Kinetic Energy Changes at fast speeds

Originally posted by sonhouse
Yes that is totally true but like an open door in front or back of the craft where mass can be ejected rocket-like is the only case where actual matter leaves the craft. In the case of heat, it is eventually like an open door to the outside, heat loss cannot be avoided, no matter what kind of technology you put in place to conserve heat, some of it will eve ...[text shortened]... ly be radiated, thus slowing down the craft if that continued. There is no way around that fact.

But you're also continuously putting energy into the system...every time the person inside accelerates they need to put energy in to do that.

Originally posted by mtthw
But you're also continuously putting energy into the system...every time the person inside accelerates they need to put energy in to do that.

Sure, but where do you think they are pushing off from? The back of the craft. That means when they push off they do so at the expense of the KE of the craft, it slows down however small that # would be. In my example for instance, I used a solenoid attached to the rear and a mass that would be shot out of it, the electromagnet tied to the frame, it shoots out the mass to the front of the ship. Think about this:
Suppose we use a solenoid in space, isolated from any other mass and we have a mass that will be shot out of the solenoid. Suppose the solenoid and the mass have the same value, say 10 Kg. We apply power to the electromagnet, what happens?
The mass shoots off at X km/second in one direction but the solenoid shoots off in the opposite direction at exactly X km/second also, they react off each other. But in the case where the solenoid is tied to the frame of the ship, it cannot move but instead will try to move the entire ship backwards from its frame of reference. If the spacecraft was at the same zero velocity frame of reference the ship would move slightly backwards till the mass hit the other end of the ship where the mass would now try to accelerate the ship in the opposite direction, canceling the velocity given to it by the solenoid, with net zero motion aside from whatever is left over after heating effects are taken into consideration.

Originally posted by sonhouse
Sure, but where do you think they are pushing off from? The back of the craft. That means when they push off they do so at the expense of the KE of the craft, it slows down however small that # would be.

I know - that's already included in my calculation - momentum must be conserved. But the additional energy (the KE of the total system increases) comes from the person (from chemical energy within their muscles). It's not just a simple transfer of KE from the ship to the person.

You can't reduce the momentum without something leaving the ship carrying that momentum.

Originally posted by mtthw
I know - that's already included in my calculation - momentum must be conserved. But the additional energy (the KE of the total system increases) comes from the person (from chemical energy within their muscles). It's not just a simple transfer of KE from the ship to the person.

You can't reduce the momentum without something leaving the ship carrying that momentum.

You should look up the 'dean drive'. In the 1950's this dude Dean came up with a device that tried to add momentum to a system without shedding mass and fooled Hugo Gernsbach at Analog Science Fiction magazine (It had another name then, forget what it was) and he published the story and showed a drawing of a submarine turned into a spaceship with this 'drive'. It was subsequently proven to be a hoax, the drive was a counter spinning set of cams that shifted its center of gravity (not sure if that is the right term) to supposedly make a one way thrust so it could independently defy gravity and such. What you are proposing is similar, you can't have an independent thrust by shaking things up internally, won't happen. You can't have like a sliding weight inside a larger object slamming back and forth between front and back and expect to start propelling the system mass through the universe if it was out in space somewhere away from Earth gravity. The only way to make the craft move is if there is an open door and the mass (human or otherwise) keeps going out the door and not contacting the front of the ship. That turns the human or the mass ejected into a rocket and it doesn't matter a whit if the energy is self-contained as in a battery or human muscle or from a power supply onboard the craft, its all the same.

Originally posted by sonhouse
What you are proposing is similar, you can't have an independent thrust by shaking things up internally, won't happen. You can't have like a sliding weight inside a larger object slamming back and forth between front and back and expect to start propelling the system mass through the universe if it was out in space somewhere away from Earth gravity. The on ...[text shortened]... as in a battery or human muscle or from a power supply onboard the craft, its all the same.

No, I don't think you've understood my explanation - in fact, what you've just said supports what I've been arguing. Momentum is conserved. You're absolutely correct that you can't have a thrust like this. You can't have a drag like this either.

