Buoyant force or water pressure?
- Thread starter jibes138
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Help! help! I'm being repressed
Come and witness the violence inherent in the system
Help! Help! I'm being repressed
They are trying to take my silly season away!
They are trying to take my silly season away!
Next someone will declare himself "King of the thread" and arbitrally end it. Just because you had some fascicle aquatic ceremony with some watery tarts does not mean you are my king.
I will not go quietly into the night!
Come and witness the violence inherent in the system
Help! Help! I'm being repressed
They are trying to take my silly season away!
They are trying to take my silly season away!
Next someone will declare himself "King of the thread" and arbitrally end it. Just because you had some fascicle aquatic ceremony with some watery tarts does not mean you are my king.
I will not go quietly into the night!
I'm feeling better now
I've moved myself, my teddy bear and my laptop under the desk and am sipping my large tumbler of purple cool-Ade.
I've moved myself, my teddy bear and my laptop under the desk and am sipping my large tumbler of purple cool-Ade.
Ok, my last post on buoyancy.
I have been trying to argue that the buoyant force is due to the weight of the displaced water rather than the water pressure.
As I've found out it is hard to convince you as far as boats are concerned, since a boat is basically a column of air pushed down into water such that the water pressure at the bottom of the column is due to the weight of the water displaced.
But I will use a fact from Roger's post #53 to explain that buoyant force is not the same as the water pressure, except in the situation where the object is floating.
Per Roger: "Even a cubic foot of lead has buoyancy in water. The weight in air is 708 pounds in air and 643 pounds in sea water."
My point is that a cubic foot of lead weighs 643 lbs in sea water, whether its bottom is at 1 foot depth where the net water pressure is 0.44 PSI, or whether the cubic foot of lead is 100 ft down where the water pressure is 44 PSI.
If water pressure was the buoyant force, then anchors wouldn't set as well at 30 feet as they do at 10 feet and lost anchors would be floating in the ocean about 1000 feet down. Steel hull wrecks would be closer to the bottom than aluminum hull wrecks, but nothing manmade would be on the bottom of the oceans.
It is only at the surface that water pressure can be substituted for buoyant force. Submariners trained in Groton wouldn't make that mistake.
I have been trying to argue that the buoyant force is due to the weight of the displaced water rather than the water pressure.
As I've found out it is hard to convince you as far as boats are concerned, since a boat is basically a column of air pushed down into water such that the water pressure at the bottom of the column is due to the weight of the water displaced.
But I will use a fact from Roger's post #53 to explain that buoyant force is not the same as the water pressure, except in the situation where the object is floating.
Per Roger: "Even a cubic foot of lead has buoyancy in water. The weight in air is 708 pounds in air and 643 pounds in sea water."
My point is that a cubic foot of lead weighs 643 lbs in sea water, whether its bottom is at 1 foot depth where the net water pressure is 0.44 PSI, or whether the cubic foot of lead is 100 ft down where the water pressure is 44 PSI.
If water pressure was the buoyant force, then anchors wouldn't set as well at 30 feet as they do at 10 feet and lost anchors would be floating in the ocean about 1000 feet down. Steel hull wrecks would be closer to the bottom than aluminum hull wrecks, but nothing manmade would be on the bottom of the oceans.
It is only at the surface that water pressure can be substituted for buoyant force. Submariners trained in Groton wouldn't make that mistake.
My last post too
DanW
You cannot consider a cubic foot of lead as an object that does not have height. Assuming a 1'x1'x1' lead block at the surface there is no pressure on the top and 62.4 lb/ft^2 on the bottom for a net upward force of 62.4 lb
at 400 ft there is a downward pressure on the top equal to
P=400*62.4
so the force pressing downward is
Ft=400*62.4*1*1=24,960.0 lb
and an upward pressure on the bottom equal to
P=401*62.4 Note the 401 because the lead block has a height
so the force pressing upward is
Fb=401*62.4*1*1=25022.4 lb
summing the forces with up being positive
Fb-Ft=25022.4-24960=62.4 lb upward. A buoying up force equal to the weight of the water displaced.
DanW
You cannot consider a cubic foot of lead as an object that does not have height. Assuming a 1'x1'x1' lead block at the surface there is no pressure on the top and 62.4 lb/ft^2 on the bottom for a net upward force of 62.4 lb
at 400 ft there is a downward pressure on the top equal to
P=400*62.4
so the force pressing downward is
Ft=400*62.4*1*1=24,960.0 lb
and an upward pressure on the bottom equal to
P=401*62.4 Note the 401 because the lead block has a height
so the force pressing upward is
Fb=401*62.4*1*1=25022.4 lb
summing the forces with up being positive
Fb-Ft=25022.4-24960=62.4 lb upward. A buoying up force equal to the weight of the water displaced.
My really last post
The purple Cool-Ade is causing me to be quite dizzy
The fact that there is a buoying up force equal to the weight of the water displaced does not tell me if the block of lead is stable or unstable, just that it will sink.
The purple Cool-Ade is causing me to be quite dizzy
The fact that there is a buoying up force equal to the weight of the water displaced does not tell me if the block of lead is stable or unstable, just that it will sink.
