Have You Ever Photographed Something You Can't Explain?

I feel I need to address this claim. If I have a high pressure water hose and I point the flow at someone, they'll get knocked off their feet and bowled along. But the water will also bounce back off their bodies towards me. That sounds like a liquid reaction force to me.

I'm not sure anything can experience no reaction force, that seems to break Newton's 3rd.

The Wikipedia entry on buoyancy you referred me to says: "upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object."

That sure sounds like Newton's 3rd to me.

If I'm floating in space and I have a bag filled with water and I push it away from me, I'll accelerate in the opposite direction. Me and the water experiencing equal and opposite forces. And isn;t that how a rocket works in space? A chemical reaction results in a high speed flow of gas. The gas and the spacecraft accelerate in opposite directions as a result of Newton's 3rd - essentially the rocket and the exhaust push each other apart.

That same wikipedia article also says: * "In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pressure at the bottom of a column of fluid is greater than at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object. The pressure difference results in a net upward force on the object. "*

It claims this is an explanation but it seems incomplete to me. Let me break it down:

• In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. *

Agreed

• Thus the pressure at the bottom of a column of fluid is greater than at the top of the column. *

** I would actually word this differently:

The pressure exerted downwards at the top of a column of fluid is less than the pressure exerted downwards at the bottom of the column" but otherwise agree. **

• "Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object. The pressure difference results in a net upward force on the object." *

** Assuming the object is a rigid solid object, this bit I find confusing.

Why does the bottom of the object exert a greater downward pressure than the top of the object? There is no water in between the top of the object and the bottom of the object, just the body of the object itself.

What this seems to me to be trying to imply is that the difference in pressure at the top and bottom of the object has nothing to do with the column of water the object is supporting or the column of water below it that it is resting on, but is caused by the material of the body itself between its top and bottom surfaces. To me this will be the case irrespective of what depth the body is at, which implies buoyancy has little to do with the depth of the water and a lot to do with height of the object. **

ps

The bolding and italics in this editor stop working after the first para

EDIT

Ok, so I have a new formulation:

1. The force that a submerged body feels on its top surface is the weight of the column of water above it

2. The force the top surface of the column of water below the submerged object feels is 1) above plus the additional force that comes from the weight of the submerged object itself. This is greater than 1) above This implies the water column above and the object itself should be treated as a single composite object.

3. The upwards thrust the bottom surface of the submerged object feels is simply the reaction force (Newton III) the top surface of the column of water below the submerged object exerts on the bottom surface of the submerged body. Therefore the upthrust is equal to the downwards force of the water above the object plus the downwards force of the object itself.

Something about this formulation feels more realistic to me.

Although we still have the problem that with this formulation, the downwards force of the combined water column + object is always equal to the upwards thrust of the water below the object which means the object is always in a static position at any depth, never sinking or rising. This is clearly counterfactual so something is missing or just plain wrong. I still have this vague sense that the something missing has to do with the object having to push the underlying water out of the way to make space for itself. Hmmm. Tricky.

ps

What you are seeing here is reasoning happening in real time. Not very good reasoning, admittedly, but you could regard it as a spectator sport....

pps

I seem to recall something Einstein said. Something about he got very confused and muddled when thinking things through. It didn't come easily. So he had to develop a very disciplined step by step approach to thinking, carefully defining each element and examining it very closely for consistency. I'd like to think I had some kind of kinship with Einstein's methods.

A kind of very stupid version of Einstein. Einstein as a dunce...

Exactly!
This is Pascal law, as the result of chaotic and (statisticaly) omni directional forces between molecules of a liquid/gas. Solids does have molecular forces, but they act different way in this aspect. So the forces transfered by the fluid on the submerged objects are just locally perpendicular to their surfaces, they don't have prefered direction beyond this. This is the very cause that water column weight acts down at the top surface of David's model cube (or my cylinder) and up at its bottom.
I was trying to explain it to David several posts ago, maybe I should have been stressed some aspects more. I also think this is the clue he lacks of here. And because the height/weight of this column is different by the height of cube/cylinder, so is the difference and net force of buoyancy pointed up.
Regards,
-J.

