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Let's try again {32pp of topic discussion by Mindbuilder v DHeyward v Tobermory}
Latest comment: 10 years ago28 comments3 people in discussion
Do you fundamentally agree that the only work done is the gravitational difference in height between the upper vessel surface and the downward pipe outlet? That the release of gravitational potential energy is why a siphon flows? --DHeyward (talk) 00:36, 28 April 2014 (UTC)
The atmosphere exerts a downward force on the surface of the reservoir and the surface of the reservoir moves downward. That's force times distance. That's work right? Or you could look at it as the atmospheric pressure pushing the liquid up the siphon is a force times distance, again work. Do you think the atmosphere does no work pushing the liquid up a barometer or drinking straw? Of course atmospheric pressure won't push liquid up an up tube alone, I don't see how that bears on the issue. The atmosphere does work pushing the liquid up. Then gravity does work on the atmosphere as it pulls the liquid out the down tube. You're right that the only significant net work done is from the decrease in liquid altitude from entrance to exit, but there is much more work done that cancels out. It's like a mass oscillating up and down while hanging on a spring. Neglecting friction losses in the spring there is no net work done, but gravity does work on the mass as it goes down, then the spring does work on the mass going up. The work just moves the energy round and round between kinetic, potential, and spring tension energy. But the work still happens. I agree that the release of gravitational potential energy is why a siphon flows.
And considering that this is a long controversial article with a long established fragile consensus, it would be best if you proposed any controversial wording changes here in talk before changing them in the article. I'll have more comments shortly. Mindbuilder (talk) 01:15, 28 April 2014 (UTC)
Talking about the bulk pressure effects is too confusing. Lets go back to the single water molecule. We need to talk about the single molecule because that is the only way to make the forces clear. Going back to my numbered questions, I asked (1)what object do you think exerts a force on our molecule in the middle of the [static] siphon at the top of the water, to prevent gravity pulling it down? You answered "The pressure difference between the air at the surface of the bucket and the air bubble supports the weight of the column." But I wasn't asking about the whole column, just the one molecule. Was it the air molecules above pulling up on the one molecule or was it the water molecule below pushing up on the one molecule or some other molecule or what? Mindbuilder (talk) 01:29, 28 April 2014 (UTC)
No, you are complicating it beyond necessity and the assertion that the atmosphere does work in the system is not correct. The sum of atmospheric forces is 0. It does no work. Work is entirely gravitational and is approximately ρgh and converts gravitational potential energy into the work performed There are hydrostatic conditions that set limits on being able to use that potential energy and it varies b liquid, but it doesn't change the fundamental fluid dnamics that the net work of the atmosphere is 0 and gravity is the driving force. No reliable sources say otherwise. Some easy principles: [1][2] and recent article that siphon flow is independant of barometric pressure until cavitation limits are reached. [3]. --DHeyward (talk) 03:58, 28 April 2014 (UTC)
If a molecule is moving up in the siphon at a somewhat steady speed, there must be a force on it other than gravity, right? Am I wrong about that last sentence? Please confirm explicitly one way or another if I am right or wrong about that last sentence. Please don't ignore it or answer it indirectly. Please answer it with a yes or no and then an explanation if you wish. And please don't answer some other question similar to but not the same as the one I asked. We need to find out what we agree on. That is a force over the distance of the molecule's movement. That is something doing work, and it can't be gravity unless you are talking about negative work. Any time an object with mass is moving up in a gravitational field, something is doing work on it, unless it is just launched balistically. What is doing that work? Pressure? Pressure is a property of a bunch of molecules. What molecules having pressure are providing that UPWARD force to do work on the molecule moving up? By what molecules are providing the force, I mean what kind of molecules and where are they? Mindbuilder (talk) 04:32, 28 April 2014 (UTC)
I read your link one at lhup.edu. I didn't notice anything in the article really wrong. Is your understanding of the siphon the same as that link, or do you see things wrong in that link?
He states at the top that it is false that "...air pressure somehow drives the liquid flow." This seems to be what you have been saying. I think it is phrased in an unclear manner, but not wrong in the sense that it was intended.
But to make clear how it was intended you can look farther down in the article where he states
"In this case we must be clear about the reason that the siphon would fail if the U-tube were too high. It is simply because AIR PRESSURE on the input side is insufficient to RAISE THE INPUT LIQUID column as high as the top of the tube. If it doesn't get to the top, it won't flow over to fall down the output tube. So AIR PRESSURE IS IMPORTANT to siphons by putting a limitation on how high they can LIFT WATER, and without LIFTING THE WATER to the top of the U-tube, no siphon flow can occur."[emphasis added]
So it seems clear to me from this paragraph that he is saying that air pressure is limited in its ability to "lift" or "raise" the water. But that implies that it IS the air pressure that is "lifting" or "raising" the water, after being enabled to do so by gravity lowering the pressure at the top.
Then near the end he says "It isn't atmospheric pressure that sustains the siphon flow. ... The siphon's flow is a result of the gravitational potential difference between the liquid level in the reservoir and at of the output tube's opening."
I don't dispute that "The siphon's flow is a result of the gravitational potential difference between the liquid level in the reservoir and at of the output tube's opening." And you could say that it isn't atmospheric pressure that SUSTAINS the siphon flow, for some limited definition of the word "sustains".
So the question is, what wording can we use in the article to satisfy your desire to make it clear that gravity "sustains" the siphon flow, while at the same time satisfying my strong desire to make it very clear that it is at the same time atmospheric pressure that is "lifting" or "raising" the water in concert with gravity? — Preceding unsigned comment added by Mindbuilder (talk • contribs) 06:06, 28 April 2014 (UTC)
The air pressure imposes a limit on column height based on the liquid used. The lhup.edu is good because it uses the same diagrams. The air pressure itself is not particularly relevant to siphon operation. It's the limit imposed on the height of the tube but it is not the driving force for the siphon. As the Nature article pointed out, the siphon flow doesn't change with altitude. If you can place the siphon in a hypobaric chamber at modulate the air pressure without affecting the flow of the siphon, it's pretty clear that air pressure is not part of the dynamic flow equation. Also a siphon works just as well if the up and down tubes are filled from the top rather than suction on the downtube. Even if you start the siphon with suction, it doesn't exhibit siphon behavior at minimum suction pressure, it exhibits it when the liquid is lower than the height of the upper container and the pressure is back to atmospheric. A sort of unrelated phenomena is the critical angle of attack when an airfoil begins to stall. It's an important limit but it isn't related to the normal lift operation until it's reached and lift degrades. In a siphon, air pressure in the inlet and outlet are nearly identical. Gravitational height is not. Gravity is doing the work. Roller coasters have many "up" sections driven completely by the potential energy of gravity. Locks built for boats can both raise and lower boats and the force is also gravity. Read the key points in each of those references:
assert or imply that air pressure somehow drives the liquid flow. This is false
the work done at each end of the siphon against air pressure is NET zero. This is true.