When the person starts walking, they do work. The kinetic energy of the system increases. But as soon as they stop it decreases again. And if the spaceship is of finite size they have to stop. The initial increase in energy is from the person, and the final decrease is due to dissipation (eventually) as heat. Dynamically, the system returns to its initial state - as it must, as momentum is conserved. Except the person is a bit more tired and the inside of the craft is a bit warmer.

If you still disagree, show me where the error in my derivation above is.

In this kind of problem, the first thing you have to do is define the frame of reference. In the frame of reference where the spaceship is moving very fast, the person walking in the spaceship also has a very high kinetic energy.

Originally posted by mtthw
No, I don't think you've understood my explanation - in fact, what you've just said supports what I've been arguing. Momentum is conserved. You're absolutely correct that you can't have a thrust like this. You can't have a drag like this either.

When the person starts walking, they do work. The kinetic energy of the system increases. But as soon ...[text shortened]... bit warmer.

If you still disagree, show me where the error in my derivation above is.

I wonder if you can set up an experiment, maybe digital simulation if all the laws of physics are set up before hand, thermodynamics, kinetics, etc., and see if I am right about losing speed in space by internal conversion to heat. It seems to me equilibrium would be upset if you lose momentum internally by a conversion to heat which would leave the craft more or less equally in all directions, there would be no net thrust due to electromagnetic radiation. If you convert momentum into heat, I don't see how any other result would result so to speak. For instance, if we take the case of a spacecraft going at some normal non-relativistic velocity, say 100 Km/second and you have a solenoid driving a mass internally from some internal energy source, a battery if you will, and the mass is being guided by a rod the full length of the ship, and when you get to the end it encounters a section of rod that greatly increases the friction, it will slow down but heat up the mass and the rod. That seems to me to represent energy leaving the system that is not kinetic in nature but radiation which leaves the ship in all directions. So tell me why the spaceship won't slow down, if only by a little bit by this process. It also seems clear you can't reverse this and accelerate by this method, but I don't see why it wouldn't slow you down.

Originally posted by sonhouse
If you convert momentum into heat, I don't see how any other result would result so to speak.

You can't convert momentum into heat. Momentum is conserved, always. We are converting kinetic energy into heat - I think that's the key point. We're then putting energy back in to replace this lost KE.

This could all change if we can find a way of losing momentum. If heat is radiated away in a preferential direction (let's say we insulate the craft all over, but leave one area uninsulated. If that worked, it would create a thrust in the opposite direction. Of course, depending on the direction that could either accelerate or decelerate the craft.

But if we assume that any radiation is in all directions equally, then the momentum must be conserved. It's a direct consequence of the laws of motion.

Or to put it another way - you can't slow the craft down without applying an external force. Where's the force?

Originally posted by mtthw
You can't convert momentum into heat. Momentum is conserved, always. We are converting kinetic energy into heat - I think that's the key point. We're then putting energy back in to replace this lost KE.

This could all change if we can find a way of losing momentum. If heat is radiated away in a preferential direction (let's say we insulate the craft all ...[text shortened]... y - you can't slow the craft down without applying an external force. Where's the force?

Well, if heat was radiated away in one direction that would just be another form of rocket, we already saw in one puzzle about the car in orbit turning on it's headlights, remember? A kw of light leaving the bulbs would move the craft a few Km in some reasonable amount of time, from a zero reference frame.
So momentum is potential energy, right? Unrealized till it is tapped by connecting to another reference frame moving at a different velocity.

Suppose we do the solenoid operation just once. We accelerate a mass on a frictionless rod from the back of the craft to the front and the back end, having no friction uses most of the energy just to accelerate the mass. That produces a backwards thrust of some small figure to the overall motion of the craft. Then it continues to the front at a constant velocity and at the other end a friction device converts say, half the KE to heat. Why is there not a reduction in the thrust it imparts to the front of the craft? It seems to me if the mass has undergone deceleration from its sliding velocity to zero relative velocity in say, one meter or so, and the conversion goes half into heat, why would there not be a reduced impact on the front of the craft?

