Acceleration and kinetic energy conflict?

[magnemoe]

This comes up in another emdrive tread however it would be relevant for other settings too at least theoretical.

Issue is that if you accelerate with an constant value like 1 m/s/s this acceleration require an fixed force and wherefore fixed power who depend on force and the efficiency of the drive.

In this setting we are operating from 0 m/s up to high orbital speeds relativity is not important, the drive is an ideal drive, with only an efficiency loss between energy and force.

Kinetic energy increased with an square of the speed, at some point the kinetic energy will grow faster than the energy you put into the drive.

Em drive should work like this, normal rockets does not, neither do practical vertices like planes, cars or trains simply because of air resistance, rolling resistance and gearing.

An long optimized coilgun might too, perhaps electromagnetic tethers. Note that none are reactionless drives however they should be able to do an constant acceleration for constant power at least theoretical or within some limited speed ranges and both are pretty effective compared to emdrive.

Anyway as I understand Newton its nothing to stop an theoretical train from moving down friction less rails in vacuum with an constant acceleration powered by constant power.

However the kinetic energy is relative compared to the point of reference, 10 m/s relative to the space station you left is non issue, as the speed is low, relative to the sun this effect would kick in almost at once as you move fast, however if you run an probe on emdrive to Pluto it would hit with more energy than it had used during the trip.

That is wrong here? Yes emdrive violate the conservation of momentum, violating no freee energy is far more serious, still the theoretical train could also be aimed at Pluto with the same result.

[N_las]

I didn't quite get how your theoretical frictionless train would move. The whole reason a train can move IS the friction of the rails against the wheels. Without friction, it wouldn't move in the first place. So the problem you want to discuss is only really a problem for a "reactionless" drive, and it is just one of the many arguments why it is nonsense. A reactionless drive would be able to produce free energy out of nothing.

Edited February 8, 2015 by N_las

[Iskierka]

Energy exchanged when a force occurs is only with respect to the two mediums exchanging force - relativity must be observed, so only the relative velocity matters in terms of the energy requirements and result.

Power is force times velocity; so the power required to accelerate is mass * acceleration * velocity - if you are pushing against the earth to accelerate, there is no conflict, as going faster will result in requiring more power to generate the force. With rocket vehicles, there is no conflict, as the only energies relevant in the generation of force are the relative energies of the propellant and rocket - which means for constant Isp and thrust, the rocket will maintain a constant thermal power output.

This also does not conflict with the energy an observer calculates, as the observer will see the acceleration add energy to the propellant, which is then expelled out the back and the energy is gained by the rest of the rocket. As a fuelled rocket has limited capability to do this, and energy added to the final vehicle will never exceed the energy available in fuel, or fuel + power source combo, in the case of ion drives.

The EmDrive is very bizarre, as it is a fuelless rocket, and so to begin with it's already violating multiple conservation of momentum laws, nevermind energy. Apparently the winning theory so far is that it reacts against quantum plasma, in which case we have no idea how conservation applies, or if it even does, as we don't really understand quantum plasma itself. A conventional rocket does not violate conservation of momentum, or energy, but the EmDrive is a mystery as to how it even works, so we cannot judge whether it violates these laws.

I'm personally doubtful of it, but it should be noted that it's possible that the EmDrive may be allowed to violate these conservations. There are many conservation laws in particle physics, most of which are absolutely conserved, but occasionally, some are not. Strangeness is conserved in all strong nuclear and electromagnetic interactions, but for unknown reasons the weak nuclear force can eliminate strangeness at will. It seems very unlikely, but there's a slim chance that whatever the EmDrive is interacting with occurs through a mechanism that does not have to conserve momentum or energy in certain cases. If it does conserve, then we need to find out what impact on the universe the EmDrive has, to determine where the momentum and energy has gone.

[Rakaydos]

This comes up in another emdrive tread however it would be relevant for other settings too at least theoretical.

Issue is that if you accelerate with an constant value like 1 m/s/s this acceleration require an fixed force and wherefore fixed power who depend on force and the efficiency of the drive.

In this setting we are operating from 0 m/s up to high orbital speeds relativity is not important, the drive is an ideal drive, with only an efficiency loss between energy and force.

Kinetic energy increased with an square of the speed, at some point the kinetic energy will grow faster than the energy you put into the drive.

Em drive should work like this, normal rockets does not, neither do practical vertices like planes, cars or trains simply because of air resistance, rolling resistance and gearing.