The fact is that the air pressure of a working siphon is pushing on both the source and outlet with an identical force (actually slightly opposing the siphon). The problem is that your wording that atmospheric pressure is lifting the water during siphon operation is not correct. It's a hydrostatic limitation on the siphon, not a dynamic driver. As long as the limitation is not reached, the outside pressure can vary widely with no effect on the siphon. The internal fluid pressure in the tube is determined by gravity and height difference, not atmospheric pressure. The Nature article pretty much covers it. --DHeyward (talk) 07:50, 28 April 2014 (UTC)
I have been reading around and there are a couple of things I will mention. Firstly the Nature article is very good and clearly explains atmospheric pressure has a negligible influence. This is summed up in this part:
'If an experiment is performed with a simple ‘kitchen’ barometer, for example, a straw pushed into the water and then lifted with a finger placed over the end, when water level in the glass is varied, the level of the water in the straw remains constant. This demonstrates that although atmosphere pressure holds the ascending column in balance it cannot push water into the inlet of a siphon.'
Another thing is that these experiments were in toto within the hypobaric chamber. However, the non-relation of air pressure to siphon action can be seen in photographs of a simple experiment in Richert and Bender (2011) 'Siphons revisited'. Here, the siphon flowed from lower atmospheric pressure into regular sea level atmospheric air pressure. In experiment three they ran a siphon and closed the plastic container that held the reservoir water. The siphon ran and as the air pressure in the container dropped the siphon continued to run. Eventually the container was crushed by outside air pressure and the siphon ran until the partial pressure within the container was so low that it resisted travel in the tube and the siphon stopped. What I saw in this was that the siphon was running out into regular sea level atmospheric pressure but the pressure in the container was dropping and thus the standard argument that it is the pressure of a column of air pushing the water up the tube must be deficient; if air pressure drives the system the regular air pressure outside the container must reverse the siphon and drive it from the lower continer in to the higher, low pressure, container.
I also wandered into reading a bit about the tensile strength of fluids. This is, as I understand it, the resistance of molecules in a fluid to being separated, when this point is reached cavitation occurs and a void filled with low pressure fluid vapour exists. There are measured tensile strengths for different materials from water to liquid helium. From this comes the cohesive forces that keep a siphon running. It seems very clear that in a siphon, fluid falling from the down tube by gravity with the fluid cohesive force (also tensile strength) of the fluid resisting cavitation draws the fluid through the tube. In that sense I would say that a siphon essentially works as a suction. This of course makes a pressure gradient down the tube but atmospheric pressure is vanishingly small effect - it is the cohesive forces of the fluid that operate the siphon. This was noted in the Nature paper:
'An important point to note is that the stability of the upper and lower reservoir levels during the operation of the siphon indicates that no energy is transferred between the siphon and atmosphere (i.e. the vector product of the force and distance is zero). In other words, the pressure of the atmosphere does not push water into the siphon, neither does the water in the lower reservoir push on the atmosphere. ... It follows from the above analysis that there must be a direct cohesive connection between water molecules flowing in and out of a siphon. This is true at all atmospheric pressures in which the pressure in the apex of the siphon is above the vapour pressure of water, an exception being ionic liquids.'
I cannot see any explanation where there is enough energy in atmospheric pressure to push a column of fluid up a siphon tube against gravity, particularly where the receiving bucket is lower than the reservoir bucket and thus has a higher atmospheric pressure acting upon it as a countervailing force. Tobermoryconferre10:31, 28 April 2014 (UTC)
@ DHeyward - I think I'm starting to see some of the reason for our disagreement here. You keep mentioning something to the effect that "the work done at each end of the siphon against air pressure is NET zero." I agree that is true. The pressure at the upper and lower surfaces are practically equal, actually higher at the bottom tending to slightly push the siphon backwards if anything.
I think our problem comes with regard to the claim that air pressure "is not the driving force for the siphon." I'm afraid that when I claim that atmospheric pressure and gravity work together, I think I may not have made clear that I don't think part of the energy to drive the siphon is provided by atmospheric pressure and part by gravity. I acknowledge that it is all from gravity. But that doesn't mean atmospheric pressure doesn't take part in providing some of the forces that make the liquid go up. The NET result of the forces of atmospheric pressure is zero, but atmospheric pressure provides some force. That you subtract the pressure at top from the pressure at bottom to determine the lifting force is proof of that. And atmospheric pressure obviously provides the force in a barometer or drinking straw, so atmospheric pressure obviously is CAPABLE of providing such a lifting force.
A similar example is the tubing wall at the top of the siphon. The floor of the tubing at the top of the siphon exerts an upward force on the liquid in the siphon. Thus it works along with gravity and all of the other things that make a siphon work. But obviously the tube wall is not the driving force of the siphon. Normally we might consider it silly to even mention the tube wall's contribution to the forces of the siphon. But if some scientists started publishing articles claiming that the tube wall exerted no force on the liquid, it would be proper for us to clarify that the tube wall does exert a force holding up the liquid above it, but that it contributes no net energy to drive the siphon, gravity provides all of the energy.
Another example would be a Ferris wheel. If sitting on a Ferris wheel, the seat of the ferris wheel will be exerting an upward force on your rear on the way up. But nobody would claim that force is the "driving force" to turn the Ferris wheel or lift your body. Because that seat force drives your body up on the way up, doing work on you and increasing your potential energy, but then your body and gravity do work on the Ferris wheel on the way down, canceling to NET zero the effect of the upward seat force. But if some physicist were to publish an article saying that the seat of the Ferris wheel is exerting no force on your rear on the way up, we would need to make clear that the force is there, and it is lifting you, but that its effects are balanced on the way back down, and thus this upward force on your rear is not the "driving" force of the Ferris wheel.
So you can say that gravity is entirely 100% responsible for "driving" the siphon or "sustaining" the siphon and I won't dispute that, but at the same time while gravity is entirely "driving" the siphon, it is still atmospheric pressure providing the force to "lift" the liquid up. Like the Ferris wheel, the energy of that lifting force is returned on the way back down for NET zero effect, but the force was still there.
