 1 m^2)
It's weight is W2 = rho2 * h < W1

Submerge it completely under water and let go.
There are 3 forces on the cylinder.
1) Its weight, -W2
2) The pressure of the water on top of the cylinder pushing down = -Ptop = -Ft*A = -Ft
3) The pressure of the water on bottom of the cylinder pushing up = +Pbot = Fu*A = Fu

Total force on cylinder of of height, h, is -W2 -Ft + Fu = -W2 + (Pbot - Ptop)

Here is the tricky part that maybe Archimedes realized.
Whatever is the top water pressure on the top part of the material 2 cylinder, obviously the water pressure at the bottom of the cylinder is greater, since if I had the same cylinder filled with just water, then the weight of that cylinder is (see above):
     W1=rho1 * V = rho1*h
The pressure at d meters below sea level is rho1*d

If the top of the material 2 cylinder is at depth d1, and the bottom is at depth d2, then the difference in water pressure is (rho1*d1 - rho1*d2) = rho1*(d1 - d2)
But (d1 - d2) is just the height of the cylinder
The difference in water pressure between top and bottom surfaces is just rho1*h

Total force on cylinder of of height, h, = -W2 + (Pbot - Ptop) = -W2 + rho1*h
             = - rho2 * h + rho1 * h = h * (rho1-rho2) > 0
               since rho2 < rho1

Now that you see there is an upward force when completely submerged, how high will the cylinder move?

When the total force on the cylinder is 0 it is in equilibrium

Since the top of the cylinder is no longer under water, Ptop = 0

If the cylinder is in equilibrium, and is b meters below sea level, then the pressure difference is just Pbot = +rho1 * b

So, we have in equilibrium net force = 0 = -W2 + rho1 * b = - rho2 * h + rho1 * b

rho2 * h = rho1 * b

b =  h * (rho2/rho1)
The force of gravity pulling down on a floating object is less than the force of the water's (or other liquid) buoyancy.
ffleismaSenior Network EngineerCommented:
yes putting another matter on top of another fluid will displace an amount equal to its own weight. perfect example would be ice cubes and water.

as you put more ice cubes on the glass, water is displaced from the glass.
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there are 2 forces acting on a floating body: the bouyant force (Fb) and the body's weight (W). for a body to float  Fb=W

.... also Fb=weight of the liquid displaced

weight of the fluid displaced = volume of the body below the surface x density of liquid

this is why an fully loaded ship rides lower in the water... needs more bouyant force to counteract it's weight .... it needs to displace more water
...therefore it has more volume below the surface....

let me know if it's not clear...
>something floats when it is less dense than water

We have ships made of steel and boats made of concrete.  Not like wooden ships, where the wood itself was less dense than water, thus floated on it's own.  You have to rely on displacement such that the volume of water displaced is less than the volume of your vessel.  The greater the surface area of your hull (while retaining the same weight) makes a shorter draft...the boat sits higher in the water.   A wide rectangular barge can carry large amounts of material and still draft little...enough to navigate shallow channels that would not be safe for traditional tankers or cargo vessels.
"I have looked at Archimedes principle and this says 'Any floating object displaces its own weight of fluid.' I was not aware that floating objects displaced any fluid. I naively thought a floating object simply sat on top of the liquid."

The principle is not self evident which is why it too so long for it to be discovered.
Objects do not float on top of a liquid but partially submerged. Take a b;lock of wood and float it in a bathtub (or sink). note that it is partially submerged. Ad a weight on top of it. Note that it is now more submerged as more water had to be displaced to offset the additional weight.
To float an object does have to be less dense than water. But it is the object not the material that the object is made of that must be less dense. Steel boats must have an AVERAGE density less than that of water.
Take an empty gallon plastic milk bottle.  
Put the cap back on to keep it from collapsing.
Fill the kitchen sink t of the way up with water.

Put the bottle in the sink.  
It isn't very heavy, it will float easily.
Now try to push it under water and watch the water level.

It takes a lot of force to push it under.     1 pint = 1 pounds  ==>  1 gallon = 8 pounds.
As you push it under, the water level rises.

Push it halfway and hold it there.

Think about the forces that have to be balanced.
You are pushing down with 4 pounds, but nothing is moving.
The 4 pounds of water that you have lifted is pushing back.

You can use a wide mouth plastic bottle and a 1 qt measuring cup to make a pretty good scale.
andiejeAuthor Commented:
Lets do a basic calculation. I don't know if the following scenario is realistic but its just to show the numbers:

50N block of wood floating on water
This will displace 50N of water = 5kg of water (using weight = mg and g = 10 N/kg)
Water has density of 1g cm^-3 so 5000cm^3 water is displaced
That is correct.