Pressure difference!!!!!!!!! We are all at atmospheric pressure and it is not a problem because the pressure is everywhere!!!!! Even scuba diving down at 120' I don't feel any pressure because it is everywhere!!!! If my ears or sinuses hurt is is because there is a pressure difference!!!!!! A floating boat only experiences pressure on the parts below the water. This is a net force upwards because the water is only pushing with a net upward force. Once the boat sinks there is no net upward force assuming that the net density is more than or equal to the water it is displacing. If it's density is less than water it will float. Anything less dense will float be it a naked woman or a log!!!! Lean meat sinks and fat floats. Of course pure water's specific gravity is 1 by definition. Adding salt increases the density such that water in the dead sea is much more dense than water falling out of a cloud!!!! Cream mostly fat floats to the top of whole milk. DNA floats higher in a cesium chloride gradient than does RNA. Cold water is more dense than hot water which is part of the reason that we have the Gulf Stream carrying warm water to the north polar regions.
My problem is that I did not get to sail long enough on Sunday and not at all on Monday!!!! Sunday we went after church and monday was too cold with a high of only 37 degrees. Once I get a good sailing fix I won't think so much about trivia.
My problem is that I did not get to sail long enough on Sunday and not at all on Monday!!!! Sunday we went after church and monday was too cold with a high of only 37 degrees. Once I get a good sailing fix I won't think so much about trivia.
Bill ,I think I can help your headache. If there was a large enough container filled with sand and a fluidizing force was applied to such sand would a boat float on the sand as long as the sand was fluidized? And when the sand was no longer fluidized could that sand be stabilized and the boat lifted out leaving hole equal to the displacement?
A cubic foot of lead
Regardless of the depth of the lead the sum of the forces acting on the lead will be exactly zero assuming it has reached the bottom. When something sinks in water it reaches some maximum velocity because the sum of the forces are a constant at all depths so there is a net force pushing the object downwards greater than the force pushing it up. This is shown clearly in Bill's diagram. A submarine has a net force pushing it downwards due to pressure differential top to bottom but it can change its density to compensate, in essence it weighs less as it reduces ballast so the downward force is reduced. This is what a scuba diver does. The forces on a submerged object are the same as on something on the surface, there are forces all over the object and whether it is in motion (sinking) or not in motion (floating) are related to whether the forces sum to zero or something other than zero.
By the way who is it that thinks air pressure is negligible? Last time I looked it was 14.7 PSI at sea level (depending on the weather). So on your cubic foot of lead you have 2,116.8 pounds of air pressure pushing on each surface of the block. Heat up an olive oil can on the stove, cap it, then dunk it in cold water and watch the air pressure crush the can like it was the saran wrap I mentioned. Our bodies are mostly water so this air pressure is pretty uniformly distributed. The pressure at depth when diving is pretty uniformly distributed also except nitrogen becomes more soluble in blood as pressure increases leading to known problems.
AT about 8,000 feet the air pressure is only about 11 PSI, if you could hold your breath from sea level to 8000 feet you might explode.
Regardless of the depth of the lead the sum of the forces acting on the lead will be exactly zero assuming it has reached the bottom. When something sinks in water it reaches some maximum velocity because the sum of the forces are a constant at all depths so there is a net force pushing the object downwards greater than the force pushing it up. This is shown clearly in Bill's diagram. A submarine has a net force pushing it downwards due to pressure differential top to bottom but it can change its density to compensate, in essence it weighs less as it reduces ballast so the downward force is reduced. This is what a scuba diver does. The forces on a submerged object are the same as on something on the surface, there are forces all over the object and whether it is in motion (sinking) or not in motion (floating) are related to whether the forces sum to zero or something other than zero.
By the way who is it that thinks air pressure is negligible? Last time I looked it was 14.7 PSI at sea level (depending on the weather). So on your cubic foot of lead you have 2,116.8 pounds of air pressure pushing on each surface of the block. Heat up an olive oil can on the stove, cap it, then dunk it in cold water and watch the air pressure crush the can like it was the saran wrap I mentioned. Our bodies are mostly water so this air pressure is pretty uniformly distributed. The pressure at depth when diving is pretty uniformly distributed also except nitrogen becomes more soluble in blood as pressure increases leading to known problems.
AT about 8,000 feet the air pressure is only about 11 PSI, if you could hold your breath from sea level to 8000 feet you might explode.
Re: My really last post
I didn't realize there were "really last post"s, but I guess I want one too. Is there something afterthat?
Bill, I like your logic with the submerged cube of lead. Can't really argue with it, so I'll ask you to explain something real, that I don't think water pressure can explain.
Submarines can dial in their depth with ballast and ballast trim tanks which are measured and regulated such that you can dial in your depth for a known temperature and salinity very well by getting the right amount of air and water in each tank. The displacement of the boat doesn't change because valves close off the tank the rest of the time such that the hull is intact.
So if water pressure is the buoyant force, how could water pressure explain a prop-stopped submarine going up to the surface very slowly while blowing out water occasionally or blow it all and take out another Japanese trawler and still get the bow out of the water.
I didn't realize there were "really last post"s, but I guess I want one too. Is there something afterthat?
