Naked Science Forum
General Science => General Science => Topic started by: Momentus on 12/12/2015 12:40:25
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I have been mythering over this idea for some weeks now. It is either trite and easily dismissed, or quite profound.
The idea is best explained as a thought experiment using a train and an electric car.
Accelerate the car up to 60 mph, the same speed as the train is travelling and move onto the train. (Like the side loading of the channel tunnel train.) Using instrumentation fitted to the car establish the amount of energy conserved/transferred from the battery to the car. Chemical energy has been conserved as Kinetic energy. Exit train, regenerative braking to conserve/transfer energy back to battery. Kinetic energy has been conserved as chemical energy, less losses to friction etc.
Do it again, but accelerate by a further 60mph inside the train travelling from the rear to the front of the train, and exit from the front of train at 120mph, regenerative braking etc.
That is the trite part. Do it again, but this time accelerate to 60mph travelling from the front of the train, exiting from the rear. This means that speed relative to starting point is zero. No regenerative braking, no energy recovery - ergo energy is retained by the car.
As this is a thought experiment, iterate, do it for a very long time, which will accumulate vast quantities of energy.
It may be of interest to include an instrument package in the car,
Atomic Clock Mechanical clock standard kilogram standard meter
Some questions, not an exclusive list I hasten to add.
Can the energy be recovered?
How does the accelerated object know that its relative speed has not altered?
What are the physical effects of conserving/transferring enough energy to accelerate it to a straight line equivalent of 0.25 C?
If it can be reversed, then I have invented the ultimate method of energy storage, remember you read it here first.
Momentus.
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if the energy is stored in a battery it can be recovered, but there will be losses due to inefficiency eg electric motor.
Remember, the enegy stored is that generated by the car using fuel. When the car accelerates inside the train the train supplies the extra energy to bring the car to 120mph by burning more fuel.
If you repeat this for a long time, you will store more energy, but you will also expend vast quantities of fuel.
How does the accelerated object know that its relative speed has not altered?
the object has no knowledge, but will react with any sufaces or objects it encounters due to the relative motion.
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A car moving at 120mph has 4 times the energy of one at 60 mph.
So where has that energy come from?
It's come from the train. When the car accelerates down the train and leaves at the front, the train has to put in extra energy, burn extra fuel, to keep it self going at 60mph, otherwise it would slow down. And that extra energy balances the books.
Conversely when the car accelerates in the opposite direction, the car is pushing the train forwards, and the train can switch off its motor, and even use regenerative braking, or brakes if it isn't. The car is then left with NO kinetic energy.
Whichever way around, conservation of energy applies. You can't, and don't get something for nothing.
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Colin2B
As you have noted, the process is a continual transfer of energy from the battery to the car and the energy needs to be constantly replenished. As you say"vast quantities of fuel".
the train supplies the extra energy to bring the car to 120mph
The train maintains a constant speed, the energy to accelerate the car always comes from the battery. It is a moot point as the energy ends up being stored as kinetic energy by the mass of the car. It is this storage of energy that is the focus of the post. When the car is accelerated from the rear of the train, its speed is zero relative to the point of origin, yet it has conserved kinetic energy. This energy is somehow contained in the fabric of the car. The energy from fleets of fuel tankers conserved by the car, with no perceptible change? Energy stored for eternity?
I am having great difficulty accepting this, but see no flaw in the logic.
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Wolfekeeper
Whichever way around, conservation of energy applies. You can't, and don't get something for nothing.
This is the very essence of the question. As there is no way to recover the kinetic energy from the car, it must stay with the car.
To move the car, the wheels react against the surface it is sitting on, exerting a force which accelerates the car. The torque at the wheels comes from the electric motor. The electricity for the motor comes from the battery. The energy can clearly be traced from the chemical reaction in the battery to the kinetic energy of the car.
When the car is on a road surface, the reaction to the acceleration of the car is the acceleration of the earth in an equal and opposite direction. As the earth is heavy and the car is light, the change in the earth's velocity is insignificant and all the energy is conserved by the change in the speed of the car.
Exactly the same mechanism applies when the car is within the body of the train, equal and opposite reactions of momentum. As the train is heavy and the car is light, the change in the train's velocity is insignificant.
This applies regardless of the direction of travel, if the car is accelerating towards the sunset, or towards the rising sun, the velocity of the road surface does not add to or subtract from the energy of the car.
The kinetic energy is not returned to the train, it is conserved by the car. Accelerate a mass at 1g for a year and it reaches light speed. Run the electric car at 1g for a year, it is still not moving. Where is the energy? can it be recovered?
Whichever way around, conservation of energy applies. You can't, and don't get something for nothing.
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Wolfekeeper
Whichever way around, conservation of energy applies. You can't, and don't get something for nothing.
This is the very essence of the question. As there is no way to recover the kinetic energy from the car, it must stay with the car.
No, that's not right. For the train to stay at the same speed, which you are implicitly assuming; the car is accelerating ON TOP OF the train, it's tyres are pushing the car in one direction, and the train in the other. The train therefore has to apply a force, equal and opposite.
And the train is moving. And energy is force times distance, and the distance is long, because the train is moving. If the car starts applying the force at position x=0, it will stop applying it a long way down the track because the train has moved in the meantime. And the further it has moved, the more energy the train will have had to apply/absorb to continue at the same speed.
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the car is accelerating ON TOP OF the train, it's tyres are pushing the car in one direction, and the train in the other.
The only force present in this scenario is at the point of contact of the car's wheels with the deck of the train. The only way that the kinetic energy can be passed from the car to the train is through this contact. Prior to the car being accelerated by its electric motor, there is no force being transmitted in any direction. The car and train are moving at the same speed.
When the car starts moving from the front of the train to the rear of the train, the force accelerating the car is reacted by a force which in turn is accelerating the train. Energy is being taken from the battery to produce torque at the wheels of the car. This force moves both the car and the train. The "distance" moved by the train is not the distance it moves relative to the ground, it is not the distance that it would have moved without the application of the force which is the distance that you are using. You need the formulae d = vt + (1/2)at² to find the distance that the train would travel under acceleration. It is much more straightforward to deal with momentum and velocity as I have done previously.
This is a thought experiment. It uses a very heavy train and a very light car, because I say so. The energy transfers to the train are consequently too small to have any significance.
There is no transfer of kinetic energy to the train from the car.
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Even a very heavy train has momentum and kinetic energy; and Newton's third Law still applies.
Your assumptions break all three of these fundamental laws.