Buoyant force or water pressure?

DanW9494 · Jan 21, 2009

I don't think that "displacement" is wrong as an answer. It's just that to me it is more of a description of what is happening than the reason it floats or sinks.

The designer of a boat has to make a hull large enough to displace enough water that the weight of the boat and cargo will not sink it. That's not hard to do since we also want the hull large enough to give protection from waves.

But let's say that the designer wants a specific waterline. All they need is the working weight of the boat and then make a specific volume of hull below that specified waterline. That weight/volume ratio has to equal the weight/volume ratio or "density" of the water she will sail in.

That's why you said displacement and I said density.

Bill Roosa · Jan 21, 2009

Semantics

So when folks say "An object immersed in a liquid is buoyed up by a force equal to the weight of the fluid displaced." we are just talking about an interesting fact that is a natural consequence of having a hole in the water that is the same volume as my hull below the waterline.

They are not saying that displacing the water CAUSES the force but that it just happens to have the same magnitude (but opposite direction) as the weight of the boat.

The statement clearly states "...buoyed up by a force..."

So where does the force come from. and please don't tell me the displacement as that would be circular, a known invalid way to debate.

Ross · Jan 21, 2009

Re: Now pull the smaller bowl out

Bill, I am agreeing with you here.

DanW9494 · Jan 21, 2009

Re: Now pull the smaller bowl out

"An object immersed in a liquid is buoyed up by a force equal to the weight of the fluid displaced." is perfectly correct. The force comes from gravity's pull on the surrounding water wanting to go down which pushes the object up.

But whether that "buoyant force" is greater than gravity's pull on the object determines whether the object floats upward or sinks. Anything underwater has a buoyant force from displacement of the water, but that anchor you dropped stays down because it's gravity force is greater than it's buoyant force.

The net force on the object is determined by it's density relative to the water's density. If you take that anchor and could somehow inject air into it, you would decrease it's density more than you increased it's displacement until it would float like a steel buoy or a steel hulled boat.

Bill Roosa · Jan 21, 2009

Boyant force

Thanks Ross
Just getting tired of "the buoyant force from the displacement of water" argument.

It is circular and doesn't happen any more than the sun causes color.

Ross · Jan 21, 2009

Rather than pulling the bowl out simply bail the water out and watch the water level in the outer bowl receed as the inner bowl rises. That is displaced water on a very small scale . Exactly the same amount of water will be displaced in a bath tub or the river. I don't know who coined the phrase "bouyant force" But I am inclined to place it in the same catagory as "personal aura" and "life force".
I love to play with mylar balloons that have lost some of the helium. If I start cutting the ribbon shorter they will soon respond and start to rise in the air. If I tie a small piece of cotton to the ribbon and wet it with alcohol I will over load it and it will drag the cotton on the floor until enough alcohol evaporates and it gains neutral bouyancy. Then it will drift around in the air currents in the house as though it has a mind of its own. With the ribbon completely removed and a bit of weight attached to one of the broad faces the balloon can be made to float flat. Then no matter what you do, it will always return to the ballast down orientation.

DanW9494 · Jan 21, 2009

If there is no buoyant force, then why are things lighter under water than above water? That's why astronauts train under water.

Bill Roosa · Jan 21, 2009

Buoyant force

I disagree Ross
Bailing out the inner bowl is no different than bailing out my boat..

The buoyant force is pressure. All the little water molecules are bouncing around at random speeds, some slow and some fast but averaging around one speed. That average speed is dependant on the temp of the water, and the weight of the water over them (proportional to their depth in an open system) They are hitting each other and transferring momentum via electromagnetic forces. They have no preferred direction to rebound and so can bounce in any direction. They are doing this not because a solid volume is immersed in them they are doing this because gravity is trying to pull all of them down to achieve minimum energy state.

If I stick a solid object into the fluid the molecules no longer have a preferred direction to rebound. The ones that hit the solid object HAVE to rebound like ping-pong balls because the molecules in the solid object can not rebound. They have momentum transferred to them and have to react like any solid object. Now if the object is light (gravity effect) enough to be buoyed up by this transfer of momentum the object floats, if not it sinks. That is because the momentum transfer is greater as you go deeper into the fluid (down in a gravity field). Some part of the object is lower and so experiences greater momentum transfer. Some part of the object is higher and so experiences less momentum transfer. The net direction of momentum transfer is upward. A scuba tank is a good example. When full it sinks (it is heaver) when empty it floats (it is lighter) (both gravity effects) but in both cases it has the same amount of momentum transferred to it by the fluid because it has the same shape and size. The density does change but that is because you held volume constant and changed the amount of mass (weight in a gravity field). It is not the amount of water that gets pushed out of the way or the fact that it has some specific density it is that the force of gravity on the object is either stronger than the waters momentum (pressure) force or it is not.

