Author Topic: Conservation of energy/momentum.  (Read 27886 times)

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #20 on: 09/15/2017 11:34 AM »
We are looking at the action, reaction of the box. The bullet has a distance to travel , therefore it will take time.
Using the level pendulum test,( no tilting) sustained deflection would be a strong indication of net thrust.
We appear to agree that although  we have a strong indication of net thrust, the device will not accelerate for the period of 60 seconds and will operate as you have said. Therefore contrary to what the test results suggest, it does not work.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #21 on: 09/16/2017 11:11 AM »
I would like to ask some questions. I am in a spacecraft (I have grip on the floor) and at one end a spring is attached to the wall. I hold the spring and begin to walk backwards, extending the spring. 1) What will happen to the spacecraft as I walk backwards? 2) What will happen to the spacecraft when I let go of the spring? 3) What will happen to the spacecraft when the spring has returned to its original state?

Online meberbs

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Re: Conservation of energy/momentum.
« Reply #22 on: 09/16/2017 02:01 PM »
I would like to ask some questions. I am in a spacecraft (I have grip on the floor) and at one end a spring is attached to the wall. I hold the spring and begin to walk backwards, extending the spring. 1) What will happen to the spacecraft as I walk backwards? 2) What will happen to the spacecraft when I let go of the spring? 3) What will happen to the spacecraft when the spring has returned to its original state?
Honestly I think it would be more helpful for you to try to work out the answer yourself, keeping in mind that the combined center of mass of the spaceship and all objects inside it will not move. In this case that refers to the hull, you, and the spring. Drawing a diagram may help with this.

Note that to simplify things in mass-spring systems, physicists often assume that a spring has no mass, which is fine when masses attached to either end of the spring have significantly more mass than the spring. Since you are letting go of one end of the spring, there is no mass attached to that end, so you can't consider the spring massless if you want to see the dynamics of the spring retracting.

Offline kamill85

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Re: Conservation of energy/momentum.
« Reply #23 on: 09/16/2017 11:25 PM »
Related to the main topic:

Test object description: spherical object with embedded power source, with outer layer composed of disks connected to motors beneath. Motors can be remotely activated at any time.

Now, if we had two test objects of the same rest-mass, accelerated both to same speed, and then remotely activated motors of one of them, would that on object have higher kinetic energy from our point of view?
Assumption: 100% efficient motors, no heat radiation, basically 100% of the battery power gets converted into spin.

Question comes from the fact that if something gets a spin in space, for example super fast rotating black holes or neutron stars, then such objects appear to have higher mass, create "deeper" gravity well.

If true: 2 saucer shaped enclosures could house rotation rigs of circularly arranged motors with certain mass to rotate at speed = X. Rotation rigs would rotate in opposite directions, with equal speed = Y, that can be changed at any time. Central unit between the two enclosures would keep track of some target, and activate motor approaching its direction in either enclosure while its at 90'c angle until 0', leaving 0' to -90' for de-acceleration. Basically, motors would be only spinning while they are on the 180' side that points toward the target. Speeds X and Y would control thrust level?

PS. Similar "drive" was proposed for capacitors, where at 90' angle they are loaded with power and unloaded at -90'. So 180' gets rotated with empty capacitor and 180' with loaded. Electricity only gets through at c from the middle outwards to capacitor at same, horizontal line towards the target, different direction between the enclosures, hence possible motion is canceled out. In this example even if mass of capacitor grows only a little bit, the difference is directly multiplied/scaled by the rotation speed.

PPS. I am aware effects described here would be barely detectable, due to mechanical stress limiting the speeds, but if there was a way to capture similar idea in less mechanical and more "solid state" fashion, it could be something.
« Last Edit: 09/16/2017 11:59 PM by kamill85 »

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #24 on: 09/17/2017 09:54 AM »
I would like to ask some questions. I am in a spacecraft (I have grip on the floor) and at one end a spring is attached to the wall. I hold the spring and begin to walk backwards, extending the spring. 1) What will happen to the spacecraft as I walk backwards? 2) What will happen to the spacecraft when I let go of the spring? 3) What will happen to the spacecraft when the spring has returned to its original state?
Honestly I think it would be more helpful for you to try to work out the answer yourself, keeping in mind that the combined center of mass of the spaceship and all objects inside it will not move. In this case that refers to the hull, you, and the spring. Drawing a diagram may help with this.