So I ask again: If a molecule is moving up a siphon at a fairly steady speed, is it not true that there must be SOME upward force acting on it other than gravity? This is basic physics. It's a simple question. And again, please don't answer it indirectly or answer some other similar question. I'll have more comments for Tobermory shortly. Mindbuilder (talk) 19:22, 28 April 2014 (UTC)
@Tobermory Womble - With regards to your first quote of Hughes, his claim that the level of water in the straw will remain constant as the level in the glass is varied, is simply not true. If the glass is lowered for example, the level of the water in the straw will drop and the bubble at top will expand downward. Of course for the altitude differences over the height of the drinking straw, the difference may not be noticeable, but it is not negligible. If you started with a drinking straw two meters tall, half filled with water, and you lowered the water level in the glass 10cm, the bubble would expand downward roughly one percent or about 1cm. So the change in the upper water level would be about 10% of the change in the lower one. Not negligible at all. Hughes's poor understanding of siphons shows up in factual mistakes like this.
Then he claims the above steady water level proves atmospheric pressure can't push water into the inlet of a siphon. That appears to be a non-sequitur, I just don't see how that follows at all. But atmospheric pressure obviously can push water into the inlet of a siphon or how else can you explain the siphon of figure 4 in our Wikipedia article? The air cannot pull up.
In the closed crushing upper container siphon demonstration, it is still the lower pressure in the crushing container pushing the liquid up the siphon. The reason it can do this is that the down leg of the siphon is tall enough that it lowers the pressure at the top of the siphon even much lower than the low pressure inside the crushing container. Though the low pressure in the crushing container is low, it is still higher than the pressure at the top. The low pressure in the crushing container gets much help in overcoming the higher atmospheric pressure at the exit from the pull of gravity on the liquid in the tall down leg.
That's why atmospheric pressure at the exit of siphons doesn't really cancel the pressure at the entrances. There is an intervening force to counter the atmospheric pressure at the exit, and that force is gravity acting on the liquid in the taller down tube. The atmospheric pressure at the exit never completely makes it to the entrance. Although in the end the net effects of atmospheric pressure come out to zero, at intermediate points the effects do not entirely cancel to zero.
It has often been mentioned that the flow rate of the siphon doesn't vary with altitude. That's true, but it does not follow that atmospheric pressure is providing no lifting force. With increasing altitude the atmospheric pressure exerts less upward force on the liquid going up, but it also exerts less upward force on the liquid coming down. These effects cancel overall in the end, but at intermediate points in the siphon, the atmospheric pressure forces are not entirely canceled.
It is cohesion pulling the liquid over in a vacuum siphon. But how can cohesion be pulling the liquid over in a siphon with a bubble at top big enough to leave the liquids on each side not touching? Or where does the force come from to lift the liquid in figure 4 of the Wikipedia article? Do you believe that gasses can pull?
It's true that in Hughes setup the atmosphere was not providing the energy to lift the water because the surface of the upper reservoir was kept constant. The energy was coming from the water being pumped up and discharged into the reservoir. And the water coming out of the siphon was not doing work on the atmosphere, it was doing work on the water being pumped out of the bottom bucket. That is a much more complicated system to analyze.
Atmospheric pressure has the energy to push the liquid up in a barometer or drinking straw. It can do it in a siphon because gravity on the down column helps it overcome the atmospheric pressure at the exit. Not all of the atmospheric pressure at the exit makes it all the way to the up side. But when the liquid does make it to the down side, that energy put in by the atmosphere on the up side is taken back out to the atmosphere again, leaving no NET driving of the siphon by atmospheric pressure, only gravity to drive the siphon.
To get a better understanding of why atmospheric pressure at the entrance and exit don't cancel completely within the siphon, consider this diagram.
Note that while it would seem the pushes of the two guys would cancel, they don't. As the big truck starts rolling back down the hill, it does not pull the little truck with it because the bumpers are not stuck together, they're just touching. The little truck is being pushed up the hill by the guy on the left, to spite the fact that you would have expected his push to have been canceled. In this diagram if both guys push half as hard, the trucks move the same. Just because the trucks move the same with half the force, doesn't imply that the guy on the left isn't providing a force. In a siphon, if atmospheric pressure pushes half as hard at each end, the siphon still moves the same. That doesn't imply that atmospheric pressure isn't providing a lifting force. Notice also that the two guys are not putting any NET energy into the trucks. All the net energy to drive the trucks is provided by gravity. But that doesn't imply that the guy on the left isn't putting any energy into the smaller truck. He's applying a force over a distance, so he is doing work on the truck. The truck is being raised to a higher gravitational potential energy. But equal work is being lost into the guy on the right. No NET work by the pushers on the trucks , but work still being done by the seemingly canceled force. Mindbuilder (talk) 21:13, 28 April 2014 (UTC)
In your picture, what if the people are only pushing 50N? The problem is that you simplify too much for your view and complicate it too much for fluid dynamics. Your gap explanation for example and the molecular movements. They are controlled by other forces at the molecular level including drift and diffusion. See osmotic pressure and semi permeable membrane for how fresh water will cross a barrier to reduce the concentration of salt water on the other side of the membrane, despite an inverted pressure or gravitational gradient. So yes, water molecules will move into a vacuum against pressure and against gravity because nature abhors a vacuum. Even if the bubble is pure nitrogen, water vapor will move from high concentration to low. Partial pressure is enough to do this at the molecular level. The type of flow will also determine the "bubble" including laminar or turbulent. Viscosity and surface tension also determine siphon action. The problem is that you are trying very to use boundary condition that is not involved in the solution. See here [4]. It is a solution to a siphon problem using normal fluid properties. There are lots of pieces missing that you claim are necessary. Atmospheric pressure and height of the tube are not given. Siphon has a closed form solution. --DHeyward (talk) 04:59, 29 April 2014 (UTC)
So are you saying that a molecule moving up a siphon at a somewhat steady speed might have no other force acting on it other than gravity? Mindbuilder (talk) 06:14, 29 April 2014 (UTC)
No, I'm saying describing laminar flow of a liquid is described by much simpler understandings of fluid dynamics and only gravity is necessary for explaining a working siphon. This is what the reliable sources say. Like I said, there are molecular forces that are very detailed such as what governs drift and diffusion in semiconductor junctions as well as things as osmotic pressure (hint, it opposes both pressure and gravity in your simplistic model). Fluids are collections of molecules with specific properties determined by more than just imagining a molecule. Boyle's flask isn't a perpetual motion siphon. --DHeyward (talk) 06:53, 29 April 2014 (UTC)
