Author Topic: Conservation of energy/momentum.  (Read 12793 times)

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #40 on: 09/27/2017 06:01 PM »
You are evading the question. The magnets will accelerate at ninety degrees to the direction of travel. As they accelerate, since they have mass, their momentum and kinetic energy will increase. Where is this energy coming from?

You had to do "work" to bring the two magnets together. That stores potential energy. When you release them, that energy that YOU put in is released as they separate and converted into their motion.
You are evading the question. The magnets will accelerate at ninety degrees to the direction of travel. As they accelerate, since they have mass, their momentum and kinetic energy will increase. Where is this energy coming from?
I didn't avoid anything. I already answered this exact question twice.

For the third time: Potential energy.
https://en.wikipedia.org/wiki/Potential_energy

And to be technical (which you should be when talking about physics) It is only their kinetic energy that increases. Total energy includes the potential energy, and therefore stays constant. And again, total momentum does not change.
I am really pleased that you have introduced potential energy, we will discuss this at a later date.
So to be technical, both of you are saying that after the two magnets have joined, the velocity of the two magnets is the same as when they were released?

Offline meberbs

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Re: Conservation of energy/momentum.
« Reply #41 on: 09/27/2017 06:30 PM »
I am really pleased that you have introduced potential energy, we will discuss this at a later date.
So to be technical, both of you are saying that after the two magnets have joined, the velocity of the two magnets is the same as when they were released?
Assuming they inelastically collide and stick together, then of course they do.

Your next question is presumably going to be what happens to the energy in this case. This question is answered by my qualification of "inelastic collision". You can read the opening of the wikipedia article on the subject for more information.

https://en.wikipedia.org/wiki/Inelastic_collision

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #42 on: 09/27/2017 07:01 PM »
No, that was not my next question. I am however getting excited here. What you are saying is that the magnets can travel the extra distance without the need for any extra energy. Just to be cautious would this not contravene conservation of energy?

Offline meberbs

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Re: Conservation of energy/momentum.
« Reply #43 on: 09/27/2017 07:03 PM »
No, that was not my next question. I am however getting excited here. What you are saying is that the magnets can travel the extra distance without the need for any extra energy. Just to be cautious would this not contravene conservation of energy?
How exactly do you think this would contradict conservation of energy?
Conservation of energy holds.

Offline whitelancer64

Re: Conservation of energy/momentum.
« Reply #44 on: 09/27/2017 07:08 PM »
No, that was not my next question. I am however getting excited here. What you are saying is that the magnets can travel the extra distance without the need for any extra energy. Just to be cautious would this not contravene conservation of energy?

Not sure why you're excited. There's utterly no need for "extra" energy. The magnets already contained all the energy needed for their movement. The total amount of energy is the same at the beginning and at the end of your thought experiment (only some of it had been converted to kinetic energy), so yes, energy is conserved.
"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 #45 on: 09/27/2017 07:33 PM »
 That little exchange has been very helpful. I am going on holiday for a few days . We can resume when I get back.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #46 on: 10/12/2017 04:44 PM »
I feel we have covered enough ground to attempt a little game of chess with nature. We are all aware that it will not be played on an eight by eight board and hopefully we are all in agreement that it is impossible to get nature in checkmate. It is however possible to manoeuvre nature so that the outcome is desirable to us. We know that we must adhere to one strict rule which is conservation of energy and nature will do whatever it takes to ensure that this rule is not broken. I would like to discuss how nature sets up a repetitive cycle that conserves energy and what will happen when that cycle is broken. I am going to add the link to the video that was earlier posted in my other thread since this clearly shows this cycle in operation. There was an objection raised that the device was using the initial counter rotation to enable the end result and I would like to counter this objection by pointing out that if a counter rotating device were to be added and that if this counter rotating device were to be switched off at the earlier mentioned point (when the tube magnet is repelled by the body magnet) in the jerk then there would be no initial counter rotation. 



I will pause to allow people who have not seen the video to view it.

Offline whitelancer64

Re: Conservation of energy/momentum.
« Reply #47 on: 10/12/2017 05:15 PM »
There was already a thread dedicated to explaining to you that this noisy motor on a string doesn't violate the laws of physics. You should re-read that.

http://forum.nasaspaceflight.com/index.php?topic=43433.0
"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 meberbs

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Re: Conservation of energy/momentum.
« Reply #48 on: 10/12/2017 05:17 PM »
I feel we have covered enough ground to attempt a little game of chess with nature.
This is a terrible analogy. Nature is not the opponent in a game, if anything it is the rules of the game.