Note that to simplify things in mass-spring systems, physicists often assume that a spring has no mass, which is fine when masses attached to either end of the spring have significantly more mass than the spring. Since you are letting go of one end of the spring, there is no mass attached to that end, so you can't consider the spring massless if you want to see the dynamics of the spring retracting.

I do like to ask questions, I find I learn things by doing so. There are other people who may be interested in the answer also and you have answered the question far better than I could have done.

Offline A_M_Swallow

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Re: Conservation of energy/momentum.
« Reply #25 on: 09/17/2017 11:48 AM »
Just a reminder.

E = m c2

Energy is equal to MASS not momentum. So it is energy-mass that is conserved. The conservation of momentum is a different conservation. When items collide two separate things are conserved at the same time, this can have some interesting side effects

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #26 on: 09/18/2017 09:26 AM »
I am on your wavelength here (no pun intended), although there is a school of thought that says a photon has momentum but no mass?

Offline dustinthewind

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Re: Conservation of energy/momentum.
« Reply #27 on: 09/18/2017 01:40 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.

Energy is the integral of momentum with respect to velocity.  int(m*v , dv) = 1/2*m*v^2.  You can also think of it as E=F*dx --> int(m*a , dx --> m*v*dx/dt) = 1/2*m*v^2 .  Of course at higher speeds one must take into account relativistic terms.  One might claim if momentum is conserved then the sum of all momentum should also be conserved. 

PS there are extra terms of the mass of the system is changing.
« Last Edit: 09/18/2017 06:08 PM by dustinthewind »

Offline dustinthewind

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Re: Conservation of energy/momentum.
« Reply #28 on: 09/18/2017 01:55 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.

For newtons cradle the kinetic energy/momentum is retained in the matter during elastic collision as kinetic energy.  There is some potential flexing of bonds in the process but it will still be kinetic I think.  If you store energy in a battery, your storing that kinetic energy/momentum in chemical bonds.  After that process I think the stored energy can be converted to different systems efficiently.  Inherent inefficiencies would probably be related to different relative masses.  Newtons cradle ~ 100% efficient at conveying kinetic energy/momentum between its systems of masses. 

Edit: I think your talking about using an EM drive powered by solar panels.  Saying the EM drive will work if solar panels are used doesn't make it suddenly an open system.  The solar panels reflecting or absorbing light can get thrust that way but its inefficient and it can be stored as power.  From the stored power you could run your EM drive but how that energy acts on something that will escape the ship so it can gain momentum is still a mystery that has not yet been solved.

« Last Edit: 09/19/2017 02:26 AM by dustinthewind »

Online meberbs

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Re: Conservation of energy/momentum.
« Reply #29 on: 09/18/2017 03:53 PM »
I am on your wavelength here (no pun intended), although there is a school of thought that says a photon has momentum but no mass?
Yes, the energy equation E = m0*c^2 uses the rest mass m0 of the object. Photons have no rest mass, but the full energy equation in special relativity is:

E^2 = (m0*c^2)^2 + (p*c)^2

Where p is the momentum.

For a massless particle, this reduces to E = p*c.

The energy equation can be rearranged for massive particles based on the relativistic momentum p =γ*m0*v as:

E = γ*m0*c^2

The relativistic mass is m = m0*γ, so this is equivalent to E = m*c^2. Note that some references just use m for rest mass, so there are 2 things that the equation E=m*c^2 can mean depending on the definition of the variables (both are true):
-Energy of an object at rest equals its rest mass times the speed of light squared.
-Total energy including kinetic of a moving object equals its relativistic mass times the speed of light squared.