So are you agreeing that a molecule moving up a siphon at a somewhat steady speed does have some other force acting on it other than gravity? Mindbuilder (talk) 07:12, 29 April 2014 (UTC)
A molecule? That's an absurd level of detail for laminar flow of a fluid. There are many. In the boundaries of a working fluid siphon, gravity is the only notable force. To get to that level of absurdity, do we count Heisenberg uncertainty regarding velocity and position? How about whether a polar molecule like water is up or down? Lorentz contraction? Tunneling? We don't describe "a molecule" because it's adequately described as a fluid with fluid properties such as viscosity. The only force in the siphon causing the collection of 1023 molecules known as a "fluid" to flow is gravity. --DHeyward (talk) 07:33, 29 April 2014 (UTC)
I don't see a need to talk about things on a smaller scale than a molecule. But I think we do need to get down to the individual molecule for my question, because bringing in a whole bunch of molecules to the question complicates it with a compound object. I'm trying to ask questions that are relatively simple to answer. I think the answer to the question is basic physics, but I have been unable to understand your thinking on these issues because you had been ignoring my questions or answering them in indirect or unclear ways. You won't even tell me clearly when you agree or disagree with me. For example on that last question I asked "So are you agreeing..." You could have answered that easily with a No. You didn't even say that you were not agreeing. You didn't say that you were agreeing. Although on the question before that one, you mercifully did answer No. I'm having to go back to the simplest foundational questions that I reasonably can, so I can figure out what you are thinking on the important ideas. We could save a lot of your time and a lot of my time if you would just answer my questions because we could get down to our core differences and agreements faster. I've been giving explanations based on my guesses about what you're thinking but, that makes for slow going because I usually guess wrong. Big long explanations that don't seem to help us get closer. I know that some questions don't admit to easy answers, so I have been trying to ask mostly ones that can easily be answered by yes or no, such as "Do you agree...?" Hopefully you know in your own mind if you agree. I'll answer your questions to, that's only fair. If I skip one of your questions just remind me and I won't skip it again.
Is the question about the force on a molecule really too absurd? I wouldn't quite know how to ask the question about a bulk of molecules in a way that would be easily answerable by you other than for you to answer that you don't know. I'm not asking yet what the forces might be. We could take it out of the context of a siphon to make it even simpler. Do you agree that if a molecule is moving up at a constant or somewhat constant speed in a gravitational field, that there must be another force acting on it besides gravity? Mindbuilder (talk) 08:30, 29 April 2014 (UTC)
Mindbuilder, I can't really follow your arguments because I find them internally inconsistent and fluid. Consider these arguments that appear in your comments:
A) So you can say that gravity is entirely 100% responsible for "driving" the siphon or "sustaining" the siphon and I won't dispute that, but at the same time while gravity is entirely "driving" the siphon, it is still atmospheric pressure providing the force to "lift" the liquid up. … So I ask again: If a molecule is moving up a siphon at a fairly steady speed, is it not true that there must be SOME upward force acting on it other than gravity? This is basic physics. It's a simple question.
B) Though the low pressure in the crushing container is low, it is still higher than the pressure at the top. The low pressure in the crushing container gets much help in overcoming the higher atmospheric pressure at the exit from the pull of gravity on the liquid in the tall down leg.
In A) you state that 'it is still atmospheric pressure providing the force to "lift" the liquid up'; yet in B) your arguement changes to, 'The low pressure in the crushing container gets much help in overcoming the higher atmospheric pressure at the exit from the pull of gravity on the liquid in the tall down leg'.
This presents two things, firstly if A) atmospheric pressure is lifting the water what need is there for gravity? Water is fairly incompressible so if air pressure lifts the water it must eject it out of the other end of the siphon. Then in B) you write 'the pull of gravity on the liquid in the tall down leg'. This explicitly acknowledges that molecular cohesion is acting and so causing the water molecules to move and so answers your Question in A),'... If a molecule is moving up a siphon at a fairly steady speed, is it not true that there must be SOME upward force acting on it other than gravity?' Molecular cohesive forces provide for this.
Now, you have argued that atmospheric pressure lifts the water column (against gravity) and that gravity acts upon the downward draining tube. Logically they must balance or water would drain out under gravity faster than replaced by atmospheric pressure. If they balance it should be fairly easy, since we know g = 9.8 ms-2 and can measure the mass of the column of the downward tube, to convert that into the force that atmospheric pressure exerts on water in the upward tube.
To summarise you have argued that atmospheric pressure lifts the up tube water column and gravity drains the down tube water column and molecular cohesion acts to draw the water on as a mass. You then introduce a third element in the case of a siphon that continues to work as atmospheric pressure drops. As you point out, 'It has often been mentioned that the flow rate of the siphon doesn't vary with altitude'. A siphon acting in that way disproves atmospheric pressure as the main force driving a siphon, however, you continue; 'That's true, but it does not follow that atmospheric pressure is providing no lifting force. With increasing altitude the atmospheric pressure exerts less upward force on the liquid going up, but it also exerts less upward force on the liquid coming down. These effects cancel overall in the end, but at intermediate points in the siphon, the atmospheric pressure forces are not entirely canceled.'
I find this a difficult last statement as the reasoning is not clear, additionally you earlier state that gravity drives the downward draining tube flow but here introduce the idea of atmospherice pressure on the falling water column; if atmospheric pressure is the force that can lift the up column of water that same force must be acting on the down draining tube, which is confounding.
The third element you add is that at the top of the siphon tube, where it switches from up to down, there is an area of lower pressure. Even as atmospheric pressure drops on the reservoir water this area of low pressure in the tube remains always lower than the atmospheric pressure.
As you write; 'In the closed crushing upper container siphon demonstration, it is still the lower pressure in the crushing container pushing the liquid up the siphon. The reason it can do this is that the down leg of the siphon is tall enough that it lowers the pressure at the top of the siphon even much lower than the low pressure inside the crushing container.' Here there is a third force you introduce: It is liquids moving from high to low pressure. This appears to be suction; yet you argue that atmospheric pressure is doing the lifting in the up tube.