There was an objection raised that the device was using the initial counter rotation to enable the end result
You apparently did not understand a thing that was said to you. The objection is that anyone who can do force balances (or torque balances in this case) would see that the angular momentum gained by the device at the end of the video was transferred to the device from the Earth through the string.

You also only talked about conservation of energy in this post. Energy that allows the device to acquire kinetic energy clearly comes from the power supplied to the motor. The issue you are not recognizing is the completely separate question of momentum conservation (specifically angular momentum).

The rest of your description fall back to your previous issues with communication, where you refer to adding a counter rotating device without explaining what part of the device it is attached to and whether it is counter rotating relative to the body (wooden part) or the arms.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #49 on: 10/12/2017 06:32 PM »
I feel we have covered enough ground to attempt a little game of chess with nature.
This is a terrible analogy. Nature is not the opponent in a game, if anything it is the rules of the game.

There was an objection raised that the device was using the initial counter rotation to enable the end result
You apparently did not understand a thing that was said to you. The objection is that anyone who can do force balances (or torque balances in this case) would see that the angular momentum gained by the device at the end of the video was transferred to the device from the Earth through the string.

You also only talked about conservation of energy in this post. Energy that allows the device to acquire kinetic energy clearly comes from the power supplied to the motor. The issue you are not recognizing is the completely separate question of momentum conservation (specifically angular momentum).

The rest of your description fall back to your previous issues with communication, where you refer to adding a counter rotating device without explaining what part of the device it is attached to and whether it is counter rotating relative to the body (wooden part) or the arms.

Can you have kinetic energy without momentum?

Offline meberbs

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Re: Conservation of energy/momentum.
« Reply #50 on: 10/12/2017 06:44 PM »
Can you have kinetic energy without momentum?
As an example, your device has angular momentum, but its total linear momentum is 0. Pieces of it have different amounts of linear momentum if you break it up of course, and it clearly has non zero kinetic energy.

"Kinetic energy" is not a conserved quantity though. Total energy (of all types) is what is conserved, and you can clearly have energy without any momentum (both linear or angular).

It appears that you still fail to understand the basic physics concepts that have been explained to you. Perhaps you would be better served if you took an introductory physics class somewhere where they could teach you these things piece at a time.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #51 on: 10/12/2017 06:54 PM »
angular momentum gained by the device at the end of the video was transferred to the device from the Earth through the string

How?

Offline meberbs

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Re: Conservation of energy/momentum.
« Reply #52 on: 10/12/2017 07:04 PM »
angular momentum gained by the device at the end of the video was transferred to the device from the Earth through the string

How?
The string applies a torque. This was explained to you repeatedly in the other thread. You can easily see the torque by taking your device (while it is off), and rotating it 180 degrees. You will see the string then cause it to rotate back to the rest position (and then oscillate back and forth until dissipative forces such as air resistance bring it to rest).

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #53 on: 10/12/2017 07:25 PM »
angular momentum gained by the device at the end of the video was transferred to the device from the Earth through the string

How?
The string applies a torque. This was explained to you repeatedly in the other thread. You can easily see the torque by taking your device (while it is off), and rotating it 180 degrees. You will see the string then cause it to rotate back to the rest position (and then oscillate back and forth until dissipative forces such as air resistance bring it to rest).

And I repeatedly told you that the string is not wound when the power is switched off. So the string applies no torque to the device at that moment.

Offline whitelancer64

Re: Conservation of energy/momentum.
« Reply #54 on: 10/12/2017 07:51 PM »
angular momentum gained by the device at the end of the video was transferred to the device from the Earth through the string

How?
The string applies a torque. This was explained to you repeatedly in the other thread. You can easily see the torque by taking your device (while it is off), and rotating it 180 degrees. You will see the string then cause it to rotate back to the rest position (and then oscillate back and forth until dissipative forces such as air resistance bring it to rest).

And I repeatedly told you that the string is not wound when the power is switched off. So the string applies no torque to the device at that moment.

However, the string did wind up while the device was on. Turning it off lets the string unwind, which is exactly what is seen in the video.

Additionally, even if the device were operating at a steady state in the "rest" position, simply switching the device off doesn't mean the device is no longer acting on the string, for example, the motor winds down without the interference of the magnets, that will rotate the device.

