Author Topic: Conservation of energy/momentum.  (Read 2053 times)

Offline chazemz

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Conservation of energy/momentum.
« on: 09/09/2017 01:11 PM »
With reference to the EM drive thread there appears to be a lot of “violates this conservation law” and “violates that conservation law” when I am of the opinion that what you are in fact doing is preserving third law. Commenting earlier on conservation of energy/momentum I put forward a simple experiment asking what would happen with a five ball newton's cradle if you raised two balls and let them go so they collided with the remaining three balls. We were in agreement that two balls must come out the other side since if one ball came out at twice the velocity momentum would be conserved but not kinetic energy I then asked if nature treats momentum and energy as the same and when nature conserves “it accounts for”.
So on this point I would like to ask a question: if we have a spaceship that has a surrounding skin that you are calling “the boundary”and I fit solar panels to the outside ( we are assuming that we have a device inside the spaceship that transfers energy/ momentum to the spaceship from the solar panels), if the energy/momentum in equals the energy/momentum stored  plus losses, since the energy/momentum comes from outside the boundary am I to assume that all is well?
For the Newtonians among you we are not looking at third law in this instance, just conservation of energy/momentum.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #1 on: 09/10/2017 05:20 PM »
So we can now place a battery in the spaceship and fit a switch. With the device switched off we can charge the battery via the solar panels. When the battery is fully charged we can jettison the solar panels and wait for a while.
I can now ask another question: The energy/momentum stored in the battery came from outside the boundary in the first instance, but when the switch is switched on nothing is passing through the boundary at that moment. So, immediately prior to switch on, is the spaceship a closed system?

Online meberbs

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Re: Conservation of energy/momentum.
« Reply #2 on: 09/10/2017 05:36 PM »
What do you mean "momentum stored in the battery"? Batteries only store energy. The miniscule momentum that the photons impart on the solar panels is spread across the whole spacecraft.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #3 on: 09/11/2017 05:31 PM »
I think it better if I use a different example.
We have a very long spaceship and at one end is fixed a cannon. Nothing is moving in the spaceship    and since we have our boundary with no external forces acting on it, we will call this a closed system. We fire the cannon and the cannon ball starts to travel along the spaceship. The recoil of the cannon means that the spaceship will begin to move in the opposite direction to the cannon ball. But as the spaceship moves it interacts with the outside. This means that exterior forces are acting on the spaceship so it is no longer a closed system. This will continue until the cannon ball hits the opposite end of the spaceship (we will have the wall really sticky so the cannon ball sticks to it on impact) The spaceship stops moving and so returns to being a closed system.
I do not need to go into detail to explain the error of your arguments regarding conservation laws and  newtonians will be pleased that this thought experiment includes third law as well.

Online meberbs

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Re: Conservation of energy/momentum.
« Reply #4 on: 09/11/2017 05:43 PM »
But as the spaceship moves it interacts with the outside. This means that exterior forces are acting on the spaceship so it is no longer a closed system.
So you actually aren't talking about a closed system at all, you are talking about an open system. To describe anything about this system's behavior, you need to define exactly what it is interacting with outside, and what these forces are.

I do not need to go into detail to explain the error of your arguments regarding conservation laws and  newtonians will be pleased that this thought experiment includes third law as well.
Error of what arguments? The only thing I stated is the simple fact that batteries don't store momentum. And yes, you have to respond to arguments if you want to have a conversation.

Offline whitelancer64

Re: Conservation of energy/momentum.
« Reply #5 on: 09/11/2017 06:03 PM »
Momentum and energy are not the same thing.
"One bit of advice: it is important to view knowledge as sort of a semantic tree -- make sure you understand the fundamental principles, ie the trunk and big branches, before you get into the leaves/details or there is nothing for them to hang on to." - Elon Musk
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Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #6 on: 09/11/2017 06:56 PM »
But as the spaceship moves it interacts with the outside. This means that exterior forces are acting on the spaceship so it is no longer a closed system.
So you actually aren't talking about a closed system at all, you are talking about an open system. To describe anything about this system's behavior, you need to define exactly what it is interacting with outside, and what these forces are.