1 cc of water = 1 gram

1 oz of water = 1 oz
andiejeAuthor Commented:
I must be reading this wrong. It says

1) 'Any floating object displaces its own weight of fluid
2) water pushes up on a weight of an object with a force equal to the weight of displaced water

So then won't everything float? Obviously they don't but my logic from reading the above is: An object will push down on fluid with a force due to its weight. It will displace its own weight in fluid and the fluid will push back with this force. So the forces on the object are balanced and it will stay put (e.g. float)

Since that quite obviously doesnt happen i'm reading it wrong
andiejeAuthor Commented:
where does shape of the object come into this?
Any object in the fluid will displace it's volume of fluid.
A floating object will displace it's own weight.
A sunken object displaces less than it's own weight.

Volume comes into play because the same mass can have different volumes.

1 ton brick of concrete will not float.  You can make a barge out of 1-ton of concrete...but it's much larger than the solid brick.

1kg sphere has less volume than 1kg cube or box shape.  Depending on densities of object & fluid, you can float the cube but sink the sphere.

If you've ever gone swimming, you know that you can float better with all limbs extended.  Tuck into a tight ball and you will lie lower in the water (or even just below the water).  In survival swimming, we were taught to lie face down with arms and legs spread out.  Low-fat, short, skinny people will rest a few inches below the water and had to periodically sweep arms to bring mouth above water for air, then sink again.
>>  So then won't everything float?

No.  A cubic foot of water weighs approx 62 pounds.

A solid cubic foot of wood weighs less.  It will float.

A solid cubic foot of iron weighs much more.  It will sink.
You can only get a max bouancy force of 62 lbs.

If a sealed, hollow lead cube weighs less than 62 lbs, it will float.
>>  where does shape of the object come into this?

If the object is solid, or (hollow and sealed), then shape doesn't matter.
The object will float or not float.
If it floats, it will do so with the lowest possible center of gravity.

If the object is dish shaped it may float evenly when it's empty, but sink if it leaks.
Many boats would be in this category as well.
>>  If it floats, it will do so with the lowest possible center of gravity.

Adding ballast (rocks) or a lead keel will lower the CG of a boar and improve stability.

Going back to the kitchen sink:  You can float a plastic water glass half full of water.  
An empty glass is likely to tip over and sink.
Try this with a bowl of water.

Put an object half-way in the water.  Notice how the water level goes up?  Well, your object is forcing the water upwards.  If you push the object in further, more water comes up.

So let's say gravity is pulling your object downwards.  Well, the water is being pushed upwards by the object.  Gravity wants to pull the water too, so there's a limit to how much the water can be pushed up.

Once the force of gravity pulling down on the object is equal to the force of gravity pulling on the water that's displaced, these two forces will balance out, and the object will no longer sink (and the water will no longer rise).  So....... once the amount of water displaced matches the weight of the object, it will float there.

You can use density to predict what will happen when you drop something in water.  If it's denser than water, then the amount of water displaced will never match the weight of the object, and it will sink all the way down (or at least until it finds deeper water that is equal in density).  If it's less dense, then it will sink until the water displaced matches the weight, and it'll float.  The less dense the object, the less it will sink before it stops.
Stuff doesn't exactly 'float' on water; it's kind of a reversal of perspective that creates the concept.

What happens is that the water sinks below a less dense object. Water is denser (heavier by volume), so gravity results in water continually being forced to be lower than the "floating" object and thereby pushing the object up out of the water's way.

From that perspective, the stuff isn't floating. It's more that the water simply sinks more.

(Lousy wording, but maybe someone else can say it better -- assuming I made any sense.)

andiejeAuthor Commented:
phoffric: i'm reading your answer and stopped here:

What is the weight of the water in this cylinder? It is W1=rho1 * V = rho1*h

Doesn't that give you the mass of water in kg not the weight? rho = kg/m^3 and V = m^3
To get the weight, multiply the mass by g.
Since this applies to both the water and the object, it does not change the result.
phoffricConnect With a Mentor Commented:
>> Doesn't that give you the mass of water
Very good detailed catch using units check. Keep checking in more detail as I wrote this down without a source.

If I got everything correct, then using the math model set up, then you should be able to figure out this problem. You hook a heavy metal block to an ideal spring and observe the spring's extension. Suspending this object from the spring, you slowly submerge the object into water.

As the block is partially submerged the spring extension decreases since there is now water pressure force upwards on the bottom of the block. After the block is completely submerged and continue dropping, the spring has reached its minimum extension and remains constant.

Thinking of the spring as a weight scale, this modeling then explains the apparent decrease in weight of an object submerged in water. The gradient of water pressure over the object provides the buoyancy force.
> 1) 'Any floating object displaces its own weight of fluid
and a sinking object displaces less than its own weight of fluid
Since the force that water apply to the object is equal to the volume of water involved a very easy way to visualize when the object will be virtually floating alike being at 0 gravity is to imagine that it's volume must displace the amount of water that have the same weight of the object.