An long optimized coilgun might too, perhaps electromagnetic tethers. Note that none are reactionless drives however they should be able to do an constant acceleration for constant power at least theoretical or within some limited speed ranges and both are pretty effective compared to emdrive.

Anyway as I understand Newton its nothing to stop an theoretical train from moving down friction less rails in vacuum with an constant acceleration powered by constant power.

However the kinetic energy is relative compared to the point of reference, 10 m/s relative to the space station you left is non issue, as the speed is low, relative to the sun this effect would kick in almost at once as you move fast, however if you run an probe on emdrive to Pluto it would hit with more energy than it had used during the trip.

That is wrong here? Yes emdrive violate the conservation of momentum, violating no freee energy is far more serious, still the theoretical train could also be aimed at Pluto with the same result.

This is called the oberth effect. Move along.

[MockKnizzle]

Not really, because the Oberth effect has to do with the fact that the rocket/propellant energy exchange is more efficient at higher speeds, and the fact that the propellant stores kinetic energy in excess of it's chemical energy. With an emdrive, there isn't any onboard propellant to store kinetic energy.

[magnemoe]

I didn't quite get how your theoretical frictionless train would move. The whole reason a train can move IS the friction of the rails against the wheels. Without friction, it wouldn't move in the first place. So the problem you want to discuss is only really a problem for a "reactionless" drive, and it is just one of the many arguments why it is nonsense. A reactionless drive would be able to produce free energy out of nothing.

Yes, an reactionless drive obviously conflict with conservation of momentum, however conflicting with conservation of energy as in perpetual mobile is way worse.

I tries to come up with other scenarios who has the same issue where constant energy gives constant force and constant acceleration.

The frictionless train might be an magelv train however the point is if an acceleration with constant force and energy use breaks newtons laws in it self not if its practical.

Closest example might be acceleration in an magnetic field.

[cpast]

Based on what I remember from GR, isn't there a relationship between conservation of energy and conservation of momentum? Or am I misremembering things?

[Bill Phil]

Momentum is related to energy, but it's not exactly the same...

or something like that.

[PB666]

I didn't quite get how your theoretical frictionless train would move. The whole reason a train can move IS the friction of the rails against the wheels. Without friction, it wouldn't move in the first place. So the problem you want to discuss is only really a problem for a "reactionless" drive, and it is just one of the many arguments why it is nonsense. A reactionless drive would be able to produce free energy out of nothing.

A maglev train is propelled by magnetic attraction and repulsion. Friction is one way of creating 'an equal and opposite reaction'.

- - - Updated - - -

Energy exchanged when a force occurs is only with respect to the two mediums exchanging force - relativity must be observed, so only the relative velocity matters in terms of the energy requirements and result.

Power is force times velocity; so the power required to accelerate is mass * acceleration * velocity - if you are pushing against the earth to accelerate, there is no conflict, as going faster will result in requiring more power to generate the force. With rocket vehicles, there is no conflict, as the only energies relevant in the generation of force are the relative energies of the propellant and rocket - which means for constant Isp and thrust, the rocket will maintain a constant thermal power output.

This also does not conflict with the energy an observer calculates, as the observer will see the acceleration add energy to the propellant, which is then expelled out the back and the energy is gained by the rest of the rocket. As a fuelled rocket has limited capability to do this, and energy added to the final vehicle will never exceed the energy available in fuel, or fuel + power source combo, in the case of ion drives.

The EmDrive is very bizarre, as it is a fuelless rocket, and so to begin with it's already violating multiple conservation of momentum laws, nevermind energy. Apparently the winning theory so far is that it reacts against quantum plasma, in which case we have no idea how conservation applies, or if it even does, as we don't really understand quantum plasma itself. A conventional rocket does not violate conservation of momentum, or energy, but the EmDrive is a mystery as to how it even works, so we cannot judge whether it violates these laws.

I'm personally doubtful of it, but it should be noted that it's possible that the EmDrive may be allowed to violate these conservations. There are many conservation laws in particle physics, most of which are absolutely conserved, but occasionally, some are not. Strangeness is conserved in all strong nuclear and electromagnetic interactions, but for unknown reasons the weak nuclear force can eliminate strangeness at will. It seems very unlikely, but there's a slim chance that whatever the EmDrive is interacting with occurs through a mechanism that does not have to conserve momentum or energy in certain cases. If it does conserve, then we need to find out what impact on the universe the EmDrive has, to determine where the momentum and energy has gone.

Its not a violation. Photons are a form of boson, it does not have mass (only fermions do).