I do not need to go into detail to explain the error of your arguments regarding conservation laws and  newtonians will be pleased that this thought experiment includes third law as well.
Error of what arguments? The only thing I stated is the simple fact that batteries don't store momentum. And yes, you have to respond to arguments if you want to have a conversation.

Well it is a closed, open, closed system. If you want to go further, take the spaceship into a region of nothing and then fire the cannon. With nothing to interact with, it will remain a closed system. So theoretically speaking you can move a closed system and conserve momentum.
I was not referring to you in the arguments. 

Momentum and energy are not the same thing.
 Mmmmm, I agree, but is momentum stored energy?

Online meberbs

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Re: Conservation of energy/momentum.
« Reply #7 on: 09/11/2017 07:16 PM »
If you want to go further, take the spaceship into a region of nothing and then fire the cannon. With nothing to interact with, it will remain a closed system. So theoretically speaking you can move a closed system and conserve momentum.
No, the center of mass of the system will not move. You can't actually move, just rearrange the ship around that point.

I was not referring to you in the arguments. 
No one else had responded to the thread.

Mmmmm, I agree, but is momentum stored energy?
No.

Offline whitelancer64

Re: Conservation of energy/momentum.
« Reply #8 on: 09/11/2017 07:32 PM »

Momentum and energy are not the same thing.
 Mmmmm, I agree, but is momentum stored energy?

No. Momentum and kinetic energy are not the same thing either. They are related, but not at all the same.

Kinetic energy is 1/2 the product of the mass times velocity squared.
Ek = 1/2 mv^2

Momentum is mass times velocity.
p = mv

Two objects with the same kinetic energy only have the same momentum if they also have the same mass.
"One bit of advice: it is important to view knowledge as sort of a semantic tree -- make sure you understand the fundamental principles, ie the trunk and big branches, before you get into the leaves/details or there is nothing for them to hang on to." - Elon Musk
"There are lies, damned lies, and launch schedules." - Larry J

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #9 on: 09/12/2017 12:50 PM »
It would be better if I made myself clearer. We will assume the spaceship is a closed system in the first instance.
If a spaceship was to be fitted with a device that could move the spaceship from within and the spaceship was to move within the confines of the known universe, as soon as the spaceship begins to move it can no longer be classed as a closed system.
If a spaceship was to be fitted with a device that could move the spaceship from within, in a region of nothing, the spaceship will remain a closed system.
An object that has constant momentum is storing energy.
The battery could be described as a rocket engine in that it converts stored chemical energy into kinetic energy.
I must make it clear, I am not validating EM Drive with any of the above statements.
 

Offline whitelancer64

Re: Conservation of energy/momentum.
« Reply #10 on: 09/12/2017 01:06 PM »
No. Momentum is not stored energy. For any closed system, the total momentum is always constant. Momentum cannnot be converted into a different form of energy.
"One bit of advice: it is important to view knowledge as sort of a semantic tree -- make sure you understand the fundamental principles, ie the trunk and big branches, before you get into the leaves/details or there is nothing for them to hang on to." - Elon Musk
"There are lies, damned lies, and launch schedules." - Larry J

Online meberbs

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Re: Conservation of energy/momentum.
« Reply #11 on: 09/12/2017 02:13 PM »
If a spaceship was to be fitted with a device that could move the spaceship from within and the spaceship was to move within the confines of the known universe, as soon as the spaceship begins to move it can no longer be classed as a closed system.
Not true, the spacecraft moving does not make it an open system. You had correctly mentioned previously that external forces are involved in making it an open system. You have not described any forces though. We are talking about a spacecraft in deep space. There are no forces defined on it, so it is a closed system, unless you describe external forces acting on it.

The battery could be described as a rocket engine in that it converts stored chemical energy into kinetic energy.
Momentum conservation says that this doesn't make sense. You can't just convert chemical energy to kinetic energy, you have to balance conservation of momentum by expelling some sort of exhaust.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #12 on: 09/13/2017 12:03 PM »
If a spaceship was to be fitted with a device that could move the spaceship from within and the spaceship was to move within the confines of the known universe, as soon as the spaceship begins to move it can no longer be classed as a closed system.
Not true, the spacecraft moving does not make it an open system. You had correctly mentioned previously that external forces are involved in making it an open system. You have not described any forces though. We are talking about a spacecraft in deep space. There are no forces defined on it, so it is a closed system, unless you describe external forces acting on it.