We have to know the specific weight of the water and of the matter we use for the experiment, specific weight of the water is 1kg. per litre, 1 litre is a volume of 10x10x10 cm. so one cubic decimeter of water weighs 1kg.

So let's imagine now to have a 1000 times heavier matter.

So imagine a cube of 1x1x1 cm of a any matter (let's call it ideal matter for this mental experiment) having a volume of 1 cubic cm. and having the weight of 1kg.  and let's suppose that this ideal matter is rigid and not deformable, impearmeable and shapeable at wish by our mind, we will imagine it as a water tight cube.

OK now we start with this solid 1kg. heavy and 1 cubic cm. cube of ideal matter, let's place it in water thank, it will immediately break the superficial tension of the water (concept that will be helpful later on to understand why is possible to "walk on water") and straight sink to the bottom.

Why this happens? As said by the physical law the object is receiving a counter force from the water equaling the weight of the water displaced, we are now displacing 1 cm cubic of water, so 1/1000 th of a kg., our ideal object is receiving a pull from the water displaced that is only of 1gram (1/1000 of a kg.) so we need still other 999 grams of force (force is not measured in this way but for this example is fine to speak so in order to keep it very simple, there ar ways to translate this value in more appropriate SI units) to reach a balance.

Just for curiosity we put our hand in the thank and try to lift the cube, we do not appreciate a any special change in the weight of the cube, the nly thing we feel is that our arm seems to be less heavy in the water (in fact it is dislpacing a volume of water and we feel the counterforce applyied , but the cube still seems to be 1000 grams heavy (not much difference between 999 grams and 1000 grams, we do not feel it)

Now as we are not really happy having our little heavy cube on the bottom of the thank we try to help it to raise it's position in this world, so now as a magician can do we move the ideal matter and give a different shape to it, let's create a hollow inside the cube without adding or removing any matter so not to change the weight of the cube that will always stay to be fixed at 1kg.

Somehow we will have to displace the matter to create this chamber inside the cube and so we discover that to keep the 1kg weight we need to change the volume, so now our new cube is still 1kg. heavy but it's volume is higher and let's say that is now measuring 5x5x5 cm. having a total volume of 125 cubic centimeters.

Let's see how big is the counterforce that 125 cubic cm of water can apply to the object, 1 cubic cm of water was applyng 1g. of force, 1x125 cubic centimeters of water will apply thus 125g. of force, so 1/8 of a kg. and therefore our object will keep staying there on the bottom.

So we really gave it a big hand, we raised the counterforce of well 125 times, but still this is not enough at all, we need more power. As before we want to see what happens now if we try to lift it, apart the same feeling on our arm seeming to be lighter, in fact this time we feel also that the cube seems to be less havy, in fact is much easier to lift it and now we try to get it out of the water, as soon as it is out of the water suddenly seems to be heavy as at the beginning of the experiment, and replacing it in the water seems to be lighter again.

What is happening? when is in the water we are helped in lifting it from the counterforce of 125g. applyied from the water, so the object appears to weight only 875 grams instead of 1000 grams, as soon as it is outside this counterforce is gone ad we feel it's real weight which in fact is still of 1000 grams (and is it is wet now even some grams more coming from the water sticking to it).

Now we drop the cube again and we see it sinking down, to the bottom again.

As we see our cube-friend there alone sitting in the bottom of the thank and we are very sorry for it, we decide to give t an extra chanche, let's displace again the matter so that the new cube will be 10x10x10 cm big.

We look at it and still sits there on the bottom, now the counterforce that receive from the water is equal to 10x10x10=1000 cubic cm of water, 1cubic cm of water weights one gram so all togheter receives a 1000 grams counterforce wich is equal to 1kg.

So now it i receiving the counterforce that equal it's own weight, why is t still sitting on the bottom? We decide to touch it again and try to lift it, this time we immediately notice that the cube is having virtually no weight and just going near it with the hand it begins to move, we grab it and we move it almost as if it is weightless, it is so easy now, we only feel the resistance of the water around it, the only forces applyied now are those of the water moving around the cube. We try to lift it outside of the water and suddenly as soon as it is partially out we feel a big weight diference and when completelly out it is again as heavy as at the beginning of the experiment, if we put it on a scale we see that it is some more of 1000 grams as some water is sticking to it, when dry will be 1000 grams again.

We still are not happy for our cube-friend we would like it to stay on top. We drop it in the water, and this time is not going down to the bottom, it is now floating freely in the water, we can place it at a certain depht and see it rolling around that position, if we move the water we see that thecube moves around with the water stream, much alike if it is done of water itself.