To sum up again, you are saying tht atmospheric pressure lifts the water column in the up tube; gravity pulls the down tube water column; molecular cohesion keeps the water mass whole; and a low pressure system at the top of the siphon tube is what allows siphons to flow at any atmospheric pressure. Am I right?
Finally, you write:
It's true that in Hughes setup the atmosphere was not providing the energy to lift the water because the surface of the upper reservoir was kept constant. The energy was coming from the water being pumped up and discharged into the reservoir. And the water coming out of the siphon was not doing work on the atmosphere, it was doing work on the water being pumped out of the bottom bucket.
A) The need for gravity is that if gravity pulling down the liquid on the taller side didn't lower the pressure at the top then the atmospheric pressure wouldn't be strong enough to push the liquid up.
When I mentioned "the pull of gravity on the liquid in the tall down leg" I wasn't implying that the liquid being pulled down was pulling anything up over the top. For example there could be a bubble at the top defeating any cohesion, yet the effect of the liquid being pulled down is to lower the pressure at the top. In fact when siphoning CO2 gas, which has been demonstrated, there is no significant cohesion anywhere in the siphon. And yes, in a vacuum siphon, liquid cohesion does provide the force other than gravity to lift the liquid. Perhaps DHeyward would consider that a notable force acting in a siphon. But that cohesion cant explain a siphon operating with a bubble at top. When cohesion isn't supplying that upward force, Atmospheric pressure must be. What other force but atmospheric pressure to lift the liquid in the siphon of figure 4? Do you think the air in a bubble can pull liquid over the top of a siphon?
I'm not clear what you're saying in your next paragraph, but the atmospheric pressure pushing the liquid up is sort of balanced by the atmospheric pressure resisting the flow down on the down side. It's not just atmospheric pressure one one side and gravity on the other. The balance is of both atmospheric pressure on both sides and gravity on a column of liquid on both sides. Or maybe I should say the lack of balance when it comes to the gravity on the column of liquid on each side.
I think we would all agree that it is gravity that drives a siphon in vacuum right? Liquid cohesive forces lift the liquid but don't provide any of the energy to drive a siphon in vacuum right? When I say the force of atmospheric pressure is an important force inside the typical siphon, but it doesn't drive the siphon, I mean it in the same sense as when I say cohesive forces in a vacuum siphon are an import force inside the siphon, but don't drive the vacuum siphon.
Two of the three clauses in your summary aren't too bad statements of my position, but I think molecular cohesion only keeps the mass of liquid whole in siphons in vacuum or where they exceed the barometric height. At the locations in siphons lower than the barometric height, the liquid molecules are repelling each other, and thus cannot exert any pull on each other.
Two questions I'd like you to answer are: Do you think atmospheric pressure is providing the force to lift the liquid in the siphon of Figure 4? and Do you think that if there is a bubble at the top of a siphon that somehow a pull can be transferred through that bubble? Mindbuilder (talk) 17:28, 29 April 2014 (UTC)
@ DHeyward - If you'd like to get away from individual molecules for a while, can I ask: Do you think liquid cohesion exerts an upward force on the molecules going up in a siphon in vacuum? Do you think that upward cohesion force is a notable force other than gravity in a siphon? I assume that since you have said that only gravity drives siphons that you would agree with me that liquid cohesion does not drive a siphon in vacuum, is that assumption correct? Mindbuilder (talk) 18:15, 29 April 2014 (UTC)
Viscosity is a fundamental aspect of a fluid in a siphon, but the only relevant force is down. Viscosity, height difference between the outlet and the source vessel, and gravity. If I have a collection of hills and roll a ball down the first hill, it reaches maximum velocity at the bottom and then heads back up the next hill. There is no force except the downward force of gravity and the various forms of potential and kinetic energy. There is no need to find an up force. Consider Newton's cradle: The cradle in motion. What forces are in play here? How many? For practical purposes the acting force is all gravity. But depending on the composition on the balls, how that force is transmitted is relevant. But there is still only one force that drives this toy. There is no need to find a force that pushes the ball up against gravity. --DHeyward (talk) 00:47, 30 April 2014 (UTC)
I'm not talking about viscosity. Liquid cohesion is different though related. Two liquids with the same viscosity could have different cohesion. But I don't want to argue about viscosity versus cohesion. And there is an important upwards force on the rolling ball from the ground in your example. Look up the "normal force". For Newtons Cradle, there are many forces, the force of the cables the balls hang from provide the direct upward force, not gravity. The cables also provide the force to bring the balls back to the center. And the repulsive force of impact is of interest as well as far as transferring that gravitational energy. The Mythbusters did a giant Newtons Cradle that didn't work because the balls absorbed too much of that gravitational energy on impact.
I've realized another issue from some of your earlier comments. You stated "Locks built for boats can both raise and lower boats and the force is also gravity." Actually gravity doesn't raise or support a boat in a lock DIRECTLY. The lifting force is from the liquid molecules under the boat pushing up on the hull. Gravity carrying liquid into the lock raises the water level, that increases the hydrodynamic pressure at the height of the boat bottom, creating excess buoyancy, and thereby an upward force in excess of the weight of the boat, so the boat moves up. It moves up until the bottom of the boat is just deep enough below the water surface for the upward force of water molecules on its bottom to match its weight. Not just you, but many of us might say the lock is gravity powered. But we would be simplifying. If some physicist came along and published an article saying something wrong like "it's not the upward force of water molecules on the boat bottom that pushes the boat up, it's the cohesion of water molecules to the SIDE of the boat powered by gravity supplying water", then we would want to start talking about not just the ultimate cause, gravity, but the direct cause, water molecules pushing up on the boat.
If you have two buckets hanging from a rope over a pulley, and you put a weight in one bucket and that pulls the other bucket up, you would say its powered by gravity, and it is, but that doesn't change the fact that the rope exerted an upward force to pull up the lighter bucket. It's not powered by the rope but the rope did exert the lifting force. The lighter bucket is not DIRECTLY lifted by gravity. We might normally not pay too much attention to the force of the rope, but if some physicist came along and published an article saying that it was powered by gravity but it wasn't the rope applying an upward force, but rather the rope contributed no force and it was a subspace distortion field transmitted to the lighter bucket telepathically, then we would start talking about how the force of gravity transferred through the rope which exerted the DIRECT force pulling up the bucket.