I was out for a couple of days doing photography and trying to straighten out my thoughts on this. I have read a hundred posts on the internet about newton's 3rd law and buoyancy and none of them make any sense to me. It seems that the arguments mix up archimedes and force level descriptions indiscriminately or they are circular reasoning or they start off proposing a rational description then suddenly produce buoyancy upthrust out of a hat. Didn't help.

However, as a result, I have been able to carefully write down a step by step model of buoyancy starting at the sea bed and working up to the surface that attempts ot explain buoyancy in terms of newton III.

It nearly, but doesn't quite work.

The problem being that it proposes that no matter what the mass of the submerged body is, the downforce form weight and the reaction force from newton iii always balance out and it is impossible for anything to sink or rise.

Clearly, there is an error. I reproduce my model below for someone to work out the flaws:

===========

The ocean is a liquid made of water molecules.

Gravity is a downwards force. Gravity pulls every water molecule downwards. They fall until they reach the solid surface of the sea bed, then they form a layer of water molecules constantly forced down by gravity and held up by the Newton iii reaction force between seabed and the layer of molecules - no net force, so they just sit there.

Gravity then pulls another layer of water on top of the first layer. The first layer has to support the second layer via its own reaction force. We now have the solid surface supporting the first and second layers via the Newton III reaction force and the first layer supporting the second layer through Newton III reaction force. There is no net upward or downward force, so everything is static in this arrangement.

Then the third layer arrives and lays on top of the second layer. The seabed now has to support the first layer, the second layer and the third layer through Newton III reaction force. The first layer has to support the second and third layers through Newton III reaction force. The second layer has to support the third layer through Newton III reaction force. Downforce form gravity and up thrust from Newton III net out, there is no net force and everything is static in this arrangement.

The increase in down force and the matching increase in newton iii reaction force with every layer added, happens right up to the surface layer (which has to support only the air). The bottom layer is now supporting all the layers above it via Newton iii. Each successive layer above the surface supports the remaining column.

The result of this is that the downwards force from the weight of the water layers increases with every layer downwards you go. In other words the downwards force due to gravity increase with depth. The newton III reaction force also increases in lockstep.

If you embed a body anywhere in the depth of the column, it will feel a downforce from all the layers of water above. And it will feel an equal newton 3 upthrust from the layer below it. This will happen at every water level (depth) you place it.

Because the body and the water cannot occupy the same volume, the body has to displace an equal volume of water. The weight of the body then replaces the weight of the displaced water.

If the density of the body is identical to the water it replaced, the downforce (weight) remains the same and the newton iii reaction force from the layer below remains the same and nothing changes.

If the density of the body exceeds the density of the water it replaced, the overall downforce on the water layer below increases. The newton iii reaction upthrust will also increase by the same amount. Nothing changes.

If the density of the body is less than the density of the water it replaced, the overall downforce on the layer below decreases. The newton iii reaction upthrust will also decrease. Nothing changes.

Conclusion: it is impossible for a body to float or sink in water, it will always remain static.

==============

The above description sets out my thinking as clearly as I can frame it. There is an error somewhere. But laid out step by step like this it should be easy to point exactly where the error is.

Go for it 😁

EDIT:

ps

I have a proposal for an additional buoyancy force that could explain lift. Although water is largely incompressible, it isn't completely so. There is a small density increase with increasing depth and therefore a small density gradient. This means that the bottom of the object is struck by more water molecules' Brownian motion than the top of the object, resulting in a small upwards force.

pps

And in terms of what might provide an additional sinking force, I can't help be drawn back to the fact that a body submerging has to push the existing water out the way. That sounds like it must drain some force away from the downwards or upwards direction...