Why this happens?

The counterforce the object is receiving is equal to it's weight (so equal to the force that it's attracting it toward the center of the hearth), thus it's weight is reduced to zero and floats in the water just as if it done of water itself.

So now that our cube-friend does not have to stay on the bottom we are happy about it, but let's help som more, we expand it some more so to have a volume of 15x15x15=3375 cubic centimeters, now that recives a counterforce from the water that is more than 3 times higher than it's own weight suddenly pops up out over the water level and finally floats on it.

If we go to see how much of the cube is under of the water level we see that moreless that volume equals the volume of water that weights 1kg, the same weight of the cube itself, let's how many cm of the cube are under the water level 15x15= 225 square cm. so then 1000/225=4,4 cm., the cube is sinking in the water for only 4,4 cm. now the rest is out of the water.

Now we try to push back the cube to the bottom, but we immediately notice that is difficult, in fact tends to come out just as if the water do not want it anymore, and i fact it is so, in fact te cube weights only 1kg., but now it's moving a volume of water that creates a counterforce of 3,335 kg. so we feel a 2,335 kg. forceback when we try to sink it.

If we expand it more the water that will be moved by it will be again 1kg. but the force that we will feel when pushing it down will be as high as the volume of water that will be moved, so let's imagine to have expanded it to a size of 100x100x100 cm,

Now the cube will be sinking for only a mere 0,1 cm. in fact 100x100x0,1=1000 cubic centimetrs which are 1kg. of water, the same weight of the cube, but if we try to push it down we soon discover that seems impossible after very few millimeters, in fact every extra mm. of depht moves a kg. of water, so a extra kg. of counterforce is added, and very soon the amount of kg. of water moved will be far to much for our strenght, after only 5 cm  we will be already pushing with a 50kg. force, and at the depht of 10cm. 100kg., to sink the whole cube we will need a big power of 1000-1=999 kg. so basically 10 heavy persons should stand on it's upper face in order to see it floating in the water just as when it was 10x10x10 cm big.

Our cube friend now is not only self floating, but is able to sustain even more that 10 persons on the water level.

We expand it once more to a very big size, keeping the weight at 1kg.
Now it is 1000x1000x1000 cm big and it sinks for only 0,001 cm.
In order to sink it under the water level we will need an amazing power of (1000/0,001)-1=999.999 kg. so we need almost 1000 tons force to sink it, WOW, that's a lot, and that is why we can build amazing big and heavy, really heavy, huge vessels.

Sorry folk.

Adding the photo I also submitted the article without having cleaned it from typos, mispellings, badly translated words and sentences, but I guess is anyway easy to understand what is written there, here are another couple of photos which shows how is possible to walk on the water thanks to the superficial tension phenomenon...
In this case we oserve a phenomenon that goes beyond the Archimede's principle, in fact it clearly evident that the coin should sink (and it does if we just lightly bump the glass of water or the coin itself), and also that the volume displaced by the legs of the insect is really far too low to make it float in this way.

We are observing here a totally different thing, it is called superficial tension and comes to be explained by other physics laws.

Every fluid have this property called superficial tension, roughly and very easily explained, this happens because moleculas forming a matter tends to stay grouped one near the other one, so in order to dip our finger in the water we first have to apply a force that exceeds the force that keeps the water's moleculas all togheter.

Normally we do not realize this force is existing, just because we are unable to appreciate such tiny forces of the water and other fluids, meanwhile we perfectly know how hard is to dip a finger in a rock, for example, as well as we know for sure that the fluid bitumen, a matter with a very high viscosity, mixed up with small rocks with wich we pave our roads have a very very high superficial tension at regular temperatures, that's why we normally tend not to bounce our head on roads out there, even if someone does it sometimes, most of us know that is not really wise activity.

This insects do feel this very light forces of the water superficial tension, and learned to live walking on it's surface, they just stand on it, walk and even run on it....

They are so light that the surface of the water, where the superficial tensions forces are present, does not break under theirs feet, so they are not simply floating on it, but walking on it just as if they had a sort of very soft carpet to live on.

So to try it out at home take a glass and pour water very very slowly and gently, when you are at the top begin to add water drop by drop and very carefully in the center of the glass, soon you will see that the water in the glass is more that what the glass can contain, and up to a certain amount the water will sort of overtop the glass it self. Stop before pouring the water outside of the glass and watch well what is happening, you will see that meanwhile before overtoping the interface at water level was concave, now that is overtopping, the interface at water level is convex, and you can add even more water, up to the point in which the weight of the extra water tending to fall down for the gravity force exceeds the superficial tension power that tends to keep all the water moleculas toghether.