Now what these intermediate and direct forces are may be of little interest to you since you think they may cancel in the end, but how a downward force like gravity can be turned to make water flow up hill is of considerable interest to almost everybody else in this debate. You are alone in finding it uninteresting what the DIRECT force is lifting the liquid. People become especially interested when they are told that siphons work in vacuum and the liquid is being pulled up. Many people don't know a liquid can pull. They don't even believe it when they hear it. Even when you show them published papers demonstrating it, they doubt the papers. But when some scientists realize liquids CAN pull, and siphons can work in a vacuum, they go too far and conclude that ALL siphons work by pulling the liquid up. The disproving example of a siphon with a bubble in it often doesn't occur to them. Your position that there is no notable force involved in the siphon except gravity is unique as far as I know. I've never seen anyone else claim that. All the scientists agree it is powered completely by gravity, but they say that the DIRECT force lifting the liquid is either liquid cohesion or atmospheric pressure. You are the only person I've heard claim that it is neither or that those forces are insignificant. Even if we allow the possibility that your view is the better view, the fact that your view is unique and you probably can't find anyone to agree with you, makes it unsuitable to Wikipedia. I'm sure you can find someone like Dr.Hughes to agree with your claim that atmospheric pressure does not drive the liquid up, but even he won't agree with your opinion that there is no other notable force but gravity acting in the siphon. Now can we put this to rest until you find one article or even one Internet post by anybody, even a nobody, that makes the bizarre claim that there is no other notable force but gravity in the siphon? Or do you prefer to take up Hughes's position that there is a notable force, liquid cohesion that pulls up the liquid, even past a bubble, by some mysterious method? Mindbuilder (talk) 02:09, 30 April 2014 (UTC)
It's not a debate and I am far from alone. I count you as being the only one asserting that air pressure is important, arguing with everyone including the journal review experts at Nature. And insofar as air pressure doesn't stop the siphon, it is irrelevant. Air pressure creates a boundary condition similar to laminar flow. Your "ball on hill" analysis is exactly wrong. You can do the vectors and see one force (gravity, straight down), gives the ball its forward and downward motion. It is always accelerating down. Even when it is moving up, it is accelerating down - and by your arguments, you don't seem to understand this. You can always break that single "down" vector into it's orthogonal points but it doesn't change the force. Down. Always. Only velocity @ time(t) are affected by the steepness of the hill, the energy and work is always the same as long as the hill is the same height. Yup, the earth acts as solid and prevents the ball from moving to the center of the earth but it certainly doesn't push the ball up with any force that would allow it to gain potential energy. You can test this by placing a ball at the bottom of the hill. It doesn't move up. The ball starts with potential energy and converts it to kinetic energy and back to potential energy. The earth didn't "push the ball up the hill." Every physicist points you to Bernoulli's equation as governing a siphon. Bernoulli's equation needs gravity, viscosity and potential energy. You don't seem to be able to follow the math or concepts or provide anything but pictures of your own ideas. Even science articles have used the Wikipedia pictures and real physics to describe them without resorting to your roundabout ideas and terminology that only you seem to use. You can prime a siphon pump using a vacuum. Once it's flowing, it's no longer using energy from air pressure. The air is not pushing it up after it starts. You should intrinsically know this because to start a siphon by suction, you stop sucking when it starts. That's the point where it's gravity (and not your lungs). You can just as easily start a siphon by filling it from the top and not use any suction or air pressure. This method is preferred by large industrial siphons like lakes because it's really hard to suck on 12" steel pipes when all you have to do is fill it from the top, turn off the fill valve and open the bottom valves. Works just as well as a vacuum and doesn't require air pressure to operate. As for a bubble, depending on the viscosity and the height of the siphon, vapor bubbles may form when the flow starts to cavitate. This is vapor from the liquid, not air and not a vacuum. Ships have this phenomenon on propellers all the time. A water vapor bubble forms when the flow departs from laminar to turbulent. Often the bubble then leaves the propeller and magically collapses back into water when the velcoity no longer supports the bubble. Makes a neat sound on sonar. . It's near the boundary condition of a liquids laminar flow. It can support some cavitation in a siphon (see the Nature article). As for flowing around a bubble. Get a bubble level, move it around. Get a pool pump and look in the top. Bubbles can form and fluid can flow around them. Now, please either find a source that says it's not Bernoulli's equation that governs a siphon or drop the air pressure argument. Air pressure doesn't push the liquid up a working siphon. --DHeyward (talk) 03:56, 30 April 2014 (UTC)
I never said that Bernoulli's equation doesn't describe the functioning of the siphon, it is a very good description. I said Bernoulli effects don't supply the force lifting the liquid and the Bernoulli effects don't CAUSE the pressure drop that allows atmospheric pressure to push the liquid up the siphon. I don't think I've seen anyone claim what you did that the Bernoulli effects CAUSE the pressure drop that drives the siphon. Although I think I did see one discussion dismissing that possibility, so I expect at least someone has proposed that before. If you are far from alone in claiming gravity is the only notable force in a siphon please provide one quote from anyone to that effect. I'm not asking for a quote that atmospheric pressure does not drive the siphon, I'm asking for a quote consistent your position that NEITHER atmospheric pressure NOR liquid cohesion exert the force that raises the liquid in a siphon.
As for my position that atmospheric pressure exerts the force to lift the liquid in a siphon, you need look no farther than the second citation of the Wikipedia article, which is second only because it is part of a list of articles demonstrating vacuum siphons. Note that this is from an author who demonstrated vacuum siphons to correct the misconception that they could not work in vacuum. He wasn't biased in favor of the atmospheric pressure theory. He was disproving that it was always atmospheric pressure.
"...it is clear that in such a broken siphon the atmospheric pressure is continuously active in pushing the liquid up the short arm." [Minor 1914]
By a broken siphon he is referring to a siphon with a T fitting at the top from which the air was sucked out and where an air bubble remained.
Even the modern instigator of this debate acknowledges that my conception that the atmospheric pressure is pushing the liquid up is very common.
"A very common misconception is that siphons work through atmospheric pressure pushing water through the tube of the siphon." [Hughes 2010]
Richard W. Ramette, L. M. Gould Professor of Chemistry, Emeritus, Carleton College, Northfield MN
Quotes "The Flying Circus of Physics"
"Contrary to much popular belief, the fluid is not pushed over the siphon by air pressure..."
Again, an opponent of my position acknowledging much popular belief of my position.
Ramette goes on to express his own understanding of what makes the fluid go upwards.