Note that the definition of momentum p =γ*m0*v does not work for a massless particle, because mass is 0, but γ is infinity when velocity is the speed of light. This makes the equation E = γ*m0*c^2 meaningless for a massless particle, since you cannot calculate the result. instead you have to go to the first one and say E = p*c. Then if you want you can define a relativistic mass of a massless particle to be m = E/c^2 = p/c, but it still has no rest mass.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #30 on: 09/26/2017 06:37 PM »
 The zero approach to conservation of momentum (what I like to call  “you can only use what god gave us”) is a newtonian third law idea that makes perfect sense. For an object to accelerate something else must decelerate or vice versa. Therefore N + -N = 0. This is why the argument will always be for there to be an exhaust of some kind. So with this in mind we can do another thought experiment:
I am travelling through nothing, I am going to give myself a direction and I am side on to the direction I am travelling. I am holding a magnet in each hand and my arms are outstretched in front of me. The poles of the magnets are opposite so when I release them they will attract each other. When I let go of the magnets one magnet will accelerate and the other magnet will decelerate. I would however like to ask a question?
If I turn so that I am facing the direction of travel and repeat the experiment, what will now happen when I release the magnets?

Online meberbs

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Re: Conservation of energy/momentum.
« Reply #31 on: 09/26/2017 06:49 PM »
When I let go of the magnets one magnet will accelerate and the other magnet will decelerate.
Both magnets accelerate. "Decelerate" is not a generally useful term in physics, since it means "accelerate in a specific direction," and using it then causes confusion, as you are demonstrating, when acceleration is not in a direction parallel to the velocity.

If I turn so that I am facing the direction of travel and repeat the experiment, what will now happen when I release the magnets?
The exact same thing, both magnets accelerate. The are accelerating in opposite directions, so momentum is conserved. You have to remember that momentum is a vector quantity, not a scalar quantity. It has a direction. (If you aren't familiar with the concepts of scalar and vector quantities, I suggest you do some research and studying on your own.)

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #32 on: 09/26/2017 06:56 PM »
What about energy conservation?

Online meberbs

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Re: Conservation of energy/momentum.
« Reply #33 on: 09/26/2017 07:35 PM »
What about energy conservation?
Do you have a specific question about it? The last scenario you described simply involves conversion of potential energy to kinetic energy.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #34 on: 09/27/2017 03:40 PM »
What about energy conservation?
Do you have a specific question about it? The last scenario you described simply involves conversion of potential energy to kinetic energy.

Well there are numerous questions that can be asked but we will start with a specific one. When I let go of the magnets they will, as you have stated, accelerate towards one another. Using inverse cube we know that the rate of acceleration will rise dramatically as the distance between them decreases. So where are the magnets getting their energy/momentum from?

Online meberbs

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Re: Conservation of energy/momentum.
« Reply #35 on: 09/27/2017 03:58 PM »
What about energy conservation?
Do you have a specific question about it? The last scenario you described simply involves conversion of potential energy to kinetic energy.

Well there are numerous questions that can be asked but we will start with a specific one. When I let go of the magnets they will, as you have stated, accelerate towards one another. Using inverse cube we know that the rate of acceleration will rise dramatically as the distance between them decreases. So where are the magnets getting their energy/momentum from?
Here is a helpful suggestion: next time you want to type "energy/momentum" DON'T

Typing that indicates that you still don't understand what you have been told repeatedly, that energy and momentum are different concepts.

What you really have is 2 separate questions, both of which I already answered:

Where do they get there momentum from?
They don't get momentum from anywhere. The total momentum of 2 magnets that both start at rest starts at 0 and remains 0 as they accelerate towards each other. Each magnet at all times has equal (magnitude) and opposite (direction) momentum to the other one.

Where do they get energy from?
The last scenario you described simply involves conversion of potential energy to kinetic energy.

P.S. Force between magnetic dipoles goes as 1/x^4, not inverse cube (1/x^3), though it gets a bit more complicated with a shaped magnet.

Offline chazemz

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Re: Conservation of energy/momentum.
« Reply #36 on: 09/27/2017 04:19 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?

Online meberbs

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Re: Conservation of energy/momentum.
« Reply #37 on: 09/27/2017 04:26 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?
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.

Offline WarpTech

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Re: Conservation of energy/momentum.
« Reply #38 on: 09/27/2017 04:44 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.

Offline whitelancer64

Re: Conservation of energy/momentum.
« Reply #39 on: 09/27/2017 05:05 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?

It seems to me the question you are trying to ask is "what causes magnets to move toward each other." The answer is their magnetic fields. The potential energy that is stored in their magnetism is converted into kinetic energy.
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