Now remove some water, take a clean small and light coin, just drop it in the water, it will immediately sink to the bottom, because it displaces a volume of water that is lighter than itself. Let's remove it and dry it properly, now let's place the same coin on the water surface, but this time very carefully, slowly and trying to lie it down on the larger surface possible at once, if we have a firm hand and we do not break the superficial tension we will see the same scene shown by the photo above, the coin will float on the water, thanks to the superficial tension of the water.

If we somehow give enough energy to the coin and the superficial tension power is exceeded thus the water surface broken, the coin sinks to the bottom.

But now let's see how this Jesus Christ Lizard learned to do even more, watch at this amazing scene (short clip from National Geographic documentary taken from youtube)....

Maybe the lizard does not know how to explain it to us, but surely does not care about and keeps running for it's life....


Surface tension is just the slight attractive force between the water molecules and certain materials.  In order for an object to sink, the force of the weight being pulled down must overcome this attractive force.  If it doesn't, it will stick to the surface of the water (notice how the water bunches up around the edges of an object that is on the surface in this way).

You can replicate the coin trick by either placing the coin very very gently on top of the water, or using a piece of (tissue) paper to help settle the coin before gently removing the piece of paper.

I'm not sure that this required three long posts.

Side note: soap disrupts surface tension by being more attractive to water molecules than whatever was floating.  You can make insects sink this way.  We used to put dishes out with soap water to prevent mosquitoes from reproducing in our backyard.

Surface tension is in fact a phenomenon that suprise and is nice to know about it, normally is discussed as a curiosity after having discussed Archimede's principle, it is a way to introduce to other concepts, it does not require 3 long posts but a whole bunch of books to be completely explained.

I wrote it as the last part of my very basic and simple divulgative text which is about the archimede principle and not superficial tension itself.

By the way not only soap disrupt the superficial tension, many substances do it, for example petroleum does reduce it and was a very interesting problem in relation to hurricanes latelly in the gulf where the disaster happened.

Yes, a soapy pond of water is a trap for certain insects, if done with a dish in your backyard is ok, but when happening in natural enviroments is not ok at all as the biological chain can be broken by that.

It is a wonderful and spectacular natural show the scene you can see at looking a crystal clear small river where thousands and thousands of waterstrides live.

Of course is not nice to know that mosquito larvae are "hanging" just below water surface thanks to superficial tension, but fishes eating them are happy about....that's why even though with genetically sterilized male mosquitos we can now exterminate this terrible creature from the face of the earth we just decided not to do it.

I guess that for whoever is asking himself something about the archimedes principle is also interesting to know that also superficial tension is out there, and that when they see something floating on the water in the way waterstrides do is not a matter of floating or displacing a volume of water exceeding it's own weight, but a matter of superficial tension, that is why is common to say some words about after discussing the archimede principle.

Side note: To prevent reproduction of mosquitos in your backyard you can also place tiny to small copper pieces in the little ponds including the flower's pots water reservoirs, and for larger water ponds a specific bacteria culture can be used, there is no need to disrupt the superficial tension with potential pollutants and ruin the enviroment for other forms of life too. It sounds funny as we are pollutting at full speed in so many ways anyway...

Last but not least superficial tension is not something occurring beetwen water and certain materials, it is a property of the fluids, every fluid have it, each one at higher or lower degree depending on it's chemistry, can be changed by other substances, and still at the end the new mixed fluid will have it's own specific superficial tension, which can be very low as in soapy water.

Can be changed also depending on what is nearby the fluid we observe, for example if a drop of water is touching a matter that is repelling water the superficial tension will tend to keep the water in a sphere.

Look at this photo, it is the effect of the superficial tension of the water when in contact with a material that is "lowly wettable" (I do not know the english correct term for this) , fascinating... isn't it?
In the seond photo you can see what happens when the water stop touching a material that is "more wettable" than the air and falls trough it, a drop is immediately formed, this due to the superficial tension effect.

Going back to archimede's principle we can make examples using air instead of water, in fact the same principle apply.

What happens when we inflate a balloon with a lighter gas than air? It begins to fly, isn'it? What happens when we fill up a ballon with air that is hotter than the air around?

It begins to float and fly in the opposite direction of the center of the heart. Why? How does come if it is filled of the same air around it? It should stay in the initial position, why is it not true?

Can the water float inside the water? And can the air float inside the air? Well we just said that a ballon of hot air floats inside a colder air system, I guess that the same happen with water, in fact if I pour hot water in a glass of cold water, even if I mix it, after a while the water on top will be hotter that the water on the hot water floats on cold water.... Why?

Let's take a black big very light plastic bag, and let's go on the beach in a very hot day, now we unfold the plastic bag and we let some air entering inside it, when it is quite full of air we close it, we tie it wit a very thin and light long fishing rope, we secure the rope to a rock and we leave it under the sunshine.