Also, since this has never been clear to me even though I asked twice in the previous thread: what is it that you are claiming this device does? You provided several contradictory statements as to how you think it works, but what exactly is it you think the end result of operating this device is?
"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 meberbs

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Re: Conservation of energy/momentum.
« Reply #55 on: 10/12/2017 07:58 PM »
angular momentum gained by the device at the end of the video was transferred to the device from the Earth through the string

How?
The string applies a torque. This was explained to you repeatedly in the other thread. You can easily see the torque by taking your device (while it is off), and rotating it 180 degrees. You will see the string then cause it to rotate back to the rest position (and then oscillate back and forth until dissipative forces such as air resistance bring it to rest).

And I repeatedly told you that the string is not wound when the power is switched off. So the string applies no torque to the device at that moment.
And you have been repeatedly told the state of the string when the device is turned off is irrelevant. While the device is on, the string applies torques. The net result of all of these torques while the device is on is the angular momentum the device has at the moment that you turn it off.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #56 on: 10/13/2017 02:56 PM »
angular momentum gained by the device at the end of the video was transferred to the device from the Earth through the string

How?
The string applies a torque. This was explained to you repeatedly in the other thread. You can easily see the torque by taking your device (while it is off), and rotating it 180 degrees. You will see the string then cause it to rotate back to the rest position (and then oscillate back and forth until dissipative forces such as air resistance bring it to rest).

And I repeatedly told you that the string is not wound when the power is switched off. So the string applies no torque to the device at that moment.
And you have been repeatedly told the state of the string when the device is turned off is irrelevant. While the device is on, the string applies torques. The net result of all of these torques while the device is on is the angular momentum the device has at the moment that you turn it off.

The state of the string when the device is switched off is everything. It is the only thing that can apply an exterior force(torque) to the device at that moment. Since the string applies zero torque at that time, it has no influence on the observed result.

Offline meberbs

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Re: Conservation of energy/momentum.
« Reply #57 on: 10/13/2017 03:25 PM »
And you have been repeatedly told the state of the string when the device is turned off is irrelevant. While the device is on, the string applies torques. The net result of all of these torques while the device is on is the angular momentum the device has at the moment that you turn it off.

The state of the string when the device is switched off is everything. It is the only thing that can apply an exterior force(torque) to the device at that moment. Since the string applies zero torque at that time, it has no influence on the observed result.
Which words in my previous post did you not understand? Your response sounds like you did not read anything other than the first sentence of my post, because you did not address the simple fact that the string had already transferred angular momentum to the device. You can clearly see in the video that the device has angular momentum just before you turn it off. Therefore, it doesn't need any more torques applied to it for it to continue to spin.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #58 on: 10/14/2017 02:26 PM »
And you have been repeatedly told the state of the string when the device is turned off is irrelevant. While the device is on, the string applies torques. The net result of all of these torques while the device is on is the angular momentum the device has at the moment that you turn it off.

The state of the string when the device is switched off is everything. It is the only thing that can apply an exterior force(torque) to the device at that moment. Since the string applies zero torque at that time, it has no influence on the observed result.
Which words in my previous post did you not understand? Your response sounds like you did not read anything other than the first sentence of my post, because you did not address the simple fact that the string had already transferred angular momentum to the device. You can clearly see in the video that the device has angular momentum just before you turn it off. Therefore, it doesn't need any more torques applied to it for it to continue to spin.

The acceleration of the rotor arms has nothing to do with the string. Since there is no counter rotation of the body when the rotor arms accelerate just before switch off, the strings only influence is to suspend the device off the ground. It is important that you and others remain sceptical. For anyone else reading this thread feel free to correct (not insult) me at any time, being wrong is good, it means I am learning.
We could go round and round (no pun intended) with the string so I am going to move on with my next post.

Offline meberbs

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Re: Conservation of energy/momentum.
« Reply #59 on: 10/14/2017 04:06 PM »
The acceleration of the rotor arms has nothing to do with the string.
This sentence makes no sense. The system under question is the combination of the rotor arms and the body.

The torques applied by the string transfer angular momentum to that system. Through the motor and magnets, angular momentum is exchanged between the rotor and the body, but the total angular momentum preset in the system of arms+body is due to torques applied by the string.

Since there is no counter rotation of the body when the rotor arms accelerate just before switch off,
What do you mean "arms accelerate just before switch off"? Looking at the video the arms appear to be rotating with roughly constant speed just before switch off, in other words, not accelerating. Just after switch off, the arms clearly accelerate (slow down) by transferring their angular momentum to the body.
 
being wrong is good, it means I am learning.
We could go round and round (no pun intended) with the string so I am going to move on with my next post.
The string is the source of all of the angular momentum in the system. If you do not understand that, it means that you are still wrong. If you refuse to further discuss the string, it means you are not interested in learning.

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