Bill, I like your logic with the submerged cube of lead. Can't really argue with it, so I'll ask you to explain something real, that I don't think water pressure can explain.
Submarines can dial in their depth with ballast and ballast trim tanks which are measured and regulated such that you can dial in your depth for a known temperature and salinity very well by getting the right amount of air and water in each tank. The displacement of the boat doesn't change because valves close off the tank the rest of the time such that the hull is intact.
So if water pressure is the buoyant force, how could water pressure explain a prop-stopped submarine going up to the surface very slowly while blowing out water occasionally or blow it all and take out another Japanese trawler and still get the bow out of the water.
The submarine first achieves neutral buoyancy and then "flies" underwater. They can come up to periscope depth and remain motionless. Or they can desend to 250 feet and lie on the bottom. When they blow ballast they become positively buoyant and rise to the surface just as a cork would.
Submarine
The submarine reduces its' weight by removing ballast. So the downward force f=mg is reduced. If it was neutral to start with the sum of the forces is now positive upward and the sub will rise. To go down it adds ballast so the force f=mg is positive downward and the sub will sink. It still comes down to summation of forces. For the submerged object :
mg+pressure force on top side of hull - pressure force on bottom of the sub = zero.
For a submerged object that argument of the water rushing back in trying to push the boat up doesn't make sense since water is trying to rush in on top of the sub and push it down.
The sum of the forces is standard engineering practice, so argue all you want.
The submarine reduces its' weight by removing ballast. So the downward force f=mg is reduced. If it was neutral to start with the sum of the forces is now positive upward and the sub will rise. To go down it adds ballast so the force f=mg is positive downward and the sub will sink. It still comes down to summation of forces. For the submerged object :
mg+pressure force on top side of hull - pressure force on bottom of the sub = zero.
For a submerged object that argument of the water rushing back in trying to push the boat up doesn't make sense since water is trying to rush in on top of the sub and push it down.
The sum of the forces is standard engineering practice, so argue all you want.
Repealing the law of gravity
Ross, why would you ask such a thing> I noted above in my equation for summation of forces that mg is one of the forces. F=mg is mass times gravitational constant for acceleration.
Now we all know gravity is a function of distance from the objects center. Perhaps this whole issue comes down to the fact that a submerged object has more gravity working on it since it is closer to the center of the earth. So as it sinks the force making it sink continues to increase. By assuming it is a constant is really ignoring the effects of the variability over distance. For a floating object that can be ignored but the sub may need to know this. Also we know the earth is not perfectly round so we may want a gravity correction factor depending on position.
Ross, why would you ask such a thing> I noted above in my equation for summation of forces that mg is one of the forces. F=mg is mass times gravitational constant for acceleration.
Now we all know gravity is a function of distance from the objects center. Perhaps this whole issue comes down to the fact that a submerged object has more gravity working on it since it is closer to the center of the earth. So as it sinks the force making it sink continues to increase. By assuming it is a constant is really ignoring the effects of the variability over distance. For a floating object that can be ignored but the sub may need to know this. Also we know the earth is not perfectly round so we may want a gravity correction factor depending on position.
Good thing too.
Submariners would learn this on the first day:
Water pressure increases with depth.
Consider a square foot cube of something with it's top one foot below the surface. Pressure on the sides creates no up or down force. Pressure on the top is 64 pounds per square foot downwards. Pressure on the bottom is 128 pounds upwards. The difference is 64 pounds upwards, exactly equal to the weight of the water displaced for the cube.
Water pressure will increase 64 pounds per foot for every foot increase in depth all the way to the bottom of the Marianas Trench so the relationship will stay the same. The math and analysis it more difficult for different shapes but the answer comes out exactly the same.
Water pressure is buoyancy both on the surface and below it.
My really really last post
see attachment
This is the same diagram as before except now we have a ballast tank that can be filled or pumped dry as desired. Note the addition of the "Wbal" term in the summation at the end. Now I have a variable that, given good design, will allow me to vary the weight (and since volume displaced is constant the density) of the "sub"
Those of you who have an eye for balancing forces will note that this is a poorly designed sub because the ballast tank is clearly not "centered" in the hull.
I would ask the group, is this a stable or unstable sub?
And
How does the fact that a sub displaces a volume of water equal to its weight give you any insight to its stability?
and
How does knowing the density of the sub tell you if the sub is stable or unstable?
I'm not trying to say those statements are incorrect, just not very useful in discussing stability.
see attachment
This is the same diagram as before except now we have a ballast tank that can be filled or pumped dry as desired. Note the addition of the "Wbal" term in the summation at the end. Now I have a variable that, given good design, will allow me to vary the weight (and since volume displaced is constant the density) of the "sub"
Those of you who have an eye for balancing forces will note that this is a poorly designed sub because the ballast tank is clearly not "centered" in the hull.
I would ask the group, is this a stable or unstable sub?
And
How does the fact that a sub displaces a volume of water equal to its weight give you any insight to its stability?
and
How does knowing the density of the sub tell you if the sub is stable or unstable?
I'm not trying to say those statements are incorrect, just not very useful in discussing stability.
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