The battery could be described as a rocket engine in that it converts stored chemical energy into kinetic energy.
Momentum conservation says that this doesn't make sense. You can't just convert chemical energy to kinetic energy, you have to balance conservation of momentum by expelling some sort of exhaust.
I see your point with reference to deep space. It would be better to use the term “at first contact”, so in low earth orbit very quick; in deep space, a little longer.
The rocket engine does it by combustion, the battery would do it by using the flow of electrical energy/power..So Watts = joules = newtons. You transfer energy from the battery onto the body of the spaceship. In low earth orbit it would then change to an open system.
It would be easier if we could define when a “closed system” is a “closed system” which is what I have tried to do in the earlier posts. If we place a charged battery into the spaceship and close all the hatches, have we got a” closed system”? So another little thought experiment! We take the cannon outside the spaceship and leave a hatch open. We fire the cannon so the cannon ball flies through the hatch and then close the hatch. What have we got?. Technically it is a “closed system” since the cannon ball has not interacted with the spaceship. But we have an “open system” in waiting since we can tell the future and know what is going to happen. And as soon as we define the system as “closed” does the cannon ball become part of the system so cannot be referred to as “exterior”
With reference to the exhaust, I need to know how you define “conserve”
Would it not be better to use the term energy/momentum are always “accounted for”?

Online meberbs

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Re: Conservation of energy/momentum.
« Reply #13 on: 09/13/2017 02:03 PM »
The rocket engine does it by combustion, the battery would do it by using the flow of electrical energy/power..So Watts = joules = newtons.
This is nonsensical. You can't just equate units that mean different things.

You transfer energy from the battery onto the body of the spaceship. In low earth orbit it would then change to an open system.
Where is the momentum balance? What specific external forces are you using? Just because it is an open system, doesn't mean that magic suddenly can happen. The open system of a satellite in LEO transfers both energy and momentum into the thin atmosphere, causing the satellite to slow down and eventually crash.

It would be easier if we could define when a “closed system” is a “closed system” which is what I have tried to do in the earlier posts. If we place a charged battery into the spaceship and close all the hatches, have we got a” closed system”? So another little thought experiment! We take the cannon outside the spaceship and leave a hatch open. We fire the cannon so the cannon ball flies through the hatch and then close the hatch. What have we got?. Technically it is a “closed system” since the cannon ball has not interacted with the spaceship. But we have an “open system” in waiting since we can tell the future and know what is going to happen. And as soon as we define the system as “closed” does the cannon ball become part of the system so cannot be referred to as “exterior”
You simply have to define what your system is. If you are going to leave the hatch open and let the cannon ball fly out you can either include the cannon ball in the system or not. If you include the cannon ball, then the center of mass of the spaceship + ball will not move, but both will fly off into infinity. If you don't include the ball, then the spacecraft transferred some energy to the ball and transferred momentum to the ball that is equal and opposite to the momentum the spacecraft ends up with. Both perspectives lead to the same result. If you are going to close the hatch before the ball can leave, then it only makes sense to consider the spacecraft and ball as a single system. You don't have to, but it makes it easier to see what would happen.

With reference to the exhaust, I need to know how you define “conserve”
Would it not be better to use the term energy/momentum are always “accounted for”?
I know someone used the phrase "account for" with you to try to get you to understand. It doesn't seem like it worked. Conserve is the standard well defined term that physicists use though, which you can look up.

I am not great at definitions, and at this point you should really just go do some research yourself, and maybe take an introductory physics class somewhere.

Basically conserve means that if you add up the momentum or the energy of some set of objects that don't interact with any other objects, it will not change with time. If they do interact with something else you can just add those objects to the system to see that momentum and energy are both still conserved. Otherwise you define the flux of momentum and energy in and out of the system as the interactions with the objects external to the system.