Hi, David!
I really don't know if You really misunderstand basic physics or just trolling pulling our leg for fun. But OK., I'll try to play a teacher once again.
This time I'll correct Your model:
You claim that upper layer of molecules just force thewir weight on lower layers and that's all. Nope. Liquid particles are no pin-pong balls that lay on each other. They are constantly moving in chaotic way. Molecular forces between them may be both reppeling and atracting, that causes maintenance of average distance between them. So, any external force pushed globaly on some liquid volume (note: this can be also the force of weight of another part of this liquid volume) can be actually transfered within the liquidin any, not only one direction as You wrongly claim. every school level experiment proves that, from the ball with holes aroundand and a piston to just U-shaped pipe... This is opposite to the solids, where molecules do not move freely, so they are maintaing also their position within a piece of material more or less (forget about thermal vibration for a moment). This causes only minimum change of the shape of the solid under a force, sometimes microsopic for us. Its own weight for example. That's why block of metal stays in one piece/shape when put on table, but if You melt it to the liquid, it will spill around to the sides like water, unless You protect it with outer vessel/form. So a solid cannot redirect the force of weight to its sides, a volume of liquid can.
Once again: This is the fact You're ignoring (by mistake or deliberately for fun/trolling) in Your models. And this fact leads to Pascal law which is the key to buoyancy force: the weight of longer liquid column giving pressure to the bottom of submerged object can create force on that bottom which is pointed up.
If Your wrong assumption (weight of liquid cannot be dissipated multidirectional) was correct, You just couldn't sink a boat by drilling a hole in its bottom. Simply the water couldn't be able to leak up into the boat. Please, do not try it at home, if the water is dangerously deep 😁😁😁
Regards,
-J.

This thread really shows why threaded view mode should be highest priority on the feature request list...

Ok, thanks. I'll have a think about both your responses and try to correct my model accordingly.

EDIT: Clearly I mis-understand basic physics, but it's been 45 years since I was taught physics. I feel that as weak as my understanding is at the moment, I bet I have a far better grasp of it today than most of my former classmates from Wyvern comprehensive school O-level physics class of 1978!

Agreed. Sub threads that pop up spontaneously need to stay on their own little branch where they can be ignored by anyone not interested in the sub thread.

Yes. I'm not saying that (another) in-depth laws-of-physics duel is without interest, but it's not what I had n in mind when starting the thread.

I think it's unfair to characterise this branch as a dual. I'm not arguing with anyone, I'm listening to what they say and trying to put together (slowly) a consistent picture that works. And making lots of mistakes in the process. The sub thread is really about educating me. I don't hold a confident position about any of this, I'm trying to build one with these guys' very kind and very patient and very appreciated help. This is probably one of the more co-operative endeavours on the whole forum. But yes, it should be on a sub branch of the thread.

Arvo: Corrected to include contribution of earth gravity-molecule gravity action-reaction pairs and seabed-water molecule electric action-reaction pairs

The ocean is a liquid made of water molecules.

The force of gravity operates between any matter. It is an attractive force that pulls matter bodies towards one another. In the ocean gravity operates between every water molecule and the Earth, pulling them together. The Earth pulls on a water molecule and the molecule pulls on the Earth. These are opposite and equal gravitational forces.

However, because of the greater mass of the Earth, we can simplify this situation by neglecting the contribution of the molecules and treating the scenario as if the Earth was doing all the pulling.

This means it's as if the water molecules are falling until they reach the solid surface of the sea bed, where they form a layer of water molecules constantly forced down by gravity. The reason they don't sink into the seabed is because they are held up by the Newton iii reaction force of electostatic/Pauli exclusion between the atoms of the seabed and the layer of molecules. Gravity pulls down on the water molecules until they are in contact with the seabed where electric forces set up a pair of action reaction forces that are equal and opposite. There is no net force, so no acclereration.

Gravity then pulls another layer of water on top of the first layer. The first layer has to support the second layer via its own reaction force. We now have the solid surface supporting the first and second layers via the Newton III reaction force and the first layer supporting the second layer through Newton III reaction force. There is no net upward or downward force, so everything is static in this arrangement.

Then the third layer arrives and lays on top of the second layer. The seabed now has to support the first layer, the second layer and the third layer through Newton III reaction force. The first layer has to support the second and third layers through Newton III reaction force. The second layer has to support the third layer through Newton III reaction force. Downforce form gravity and up thrust from Newton III net out, there is no net force and everything is static in this arrangement.