"The ambient atmospheric pressure at the input point responds to this reduced pressure by forcing the fluid upwards, sustaining the flow..."
Of course I could quote hundreds of random Internet posters for what that's worth. Can you quote even a single random internet post that expresses the belief that there are no notable forces in the siphon other than gravity?
An interesting thought about the articles that deny the atmospheric pressure theory is that I don't think a single one of them mentions the case of a siphon with a bubble in it. I think if they'd considered that before solidifying their opinion, they wouldn't have published those articles.
When I talk about a bubble in a siphon, I'm not talking about the liquid flowing around the bubble. I'm talking about how there can be a pull with a bubble present and that somehow the bubble can both transfer the pulling force and/or not expand to unlimited size as long as there is pulling.
Mindbuilder, I am trying to understand you thinking here but you shift your concepts and arguments to fit an arguement that atmospheric pressure plays a significant role in the operation of a siphon. For example you write, 'I think we would all agree that it is gravity that drives a siphon in vacuum right? Liquid cohesive forces lift the liquid but don't provide any of the energy to drive a siphon in vacuum right?' yet then go one to argue that atmospheric pressure is the force that lifts fluid up in a siphon. I suggest that you are coming from a perceptual scale misapprehension. Several times you have presented a siphon with a bubble as proof that cohesive forces do not operate, citing 'figure 4' I am well aware of bubbles in siphon tubes; they do not invalidate the operation of siphons. Consider the siphon being used to clean a fish tank[1]. They entrain matter and it travels with the siphon flow, same with bubbles. Even large bubbles, to cite figure 4, feel the same effect. You go on to argue that air, a gas, has no cohesive forces. That is not so. Just get a syringe (sans needle) and pull a little air in it, block the entrance with your finger and pull on the plunger there is a resistance and if you let go the plunger will pop back to its prior position.
The operation of a siphon at the end is at a molecular scale. Consider you comment about CO2 being siphoned 'without cohesion'. CO2 is heavier than air and acts as a liquid in this situation, one can have a beaker of CO2 and drop a lit match into it and it goes out. That beaker of CO2 can be poured over a lit candle and put the candle out. When the down draining tube of a siphon is opened gravity acts on all the molecules and they start to drop - do you expect them to seperate and fall with vacuum created between the molecues?
No, we have a fluid flowing under the force of gravity; a fluid flowing along a pipe and along this pipe there is a drop in pressure (Poiseuille's Law) and this is what is is happening in a siphon. Earlier on I wrote that 'in essence, a siphon functions as suction'. My father used to siphon petrol out of the car for the lawnmower. He would put the tube in his mouth and use suction to draw the petrol out and set the siphon going; once this happened gravity took the place of suction and the pressure differential along the tube made by this is seen. Cohesive forces see that fluid travel along the length of the pipe, from source to exit, in the direction of the pressure gradient
Now to get back to you misapprehension, at different scales fluids are not the same. Consider, sea water at the scale of plankton is a thick, viscous, 'soup'. For an airborne gnat or midge, they are so small that to them the air is a thick matrix that they labour away within. To us, pitch is a solid at room temperature but the pitch drop experiments have shown that it behaves as a liquid, just much more viscous. Siphoning pitch would be possible but very slow! At the other end of the spectrum, physicists are reinvestigating the idea that space-time can be considered a fluid and apply hydrostatic principles to it.[2]
Fluid behaviour follows behaviours that we have discovered, and at the molecular level within a fluid, other forces (like cohesion, van der Waal's, tensile strength, etc) are also operate. Pascal's explanation comes from a time when atoms were unknown, except as one of a many theories and the explantion given by what was visually perceptible. I think we know much more and the explanation of 'atmospheric pressure' has been superceded by increased scientific understanding. Gravity and cohesive forces are all that are required to explain the fundamental operation of a siphon. Fluid dynamics explains how fluids flow within a siphon. Air pressure has no role in this.
To answer your questions- 'Do you think atmospheric pressure is providing the force to lift the liquid in the siphon of Figure 4? No, intramolecular cohesive forces are all that is needed. ... Do you think that if there is a bubble at the top of a siphon that somehow a pull can be transferred through that bubble? Yes, gravity is being felt as a pressure drop (suction) by all matter within the tube.
As a side note, I've mentioned tensile strength a couple of times. Cohesion can be considered the 'stickiness' that holds fluids together, tensile strength the amount of force that will cause cavitation in a mass of water, exceed that and the stress causes water to form a cavity of water vapour. In what I've been reading, depending on the condition of the water, the tensile strength lies between 0.5 atmospheres and 60 atmospheres, though one paper used 3000 psi as a value. Water appears loose and splashy to our perceptions but at the molecular level it is quite closely bound.
To comment on your latest comment: You write, 'As for my position that atmospheric pressure exerts the force to lift the liquid in a siphon, you need look no farther'. Then you write, ' Even the modern instigator of this debate acknowledges that my conception that the atmospheric pressure is pushing the liquid up is very common. "A very common misconception is that siphons work through atmospheric pressure pushing water through the tube of the siphon." You contradict yourself. Tobermoryconferre13:08, 30 April 2014 (UTC)
You quote me thus:
For example you write, 'I think we would all agree that it is gravity that drives a siphon in vacuum right? Liquid cohesive forces lift the liquid but don't provide any of the energy to drive a siphon in vacuum right?' yet then go one to argue that atmospheric pressure is the force that lifts fluid up in a siphon.
Those statements are not inconsistent. In a vacuum siphon, cohesion exerts the force to lift the liquid. In a typical siphon under typical atmospheric pressure, atmospheric pressure exerts the force to lift the liquid. In both cases gravity is entirely the ultimate source of energy to drive both siphons.
I don't claim that bubbles invalidate the operations of siphons. In fact my point is just the opposite, that typical siphons work fine with small, medium, and even big bubbles, if they're not too big. This invalidates the cohesive force explanations for typical siphons because the gases in the bubbles would happily expand to empty the entire tube if there was any pulling going on, as they do if they get into a vacuum siphon.
In your syringe example, the resistance you feel pulling the plunger is not from the air pulling but from the external atmospheric pressure pushing on the outer surface of the plunger. In fact the air inside the syringe, if it's not a perfect vacuum, would be pushing out helping you pull the plunger, making it easier to pull. Consider the first paragraph of the Wikipedia article on suction:
Suction is the flow of a fluid into a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Suction is popularly thought of as an attractive effect, which is incorrect since vacuums do not innately attract matter. Dust being "sucked" into a vacuum cleaner is actually being pushed in by the higher pressure air on the outside of the cleaner. The higher pressure of the surrounding fluid can push matter into a vacuum but a vacuum cannot attract matter.