Originally posted by sonhouse
Well, if heat was radiated away in one direction that would just be another form of rocket, we already saw in one puzzle about the car in orbit turning on it's headlights, remember? A kw of light leaving the bulbs would move the craft a few Km in some reasonable amount of time, from a zero reference frame.
So momentum is potential energy, right? Unrealize ...[text shortened]... version goes half into heat, why would there not be a reduced impact on the front of the craft?

No, momentum is not potential energy. (4-)Momentum is conserved, always.

Originally posted by KazetNagorra
No, momentum is not potential energy. (4-)Momentum is conserved, always.

Under the circumstances I described, the Ke turns part into heat and part into momentum? Where does the energy come from that made the heat if not from the motion of the mass on the shaft? If it comes from the motion of the mass, how can there be the same amount of impact energy when it stops?

Originally posted by sonhouse
So momentum is potential energy, right? Unrealized till it is tapped by connecting to another reference frame moving at a different velocity.

Suppose we do the solenoid operation just once. We accelerate a mass on a frictionless rod from the back of the craft to the front and the back end, having no friction uses most of the energy just to accelerate th ...[text shortened]... nversion goes half into heat, why would there not be a reduced impact on the front of the craft?

I still think that any concept of "converting energy into momentum" is misleading. They are different physical quantities. It's like saying you're converting mass into distance.

In the example you've given, any idea of "converting half the KE to heat" when the mass stops isn't going to work. If you bring it to a stop all the additional KE that you put into the solenoid is going to be converted into heat (either through your friction device or through vibrations that will eventually dissipate as heat). The impact on the front of the craft might be lessened, but the friction device has to be connected to the craft, and so as that decelerates the solenoid the opposite reaction will cause a force on the craft. The net result (the impulse) is exactly the same - all you've done is make it act over a longer timescale.

Originally posted by mtthw
I still think that any concept of "converting energy into momentum" is misleading. They are different physical quantities. It's like saying you're converting mass into distance.

In the example you've given, any idea of "converting half the KE to heat" when the mass stops isn't going to work. If you bring it to a stop all the additional KE that you ...[text shortened]... impulse) is exactly the same - all you've done is make it act over a longer timescale.

If it is exactly the same, then aren't you getting free energy somewhere? Say it left with 1000 joules of Ke as it left the solenoid and because we specified the trip would be frictionless, several ways to do that in reality, and at the end it encounters friction that slows it down and the device doing that gets hot. So if the resultant impact which now happens over a longer time than the original acceleration has heated up an object so how can it have the exact same amount of kinetic energy to impart to the inside of the craft?

In an electric car, they can use regenerative braking, where the Ke of the car gets converted to electricity to re-use some of the kinetic energy which turns into electricity by the change of velocity as the car slows down. So why would my example be any different?

For instance, suppose the device that slows the mass down now is a generator that makes a quick pulse of electricity which powers a series of LED's in front and back of the craft. That energy will now shoot off as electromagnetic radiation in both the direction of motion and the reverse, so how could the mass still have the same amount of Ke at the end of its travel?

You haven't gotten something for nothing here, the change in velocity of the mass powered a generator briefly so we have conserved energy, it would all add up and be accounted for.

Originally posted by sonhouse
If it is exactly the same, then aren't you getting free energy somewhere? Say it left with 1000 joules of Ke as it left the solenoid and because we specified the trip would be frictionless, several ways to do that in reality, and at the end it encounters friction that slows it down and the device doing that gets hot. So if the resultant impact which now hap ed a generator briefly so we have conserved energy, it would all add up and be accounted for.

You have to put energy into the system to accelerate the mass. Presumably from a battery or something. You might be able to reclaim some of that at the end - the rest would be lost as heat - but you certainly aren't getting something for nothing.

So energy is conserved.

Energy used to accelerate = Increase in total KE = Energy reclaimed while breaking to rest + Energy lost as heat/radiation/whatever.

Originally posted by sonhouse
Under the circumstances I described, the Ke turns part into heat and part into momentum? Where does the energy come from that made the heat if not from the motion of the mass on the shaft? If it comes from the motion of the mass, how can there be the same amount of impact energy when it stops?

You cannot turn energy into momentum. They don't have the same unit, it's fundamentally impossible.