Offline whitelancer64

Re: Conservation of energy/momentum.
« Reply #14 on: 09/13/2017 02:41 PM »
*snip*
With reference to the exhaust, I need to know how you define “conserve”
Would it not be better to use the term energy/momentum are always “accounted for”?

In science, the word "conserve" means that the total amount is unchanged through the course of a process. The amount of energy in a given system can change forms (for example, a chemical reaction can give off heat), but the amount of energy in the system always remains the same before and after the reaction. Same with momentum, in an interaction between two bodies, the total amount of momentum always stays the same before and after the interaction.
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Offline Rocket Science

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Re: Conservation of energy/momentum.
« Reply #15 on: 09/13/2017 02:49 PM »
Please note not all collisions are equal: elastic vs inelastic collisions. A perfectly elastic collision is defined as one in which there is no loss of kinetic energy in the collision. ... Momentum is conserved in inelastic collisions, but one cannot track the kinetic energy through the collision since some of it is converted to other forms of energy.
https://www.khanacademy.org/science/physics/linear-momentum/elastic-and-inelastic-collisions/v/elastic-and-inelastic-collisions
« Last Edit: 09/13/2017 02:50 PM by Rocket Science »
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Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #16 on: 09/13/2017 03:46 PM »
You simply have to define what your system is. If you are going to leave the hatch open and let the cannon ball fly out you can either include the cannon ball in the system or not.

The cannon ball flies INTO the spaceship, then the hatch is closed.
Is it then classed as a "closed system".
If it is a "closed system" what will happen to conserve momentum in the system.

Online meberbs

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Re: Conservation of energy/momentum.
« Reply #17 on: 09/13/2017 03:59 PM »
You simply have to define what your system is. If you are going to leave the hatch open and let the cannon ball fly out you can either include the cannon ball in the system or not.

The cannon ball flies INTO the spaceship, then the hatch is closed.
Is it then classed as a "closed system".
If it is a "closed system" what will happen to conserve momentum in the system.
If you are defining it as a closed system, then the system is spacecraft + ball. The system starts with net momentum, and ends with the same net momentum.

The definition of a system boundary is arbitrary and based on whatever makes the solution easier to calculate. Often this means finding a set of objects that can be considered a closed system.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #18 on: 09/14/2017 12:06 PM »
Jamie and I did discuss this data with Roger. He commented it was representative of what he would expect from Jamie's test rig.
But it is not representative of what would happen if the force generation stopped when the acceleration stopped. In that case, the drive would return straight back to the null position from the first peak (and oscillate around it) rather than oscillating around the forward position.

As for what could be causing the displacement, you have to remember that while it is only 2W of power, it is also only equivalent to a couple of microNewtons of force. The end of the generation of the apparent force does not correspond to the drive reaching 0 acceleration or to the end of the RF power. This is not an encouraging piece of data for the emDrive working.

I would like to introduce another thought experiment that may be helpful.
I have a long sealed box and attached to one end is a small very rapid firing machine gun (10,000 rpm) with a magazine of 10,000 rounds. I place the box on a sensitive thrust measuring device and turn the machine gun on. The box will show a slight deflection which would indicate thrust and the deflection will last for 60 seconds. If the box is taken into space and the experiment repeated, I am of the opinion that the box will initially accelerate but then the box will move at a steady (if not rapid jerky motion) rate (Not accelerate) until the magazine is exhausted.  Are we not exploiting the time lag between action and reaction which may give us a false impression on the measuring device?

Offline ppnl

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Re: Conservation of energy/momentum.
« Reply #19 on: 09/14/2017 09:26 PM »

No, there is no time lag between action and reaction. The gun fires a bullet. Action. The box is instantly pushed in the other direction. Reaction. The bullet hits the far wall. Action. The impulse of the bullet instantly stops the motion of the box. Reaction. Action and reaction are connected instantaneously.  You just have a pair of them.

The center of gravity of the box will not move at all. The box will appear to move but no matter how many bullets you use it will always move by less than the length of the box. You are just shifting weights inside the box causing its position with respect to its center of gravity to shift. The center of gravity itself does not shift. Get a broom and sweep the bullets back to the other end and the box will move back to its original position.

Your measuring device will show all of this perfectly.

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