Let's go to have fun now we will come back later on to see what happens to tjhis lousy bag.
After bathing eating ansd having fun we go to give a look to he bag, and suprise, it is not there anymore, but it is flying high over our head, it is retained by the fishing line, otherwise would have already gone far away.

What happened meanwhile bathing? The sunshine heated the air inside the bag and expanded it's volume, first the bag was completelly filled by this expanding volume of air, the when he pressure rised inside much air begun to get out of the bag, as the heat was increasing the volume of the air kept inside expanded more and more and so more air was escaping the bag, up to the point in which the volume of the air displaced by the volume of the bag became to weight more than the bag with the hot rarified air inside the bag itself.

At that point the bag begun to float, as the heat was still increasing and the air inside the bag expanding further, more hot air was coming out of the bag, so the weight of the whole bag+air inside was decreasing, so the counterforce of the volume of air outside the bag was becoming even enough to lift the weight of the fish line to, so the bag begun to fly.

Finally the bag is flying high and is kept near us only by the rope, this happens because now the air inside the bag is weighing much much less than the air outside the bag.

So the archimede's principle works in every system, even in our lungs for example, that is why it is very healty to stay also some minutes upside down when making gimnastic, in this way we can discharge heavy toxic gases which tends to drop down in the bottom of the lungs and that normally in the erect position are difficult to expell.

Absuming that position which is typically joked as a strange thing of the yoga discipline we help mixing and expelling heavy toxic gases we waste from our blood, we do not think that this can help preventing many diseases for example, commonly it is seen as a funny thing that some strange persons do.

As we see the Archimede's principles is useful to understand a huge variety of phenomenons and things that happens everyday around us..... and inside us too...of course.

You probably don't get as many mosquitoes than I do where I'm from.

Yes, surface tension is a chemical property that extends to other fluids. I chose not to expound on it as it was not the topic in question.

From whence globalization mixed up things we have got now the terribly day active TIGER mosquitos, these are so aggressive they do not care to die at all, they just attack you and pinch the meat straight right, it seems that they have a mass group strategy, you reach to kill some (very easy to stamp them down to hell), but in the meantime many others drank blood already and they are gone.

It is a nightmare, I you are unprotected (picaridine, clothing, permetrine aerosols, geranium citronella and other essences in the air/on the skin etc.) you get sick of it in just 15-20 minutes, allergic people really risk immediate health hazards....

It is a serious problem, and we are not in a tropical area, but in the middle of the mediterranean area....

So I do not know how many are there where you live, but unluckily we are suffering a lot about mosquitos...and disappointgly reducing their reproduction is a measure as strong as a grain of sand in the desert in our areas, only well organized major disinfestations give some results, but it is not possible di disinfest every now and then for other reasons, so.....big problem...


If you're in a small boat on a small pond and you have in that boat a large brick, does the water in the pond go up or down if you throw the brick into the pond?
>>  If you're in a small boat on a small pond and you have in that boat a large brick,
       does the water in the pond go up or down if you throw the brick into the pond?

Floating in the boat, the brick displaces its weight in water.
Sitting on the bottom, the brick displaces only its volume.
So the water level goes down.
Very nice question
Since the force that water apply to the object is equal to the volume of water involved a very easy way to visualize when the object will be virtually floating alike being at 0 gravity is to imagine that it's volume must displace the amount of water that have the same weight of the object.

It might also be useful to compare and contrast a situation where "floating" material displaces water and a special case situation where floating material does not displace any water.

In the first case, a cylinder partially filled with water will show displacement when a block of wood is allowed to float at the water's surface. But what if instead of a block of wood we "float" a volume of an oil that is less dense than water? Is it still "floating"? Is there displacement equal to the weight of the oil?

The differences between the cases and the reasons for the differences can also lead to how and why displaced volumes of water can equal the weight of the floating material. The rigidity of a block of wood is important.

>The rigidity of a block of wood is important.

I do not see how rigidity of wood affects its ability to float. It is its micro structure that enables wood to float. It has large numbers of air filled spaces into which water only penetrates slowly. The main substance from which wood is made is cellulose and that is heavier than water. In fact once fine sawdust is properly wetted out it sinks in water. There are also many extremely hard, rigid woods which are heavier than water.

The example of oil and water is only a matter of specific gravity. As it is lighter than water it will rise to the surface of the water - à la Deep Horizon. However even that example is flawed because the surface tension of the oil makes it clump together so creating a mass some of which is below the surface of the ocean and some above - so it does actually displace water. It is a not good to assume that a lighter-than-water liquid necessarily covers the whole surface of the water on which it is floating. In a small container that might be the case but in the general environment that is rarely the case.
Let's take some talco powder and let's very finelly spread it onto the surface of the water in a big cylindric thank of 1 meter of diameter.

