# Would this violate or challenge Newton's laws?

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• li dan
In summary, The phenomenon of diffusion is a transport phenomenon that involves the movement of molecules from a region of high concentration to a region of low concentration, driven by thermal energy. This conversation discusses the use of diffusion for propulsion, where a container with a solution of solute and water is moved by launching the extracted solute in one direction and then waiting for it to diffuse back to the other side. However, this setup does not work due to Newton's laws, which state that the center of mass cannot be moved without an external force acting on it. Additionally, diffusion is a small force that is usually negligible, and pressure from surrounding particles may play a role in the movement of the container.
li dan
TL;DR Summary
Is there a problem with the principle of using diffusion phenomenon for propulsion?A challenge to Newton's law! ？

The phenomenon of diffusion is a transport phenomenon based on the thermal motion of molecules, a process in which molecules are transported from a region of high concentration to a region of low concentration by Brownian motion.

Let's assume that there is a car, the road under the wheels is smooth and frictionless horizontal surface, the car as a container, the interior of the car is a solution of solute and water, the density of solute is greater than the density of water, and the solubility of solute increases with the increase of temperature. It is assumed that the solute is one of barium chlorate, barium hydroxide, ammonium bicarbonate, ammonium chloride, and the carriage has a launcher above the middle of the carriage.

First, the solute in the solution is extracted, for instance, by using the crystallization method, specifically by setting a temperature regulation device on the carriage to lower the temperature of the solution to extract the solute. Secondly, the extracted and collected solute is launched to the left using the launcher, and the solute hits the baffle on the left side of the carriage (assuming no bounce). Let's assume that the vehicle moves to the right at this time the distance s1, and then solutes fall into the water (or solution) below the left baffle, from the solutes fall into the water (or solution) to the water (or solution) smooth process, the vehicle will move to the left s2 distance, s1>s2, overall, the vehicle moved to the right s1-s2 distance. At this point, it is necessary to increase the temperature of the solution to increase the solubility as well as speed up the diffusion rate, and then wait for the solute to diffuse to the right until the solute is uniformly distributed within the solution. By repeating the above process, extracting the solute from the solution and launching it to the left...etc., the cycle repeats itself so that the vehicle can be made to move to the right one again and again.

This is an unnecessarily complicated setup equivalent to sitting on the cart and throwing a yo-yo toy horizontally. Or bouncing a ball off the baffle and then catching it.
I.e. there is a mass element that moves to and fro between the launcher and the baffle. First, the mass element leaves the launcher with some momentum, and the cart moves with equal but opposite momentum. Then, the mass hits the baffle, and exchanges momentum with the cart (whether with a bounce back or not). Next, the mass moves towards the launcher, by whatever means, while the cart moves in the opposite direction. Finally, the launcher catches the mass, bringing the system back to the initial state.
The nett change of momentum in the system at each step is zero. The entire diffusion process only serves to obfuscate the analysis.

Frabjous, Dale, nasu and 1 other person
Newton's laws say that you cannot move the center of mass of an object (here the vehicle and its contents) without an external force acting on it. If part of the mass moves is made to move one way, the rest will move in the opposite way. If molecules move from one side of the container to the other, the rest of the mass will move in the opposite direction to keep the center of mass at the same point with respect to the external world.

I see that @Bandersnatch post says the same thing so I will stop here. Your contraption will not work for the same reason that grabbing your collar and pulling up won't lift you off the floor no matter how hard you pull.

Delta2 and Frabjous
kuruman said:
Your contraption will not work for the same reason that grabbing your collar and pulling up won't lift you off the floor no matter how hard you pull.
What if I pulled by my own bootstraps? I've heard it works for some people.

phinds, davenn, Frabjous and 1 other person
Bandersnatch said:
What if I pulled by my own bootstraps? I've heard it works for some people.

li dan said:
Summary:: Is there a problem with the principle of using diffusion phenomenon for propulsion?A challenge to Newton's law! ？

wait for the solute to diffuse to the right until the solute is uniformly distributed within the solution
During this time the car on the frictionless surface will move back to where it started. You seem to think that diffusion cannot exert a force simply because it is small enough to usually be negligible.

Bandersnatch
Dale said:
During this time the car on the frictionless surface will move back to where it started. You seem to think that diffusion cannot exert a force simply because it is small enough to usually be negligible.
What force acts on the container and causes it to move?

li dan said:
What force acts on the container and causes it to move?
Pressure from the fluid

Ibix
I find this easier to understand in a molecular picture. If the dividing wall is not permeable then for every molecule bouncing off the front wall there's one bouncing off the dividing wall and no net momentum transfer.

However, if the divider is semi-permeable then not every molecule bouncing off the front wall is balanced by one bouncing off the divider because some pass through the divider. Thus there's a net force on the tank walls that doesn't completely even out until the concentrations equalise, at which point molecules bouncing off the front wall are balanced by molecules bouncing off the back wall.