Imagine an atom that undergoes radioactive decay and releases a photon, in doing so the atom loses mass (E=mc^2). The photon carries the energy, while the photon is traveling it imparts no energy, while it is traveling it is simply a massless field. From the photons perspective time does not pass, when it reaches its target though it reveals its source and momentum. The photon is a force carrier, its almost if the photon receiver were next to the decaying particle when it decayed. It causes some bizarre paradoxs. If an event produces two photons, no matter how far they travel, the reception of one will effect the reception of the second no matter the distance or time elapsed. The true strangeness of the universe is masked by the fact that we live in a fermion dominated world this is dominated by the fact that things with 1/2 spins like to interact with everything, particularly in the unpaired state (e.g. Opaque epoch).

[N_las]

I tries to come up with other scenarios who has the same issue where constant energy gives constant force and constant acceleration.

The frictionless train might be an magelv train however the point is if an acceleration with constant force and energy use breaks newtons laws in it self not if its practical.

I am pretty sure a maglev train needs AC to propel, and the faster the train, the higher the needed frequency to impart a constant force. I am fairly certain there is no real live example or realistic thought experiment of any system were a constant energy supply produces a constant force. Even if you make your enviroment ideal (no friction and stuff), there is no way.

[YNM]

From what I read at the end of your post, your problem isn't with conservation of energy. Your problem comes with not using reduced mass.

But from the first part of your post... Do some integral I guess ? The half and squared velocity have a reason to exist, not just spawned. Also, relativity is a more important thing in any ways.

[Lukaszenko]

This comes up in another emdrive tread however it would be relevant for other settings too at least theoretical.

Issue is that if you accelerate with an constant value like 1 m/s/s this acceleration require an fixed force and wherefore fixed power who depend on force and the efficiency of the drive.

In this setting we are operating from 0 m/s up to high orbital speeds relativity is not important, the drive is an ideal drive, with only an efficiency loss between energy and force.

Kinetic energy increased with an square of the speed, at some point the kinetic energy will grow faster than the energy you put into the drive.

Em drive should work like this, normal rockets does not, neither do practical vertices like planes, cars or trains simply because of air resistance, rolling resistance and gearing.

An long optimized coilgun might too, perhaps electromagnetic tethers. Note that none are reactionless drives however they should be able to do an constant acceleration for constant power at least theoretical or within some limited speed ranges and both are pretty effective compared to emdrive.

Anyway as I understand Newton its nothing to stop an theoretical train from moving down friction less rails in vacuum with an constant acceleration powered by constant power.

However the kinetic energy is relative compared to the point of reference, 10 m/s relative to the space station you left is non issue, as the speed is low, relative to the sun this effect would kick in almost at once as you move fast, however if you run an probe on emdrive to Pluto it would hit with more energy than it had used during the trip.

That is wrong here? Yes emdrive violate the conservation of momentum, violating no freee energy is far more serious, still the theoretical train could also be aimed at Pluto with the same result.

Acceleration in cars/ trains/ boats/ railguns/ planes* is NOT constant with power. Acceleration decreases the faster you go. The only time that acceleration is constant with constant power is with things like rockets that bring their reaction mass with them. This is a special case and it has to do with many things, among them the fact that kinetic energy depends on which frame of reference you use. With a car that pushes against the earth, it makes sense to reference the earth. A rocket pushes essentially against itself, so you can easily jump between frames of reference and come up with apparent paradoxes.

Of course with a rocket you can resolve the paradoxes, but as you pointed out it nevertheless is still a problem with the emdrive since it doesn't seem like it pushes against anything.

[magnemoe]

Acceleration in cars/ trains/ boats/ railguns/ planes* is NOT constant with power. Acceleration decreases the faster you go. The only time that acceleration is constant with constant power is with things like rockets that bring their reaction mass with them. This is a special case and it has to do with many things, among them the fact that kinetic energy depends on which frame of reference you use. With a car that pushes against the earth, it makes sense to reference the earth. A rocket pushes essentially against itself, so you can easily jump between frames of reference and come up with apparent paradoxes.

Of course with a rocket you can resolve the paradoxes, but as you pointed out it nevertheless is still a problem with the emdrive since it doesn't seem like it pushes against anything.

You are 100% correct, I also know cars and trains got harder to accelerate as the speed and potential energy increases however this is overshadowed by air resistance and frictions who also increases with speed.

How would this affect an solar sail or electromagnetic tether, their acceleration should also go down as speed go up, keeping distance to sun or planet pretty constant.

[N_las]

The faster the solar sail moves, the more redshifted are the incoming photons, the less momentum do they carry -> less acceleration.