The increase in down force and the matching increase in newton iii reaction force with every layer added, happens right up to the surface layer (which has to support only the air). The bottom layer is now supporting all the layers above it via Newton iii. Each successive layer above the surface supports the remaining column.

The result of this is that the downwards force from the weight of the water layers increases with every layer downwards you go. In other words the downwards force due to gravity increase with depth. The newton III reaction force also increases in lockstep.

If you embed a body anywhere in the depth of the column, it will feel a downforce from all the layers of water above. And it will feel an equal newton 3 upthrust from the layer below it. This will happen at every water level (depth) you place it.

Because the body and the water cannot occupy the same volume, the body has to displace an equal volume of water. The weight of the body then replaces the weight of the displaced water.

If the density of the body is identical to the water it replaced, the downforce (weight) remains the same and the newton iii reaction force from the layer below remains the same and nothing changes.

If the density of the body exceeds the density of the water it replaced, the overall downforce on the water layer below increases. The newton iii reaction upthrust will also increase by the same amount. Nothing changes.

If the density of the body is less than the density of the water it replaced, the overall downforce on the layer below decreases. The newton iii reaction upthrust will also decrease. Nothing changes.

Conclusion: it is impossible for a body to float or sink in water, it will always remain static.

==============

Yar

Many years ago when my nephew was a little boy learning science, I challenged him to explain the forces at work with hot air balloon flight. I never got an answer from him but I thought about it a lot myself. And I came up with a model that used the kinetic motion of warm molecules to explain it. It involved differential forces from Brownian motion applied to the top and bottom of the balloon because of the small density differences of the air at different altitudes. I'm afraid that this model attracted a degree a ridicule so I kept quiet about it while I gave the matter further thought. I have been tempted to bring this back into this discussion but dissuaded from doing so by the insistence that water is in-compressible and there can be no density gradient and thus no mismatched forces at depth.

You seem to be saying that the kinetic motion of molecules in a liquid does play a role, even if a slightly different one from what I envisaged. I can't quite discern from your post exactly what that role is, however, other than that it allows for forces that in my simple model can only be directed upwards to be directed in other directions. If that is the case, it provides a mechanism for solving the static action-reaction problem my model creates. This is good! I will look up this Pascal Law you mentioned, but in the meantime, perhaps you could clarify how you think the kinetic motion of molecules changes the distribution of forces? In particular, how it increases them so there can be a rising force.

EDIT: Wikipedia talks about Pascal's Law as something that applies to liquids that are confined within a container. Is this the situation when talking about the ocean? The ocean isn't really confined, it can slosh about and rise and fall relative to the land. Maybe this doesn't matter for this discussion, I couldn't say.

Imagine a sphere made of a thin, ultra lightweight exotic metal of infinite strength. The entire skin of the sphere masses 1g and is 100m in diameter.

It has a valve.

You connect this valve to a pump and remove all the air from within the sphere so it contains a hard vacuum. The skin is so strong it holds itself in shape despite the removal of the air.

You release the clamps holding it in place and it flies rapidly up into the sky.

Why does it do this? Why does removing the air suddenly provide an upthrust when there was none before?

Can we draw before and after diagrams portraying the forces impinging on the sphere prior to and after evacuation of the air...

I'm really struggling to understand why it not as simple to do this as it is to draw force diagrams for 2 colliding billiard balls, and yet it clearly isn't. I guess the reason is because there are multiple forces that resolve to the upthrust and this makes a forces picture very complicated. I'd still like to see it though. There is nothing complicated about my desire to model this scenario using forces. I don't understand why the rest of the world thinks it is very strange to want to do this. Introductory mechanics is full of colliding billiard balls and force diagrams. This is just one more example...

Pressure is point forces averaged over a surface so it's just a more complicated version of force. But for me, a more difficult concept to work with because when I think about pressure I'm automatically trying to reduce it to a single point force. And if we go down that route we'll end up measuring buckets of water displaced by a body. i can already work with the Archimedes approach but, to me, while it sounds like an excellent description of what happens, it lacks step by step explanatory power. I don't need to reduce things to quantum field theory, but point forces seems like a good explanatory level.

It's what we use to explain rocket ships, a balloon should be easy!