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Bandersnatch
Dale said:
Pressure from the fluid
Assuming the solution has an infinite number of vertical dividers, and the pressures on both sides of the dividers must be equal to comply with Newton's third law. Moreover, unequal pressure will cause the solution to flow, but diffusion is caused by temperature, so the pressure of the solution on both sides of the containers will be equal.

li dan said:
pressures on both sides of the dividers must be equal to comply with Newton's third law
That sounds like Newton's second law. Newton's third law would equate the force from the fluid on one side of one divider with the force of that divider surface on that same fluid.

One must be careful about invoking Newton's second law to declare that a force difference is zero because a mass element is infinitesimal and the acceleration is finite. The correct conclusion is that the force difference is infinitesimal. Integral calculus is [roughly] about adding up infinitely many infinitesimal pieces and getting a finite result.

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li dan said:
so the pressure of the solution on both sides of the containers will be equal.
This is incorrect. The pressures in the two sides of the container are clearly unequal since the center of mass accelerates for the solvent/solute.

Dale said:
This is incorrect. The pressures in the two sides of the container are clearly unequal since the center of mass accelerates for the solvent/solute.
According to " In a phase with uniform temperature, absent external net forces acting on the particles, the diffusion process will eventually result in complete mixing" in Wikipedia's "molecular diffusion",Therefore, when the mass is transferred to the right due to diffusion, no reaction force will act on the bottle.

li dan said:
According to " In a phase with uniform temperature, absent external net forces acting on the particles, the diffusion process will eventually result in complete mixing" in Wikipedia's "molecular diffusion",Therefore, when the mass is transferred to the right due to diffusion, no reaction force will act on the bottle.
You are misunderstanding that. It is talking about forces acting specifically on the diffusing particles. For example, a magnetic field acting on diffusing iron particles.

It is not saying that pressure is not present (acting on the fluid as a whole, not just the diffusing particles). Only that there is no additional force on the diffusing particles specifically.

The center of mass moves so by Newton’s 2nd law there is a net external force ##\Sigma \vec F = m \vec a##. That is the pressure gradient. No amount of wishing or misunderstanding Wikipedia quotes on your part changes the obvious fact.

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Bandersnatch said:
What if I pulled by my own bootstraps?

This is not the first time. The thread is now closed, but see Message #7.

Bandersnatch
Clearly, it's just a matter of redesigning the bootstrap.

Dale said:
Yes, the pressure gradient can cause convection, and the diffusion is caused by Brownian motion, both of them can exist at the same time, but the diffusion is not caused by the pressure gradient, so the mass transfer caused by the diffusion will not react to the vessel through the pressure gradient.

li dan said:
so the mass transfer caused by the diffusion will not react to the vessel through the pressure gradient.
...no. The pressure gradient exists because there's a concentration gradient. The pressure gradient causes the container to move as long as it exists, and it exists until the concentration equilibrates.

li dan said:
Yes, the pressure gradient can cause convection, and the diffusion is caused by Brownian motion, both of them can exist at the same time, but the diffusion is not caused by the pressure gradient, so the mass transfer caused by the diffusion will not react to the vessel through the pressure gradient.
I don’t care if you try to split the motion up into different parts, diffusion and convection, or not. If you do separate it then you must analyze both parts. The important thing is that there is a pressure gradient and that pressure gradient causes the motion keeping the CoM stationary. Do you understand that now?

If pressure gradients are to be considered, it will probably be easier to imagine this contraption in free space where the pressure gradient due to gravity does not come into the picture.

## 1. Would an object moving at a constant speed violate Newton's First Law of Motion?

No, an object moving at a constant speed in a straight line does not violate Newton's First Law of Motion. This law states that an object will remain at rest or in motion with a constant velocity unless acted upon by an external force.

## 2. Can an object moving in a circular motion challenge Newton's Second Law of Motion?

No, an object moving in a circular motion does not challenge Newton's Second Law of Motion. This law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

## 3. Would an object falling at a constant speed violate Newton's Third Law of Motion?

No, an object falling at a constant speed does not violate Newton's Third Law of Motion. This law states that for every action, there is an equal and opposite reaction. In the case of an object falling, the action is the force of gravity pulling it down, and the reaction is the object pushing back against the force of gravity.

## 4. Can an object at rest challenge the concept of inertia in Newton's First Law of Motion?

No, an object at rest does not challenge the concept of inertia in Newton's First Law of Motion. Inertia is the tendency of an object to resist changes in its state of motion, whether it is at rest or in motion.

## 5. Would a force acting on an object in a vacuum violate Newton's Second Law of Motion?

No, a force acting on an object in a vacuum does not violate Newton's Second Law of Motion. This law applies to all objects, regardless of their environment, and states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

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