This transfer of momentum in a fluid is called pressure. You could just as easily (well not so easy really) suspend your boat with a mess of air jets.

Buoyant force is the pressure of the water. If the displaced water was put back the pressure would still be there.

Bill Roosa · Jan 21, 2009

correction

Sorry I said in para 2
"If I stick a solid object into the fluid the molecules no longer have a preferred direction to rebound. "

I should have said
If I stick a solid object into the fluid the molecules have a preferred direction to rebound.

Ross · Jan 21, 2009

I went else where on the web for another opinion and found this.
http://www.answers.com/topic/buoyancy

Bill Roosa · Jan 21, 2009

Upward force

Right upward force.

When pressures acts on a surface you get force.

pressure * area = force Basic stuff

lb/in^2 * in^2 = lb

The area is the hull. So pressure on the hull is buoyant force. That would be water pressure outside the hull not some imagined water that the hull pushed (displaced) out of the way because it is a solid.

Displacement happens because it has to. But the amount of water being pushed out of the way does not cause the pressure. The two bowls experiment proves that. We produced the same buoyant force with less water. The amount of water is irrelevant. It just happens to always be exactly the size of the object that you put in the water because that is the way things work. The hole you produce can be filled with water and I'll agree that that amount of water you put back in the hole will weigh the same as it did when you pushed it out of the way in the first place. its the same volume of water. Of course it weighs the same.

The fact that the boat and the water weigh the same is to be expected because the hull is now intercepting the momentum transfer that was already happening through the hole's surface. By hole's surface I mean the hole that the boat makes has a surface that is shaped like the hull. You would HAVE to intercept that much momentum to hold the boat up. And it would HAVE to be that exact shape to hold the boat up in that position.

This is classic cart before the horse stuff.

DanW9494 · Jan 21, 2009

Re: Upward force

If water pressure was the buoyant force, then submersibles would be made of Osmium, and nothing would sink to the bottom in the deepest parts of the ocean where the water pressure is extreme. Wrecks would float around like ghosts. Fish couldn't swim down there, etc.

Rather, the density of water at the bottom of the sea is only slightly higher because of cold and pressure, because water is essentially not compressible.

Ross · Jan 21, 2009

jibes138 said:
Lets say you have water in a closed container that has a cylindrical bore in it. You insert a cylindrical boat into the cylindrical bore until it floats. The boat is sealed with a seal in the ID of the cylinder. Now you open a valve that allows you to apply more pressure to the water with a pump. Do you think the boat will rise in the bore or stay constantly in the same position? Pretty obvious the boat will rise. So position of the boat in the water, or depth of submersion is a function of water pressure. Water pressure is a function of the air pressure applied to the water surface plus the head of water above the measurement point. If the boat in the experiment was an oddball shape and you had a seal that would conform to the shape then the boat will rise and there will be less displaced volume of water because the pressure is greater so acting on a smaller surface area will float the boat. So the only reason the volume of water is equal to the weight of the boat in an open container is because everything is at atmospheric conditions.

Then it follows that you could pull a vacuum on the cylinder and sink the boat. :deadhorse.

DanW9494 · Jan 21, 2009

If I am understanding your experiment right, then you are demonstrating that air can compress. If you could replace all the air above the boat in the cylinder with oil, which floats since it is less dense than water, I don't think the waterline would change on the boat, after everything comes back to equilibrium, because I don't think the density of the water changed, because water cannot be compressed except at extreme pressure.

Now if you took that contraption and put it into the freezer, just before the water froze the waterline would lower as the water got denser, just as your boat floats higher in sea water than fresh.

Bill Roosa · Jan 22, 2009

How water works

Density is one way of looking at it. When I add air to re inflate my BCD as the water PRESSURE increases with depth to achieve neutral buoyancy what I'm really doing is changing my volume. If I hold mass (weight in a gravity field) constant and change my volume then yes my density changes. That density change is just like the displacement issue, it is a necessary consequence of the fact that 2 things cannot occupy the same space at the same time.

Moving on;
when you immerse an object in water you get the following;
a force from the water pressing down on the top
a force from the water pushing up on the bottom
a force on each side pushing left, right, forward, and aft on those sides.

The force on the top is NOT equal to the force on the bottom. Which is why when a ship sinks in deep water it makes it all the way to the bottom.

BTW if you examine the formulas in the picture you will see that the pressure at any level is exactly equal to the weight of the column of water above it.

Comments?

Ross · Jan 22, 2009

Often a little knowledge is a dangerous thing.

Bill Roosa · Jan 22, 2009

You got that right brother

Sometimes I hate it that I took 4 quarters of hydrodynamics.

Dean5735 · Jan 22, 2009

for every action there is and equal and opposite reaction
what goes up must come down

Wow this topic got em goin!!!!

Buoyant force or water pressure?

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