You will understand things better if you adjust your mental model to leave out the concept of suction. Where you see suction, generally you should try to see how the effect is actually a result of pressure, a pushing, not a pulling. The vacuum in a barometer doesn't suck up the liquid. A vacuum cleaner doesn't suck, atmospheric pressure blows the dust in. A suction cup doesn't suck, atmospheric pressure pushes the back of the suction cup towards the surface. Of course rarely liquids can exert a pull that might legitimately be considered a suction, like in a vacuum siphon. But I can't think of any situations where a gas does.
By the way, you think that pitch that takes a hundred years to move a centimeter can be siphoned? I don't believe it. Prove it with a demonstration. I'll await your results. :) (I'm kidding of course. You're right, it should work - kinda slow though) Mindbuilder (talk) 23:14, 30 April 2014 (UTC)
Weight (W), the frictional force (Fr), and the normal force (Fn) impacting a cube. Weight is mass (m) multiplied by gravity (g). @ DHeyward - Vector force diagrams often omit the normal force if you don't need that information, leaving only gravity in the diagram, but the table and ramp do push up on the ball, and to be complete you need to put the normal force in. From Normal Force:
"In a simple case such as an object resting upon a table, the normal force on the object is equal but in opposite direction to the gravitational force applied on the object .... The normal force here represents the force applied by the table against the object that prevents it from sinking through the table"
Likewise, in a siphon you need to put in the force of atmospheric pressure or cohesion to be complete. If gravity was the only notable force in the siphon, the liquid would be in free fall. The force of gravity never makes anything go up directly (or sideways as on the ramp) There has to be some other intermediate force for gravity to power an upward movement. Gravity can only directly make water flow down a tube. There has to be some other force to make the liquid go up the tube. Everyone else argues about whether that force is atmospheric pressure or liquid cohesion. You're the only one that argues neither is significant. You seem to acknowledge that atmospheric pressure is exerting an upward force on the liquid in the siphon while it is being vacuum primed, but then you seem to think as the siphon is filled, the force transitions to gravity. We know that gravity normally tends to make water run down out of a tube. How is the force of gravity made to change direction to get the liquid to flow up? When a molecule is going up a siphon at a somewhat steady speed, what does the vector force diagram on the molecule look like? If you just can't stand to do the force diagram for a molecule, then how about the force diagram for a cubic millimeter of liquid. The gravity vector would be down. How would you label the force vector corresponding to the normal force - atmospheric pressure, liquid cohesion, or what? Do you turn the gravity vector around to point up? How does the liquid on the up side of the siphon know that the liquid on the down side is getting pulled down by gravity? When the liquid on the down side is getting pulled down, it causes something to happen at the top of the siphon, right? What does it cause to happen? It causes the pressure at the top to go down, right? Is there some other effect I'm not thinking of other than just the pressure reduction that is transferred over the top of the siphon that tells the up side that gravity is pulling down the down side? Something like a magnetic or electric force or something? If there is, then do you know what it is, or do you just suspect something unknown other than a pressure drop communicates over the top?
Consider a ballistic pendulum. Gravity powers a ball rolling down a ramp to gain speed. Ball hits the cup in the ballistic pendulum. Ballistic pendulum swings up some distance. Is there no tension in the arm of the pendulum pulling on the ball applying a partly upward force on the ball towards the pendulum pivot? This ballistic pendulum example is kind of like what we're talking about with siphons. Upward motion powered entirely by gravity but some other force like the pendulum arm or atmospheric pressure providing the upward force.
Or how about we do a variation of the chain model of the siphon. You have a rope over a pulley. A bucket tied to each end. One bucket on the ground half filled with water, the other bucket empty about 2m above the ground. You fill up the higher empty bucket with water. The heavier bucket pulls on the rope over the pulley, the rope pulls up the half filled water bucket. This is powered entirely by gravity. Since this was entirely powered by gravity does that mean there was no notable upward force by the rope on the half filled water bucket? Mindbuilder (talk) 01:25, 1 May 2014 (UTC)
Mindbuilder; Sorry, but you your posts have become confused and erratic. Either you are just warping and shifting your concepts to always put your belief that 'air pressure drives siphons' unconsciously or you are deliberately spinning with that agenda. You are not even being logical. Could you please do me a favour and actually write a paragraph or two that clearly and concisely explains exactly how a siphon works in your own mind so that I can have a skerrick of comprehension on how you concieve a siphon to work. Tobermoryconferre11:08, 1 May 2014 (UTC)
Mindbuilder, you are aware that flow itself lowers liquid pressure in the siphon, correct? That regardless of air pressure, the liquid pressure in the tube of moving fluids is lower in sections that move vs sections not moving? The main problem with your argument is that you seem to think that because a particular force is used to start a process, it must necessarily continue. This is false. If you pull a cart up a hill, against gravity, with a rope, you can analyze all the forces in the rope, the work against gravity, rolling friction, etc. Let the cart go and it travels back down the hill, gains speed, climbs the other hill to mgH, and the continues to oscillate. It is dead wrong to assert that you need the rope for each upward leg of the cart after it starts it's motion. Air pressure, like a rope in the cart example, can be used to start the siphon. Once it starts, it's purely a conversion of potential energy. As I said when you start a siphon by evacuating one end, the air pressure difference starts it. You can feel the pressure difference with your lungs. You can also feel when the air pressure is no longer acting on the siphon because the max pressure difference isn't when the siphon is working, it's at the maximum height of the liquid. Once there is a height difference between the outlet tube and the source vessel, the flow sustains itself COMPLETELY through the the conversion of potential energy of the upper reservoir to moving liquid at the exit. Even your own experience regarding the feeling of your lungs telling you there is no air pressure role when the siphon is running, you insist that a force that is doing no work is actually lifting liquid. It is not. The fact you cannot wrap our head around gravitational potential energy being converted to work is your problem. It is no different than watching a ball roll down from a 10 foot hill, accelerated by gravity, climb a 5 foot hill, slowing velocity because of the single force of gravity. We don't need a rope to explain how it climbed a 5ft hill when it started from a 10 ft hill even if we used a rope to pull it up the first 10 ft hill. --DHeyward (talk) 12:36, 1 May 2014 (UTC).
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