Let's now take a sngle drop of oleic acid and we drop it in the center of the water thank, the oleic acid will float on the water forming a moreless somehow oval patch the talco powder is displaced and we can see how much of the surface is now occupied by the floating oleic acid.

Is the oleic acid not spreading around and tending to stay in a determined area because iit's way is closed by the talc powder or it is tending to stay grouped with itself for other reasons?

Is it displacing water or not?

The oleic acid is so fluid that goes to create a so thin film on top of water that virtually we can think of it having one molecula nearby the other and none one on top of the other. If the moleculas have forces that attract one to the other they tend to stay togheter otherwise they can spread around freely.

I think that in a small cylinder adding oil on top of water there is not evident displacement of course, but because this happens in a special situation for which the heavier NON miscible (with the other) fluid goes toward the center of the heart simply for gravity effect.

As for chemical reason the two fluid cannot really be mixed but only emulsionated they will always tend to separate one from the other.

So if we close the cylinder and shake it as much as biking with hellrider on the rocky road and finally we obtain a fluid that seems to be mixed, and then we look at it with a microscope we discover that it is still not mixed, but only reduced in very small particles mixed togheter, if we leave it standingl for a while we will see the two fluids going back to their original positions, one on top of the other. All this happens for chemical reasons.

In a different situation the same oil would displace the water unless it's moleculas can freely spread around, but then again is  a single molecula displacing the water or not?

I think yes in an ideal enviroment, in the real world might not for other reasons. Do you agree?

Let's use a large low transparent cylinder which have the internal surface virtually "frictionless" just to make it more fun.

Instead of two fluids we pour in it an ideal mix of two differently coloured solid materials of very different specific weight, so one material pretty heavy and the other one pretty light, both shaped in particles having the form of extremely smooth small perfect same size spheres with a glidy surface.

Let the mix stay there for a while. What will happen?

I guess that if we do not give any energy to that system nothing special will happen at least appearently.

We give energy to the system and control if something happens.

We start irradiating the system with solar light, and we will see it better, but those materials are not influenced by the light so much that we can see something.

As the sunlight heated it a bit we decide to try with some more heat so we place it on the fire for a while, we might see something happening if the two material could melt at that temperature, or swollen so much to move, who knows what else, but those two ideal materials are just not showing special behaviours at this temperatures, so nothing happens.

Well,  try now to shake it circularly on the horizontal plane, that's also a way to give energy to the system, isn' it?

What happens now?

I guess that now the heavier material might tend to move down and the lighter one, looking as able to win gravity, seems to go up.

Is it true?

And why would not happen when not shaked? What stops the heavier particles from taking a lower position?

I think that the friction is the cause of the strange behaviour of this particles.

As we are able to imagine we take a time to do it, so let's coat the spheres in order to make them ideally "frictionless", on top of it we say that the spheres are soft and fragmentable but still always recreating smaller spheres with the same original qualities as said up to now.

The two different materials' spheres cannot be recombined if not with one of the same kind.

We shake it again, we even stirr it for a while.

What will happen now?

I guess that will be something similar to what happened with the water and the oil, so the spheres will be divided into two phases were the lower is occupied by the heavier material.

Is it true?

We add a third colured material having the same identical qualities of the others but the specific weight that will be half of the heavier one.

Will the third material place itself in a new middle phase?


Now let's take a solid frictionless rigid cube of a impermeable material that have a specific weight being the half of the heavier too.

We drop the cube in the cylinder.

Which position will take the cube when still?

Let's say that water is heavier than the three materials and let's pour a quantity of it inside the cylinder. Will it not go to the bottom forming another Phase?

If we place the cylinder in an ideal sea of water will float or sink?

Now we instantly remove the cylinder, what will happen to the system?

Your example is far too long to hold my attention.
>If we place the cylinder in an ideal sea of water will float or sink?

All depends on whether the sea water displaced by the cylinder weighs more or less than the weight of the cylinder.
Yes in fact it might be better to split it in two parts, to be valued, but still is possible to handle it at once it is not at all beyond our typical human possibilities we can add more and more ingredients before to overwhelm most minds, of course need concentration, should be done in a moment of full energy and attention

Yes correct....... we cannot say it as we do not have enough information about the cylinder's properties    (this tricky questions sounds strange but in reality are a very good way to check out )

In the last sentence of the example I did not precise that absumed the cylinder could float I was imagining to make disapper the cylinder only so to leave the layered cake of materials inglobating the floating cube without contention walls.

I would like to have a classic physics simulator anyone knows if nowdays exists a nice one having a comfortable GUI?
>In the last sentence of the example I did not precise that absumed the cylinder could float I was imagining to make disapper the cylinder only so to leave the layered cake of materials inglobating the floating cube without contention walls.

What on earth does that mean?
Move along everyone...
This thread is over, dead, and starting to smell...
Move along.  Move along...

It means that is funny to imagine how things could move from that point on, imagine the cylinder disappearing.

Let's say that the cylinder was of transparent material lighter than water, so when we placed it in the sea was floating.

Let's say that has 1cm thick walls, suddenly disappears what happens to the water around it and to the layered system inside it? How will they move around?

One of the ways to learn classic physics concepts is to try to visualize what happens.

If we do experiment to start with in laboratory but also in virtual, much more complex and fantasy friendly, laboratories auch as simulators, and why not, also using our mind, so to exercise the comprehension of this phenomenons in a more intuitive and analogic way, I guess it is much easier to pick up this knowledge.

It is easier then to study the math related to the laws we are examining as it is also easier to understand why what we thought was true or false when controlling the previsions.

Does anyone know about a good simulator?
It would be nice to reproduce this experiments and visually observe all of this.

Here some links to video I found showing what I mean:

Imagination  simulation and real experimenting is extremely important to make research and study. I remember the case of a research project stuck from a long time on the unexpected behaviour of a particular kind of particles conducted in a particle accelerator in Europe, the person who told me about was working there as young researcher and got involved pretty soon.

As he imagined that the particle had to behave as expected and that the machine had a defecftive part somewhere cause of the rpeated anomalous behaviour of the particle, overwhelmed by the complexity of the machine decided that first was wise to mathematically prove it's intuition.

This happened more than 30 years ago and first computers were available, being also a good programmer simulated the experiment with the computer, obtaining well different results.

He so went further with the simulation pointing out where in the machine could be found a problem.

A metal bar was found in the wrong place.
Finally the machine was repaired, the experiment repeated correctly, everyone was happy and the main project went on.

Imagination, fantasy, mental simulation, experimentation, virtual simulation, verification, repeatition etc. and then language, math, illustrations, videos, graphs etc. are fundamental in science studies, all what we can learn now on books was discovered in this way using the means available at the time of the discovery, and at the very beginning the lab logically had to be only the mind.

Give up - this subject is exhausted.
I do not see how rigidity of wood affects its ability to float.

It doesn't. Rather, it affects how displacement of a specific volume of water gets into the whole question of what is meant by "floating".

The "floating" on water comes about due to gravity having a stronger effect on the denser water. The water goes to the lowest level, effectively forcing the rigid block of wood out of its way. The actual thing that happens is that water sinks more than the wood. But we mentally reverse it into the concept of "floating" because it's hard to visualize the water "sinking".

In the case of a light oil, non-rigidity allows the oil to spread over the entire surface in the container. Not so with the block of wood.

The sinking of water is easily seen when a 10" tall 3" diameter cylinder has a 1" layer of the oil, and water is added. The water will 'sink' to the bottom.

Water also 'sinks' to the bottom when it's poured into a cylinder with a 1" cube of wood sitting on the bottom. But it can't "displace" the block of wood immediately. The weight of the wood holds it in place until the water level rises to the point where the volume of submerged wood reaches the volume of water that equals the weight of the wood. At that point, the water has reached enough pressure to force movement of the cube, and water can physically "sink" to the area below the wood. The pressure needed to move (displace) the cube is the pressure that is equal to the weight of the cube.

The rigidity doesn't affect floating. It simply lets us see how displacement volumes are related.

andiejeAuthor Commented:
What's happened to my post? I can't read all this information just for a simple question. Whilst I appreciate the lengthy answers I simply don't have the time to read them.

phoffric: are you still around if you i look at your comment and work through it?
Sorry, I am giving up on this thread, as it appears to have been hijacked onto interesting topics other than Archimedes principle.
It is true that the last umpteen comments were not on the original subject but the first 6 answerers were trying their best to answer the original question.
Then the author enlarged the question and the subject took off.
If the author had graded an early answer (if not truely satisfactory give low grade) she would have avoided the digression and some of the subsequent off topic essays.
She at least got some of her money's worth in the first answers.
Have her grade the relevant answers and move on.

I Think also that the asnwer was given in more ways and also that a good amount of extra information was given including digressions, links to other pages, illustrations, exercises and suggestions on how to approach in alternative and helpig ways this topics.

The author of the question should be happy to have got much more than enough.

I suggest to accept one or more comments or to PAQ the questions sharing beetween all those who contributed as I see that everyone added good information.

Author wrote in OP:
>> But i was wondering how floating is explained from a forces point of view.
>> Archimedes principle

http:#33545472 and http:#33550829 describes how floating is explained from a forces point of view w.r.t. Archimedes principle (as opposed to some other comments dealing with surface tension).
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