by ShieTar » Tue Jun 02, 2015 7:50 pm
Guest wrote:I guess... In much the same way that physics teachers often explain about blocks on inclined planes without accounting for the curvature of the earth, or explain the parabolic motion of a ball without using relativistic equations.
Well, but the teachers usually do the experiment and verify that their planes are not actually affected by the curvature of earth, and the do throw their balls slow enough so that the non-relativistic equations remain sufficiently accurate.
The teacher in the comic on the other hand talks compares the small gravity wells of the moons while ignoring the fact that they are themselves at different positions of the larger gravity wells of their planets, which in turn are also at different positions of the even larger gravity well of the sun. In this case, the moons are actually the negligible part of the problem, not the important one. Its like saying it takes more work to get from a 10m deep hole in the Netherlands to the peak of Mt. Everest than it takes from a 20cm deep ditch in the North Base Camp, because the hole is deeper. Its just very much not true, unless you make some assumption like "You only need to get out of the hole yourself, the rest of the way is done by helicopter". Yes, such assumptions exist and can even be realistic, but you can not just leave them out of your teaching.
Guest wrote:
Also, interestingly, if you say, take an asteroid from approximately Mars orbit and move it in to approximately earth orbit, you'd be adding energy to the rest of the solar system, not reducing it, as the system is conservative and you've reduced the energy of the asteroid. Probably, you would increase the orbital energy of the earth and decrease that of Mars, as the asteroid would be dragging Mars inward and the earth outward. It's difficult to say in such simple terms, but at any rate, no energy would have to be lost by the system until you exert a non-conservative force such as a rocket engine.
Fair enough, though reducing and increasing are rather irrelevant terms regarding the energy of a system, as the 0 is a completely arbitrary definition (Do you have 0 kinetic Energy because you just sit in your chair, or do you have a lot because you travel quickly around the sun?). Also as long as you change the sign on all your definitions, the physics remains exactly the same.
But granted, the general definition will state that the Asteroid now has less energy because it is on a lower orbit around the sun. So depending on where and how you have done your swing-bys, either Mars or Earth or both will now have a larger orbit about the sun with more energy, or maybe the Earth-Moon Distance is now larger. Dragging Mars inward would not help you, because that would push the asteroids out, not pull them in.
But where you are incorrect here is by calling the rocket fuel "non-conservative". It is not, if you use the rocket engine to push the asteroid in, you push the rocket fuel into a higher solar orbit. So the entire solar system, including the now interplanetary fuel cloud, would still keep the same energy. Some of it will be transferred from chemical potential energy to heat due to the way effective rockets work, but there is still no overall loss of energy. There never is.
Unless you push earth a tiny bit to far out. Our solar system is "dynamically full". If the distances between the planets get smaller, they start to interact with each other (much like a swing-by), and there is a high chance that one of the two planets will eventually get pushed on an increasingly eccentric orbit, and finally get removed entirely from the solar system. I believe I read that Mercury is destined to be kicked out in a few billion years, as its still a tiny bit too close to Venus for its own good.
guest2 wrote:The same could be said about Solar Power. It's not free energy; instead of heat that would have radiated to the atmosphere, it is instead converted to electrical energy and then used elsewhere. Eventually we will create a global weather shift due to Solar Power... Of course, as you pointed out, the "eventually" is very, very far out. Also (assuming we don't use it for rockets) the system we are in is closed for the most part.
Absolutely true. Firstly though, rocket fuel is also solar power, just photochemical rather than photo-voltaic. Secondly, I would not necessarily agree that Solar Power will be very slow to change the weather. When you remove heat from a sufficiently large area, you create artificial low pressure centers. Our complete weather system is created by nothing but local differences in ground absorption, water evaporation and wind speed resistance. The sun itself would only quiet homogeneously heat up half the planet at a time, so all the weather effects are created by local differences in cooling and convection. Play around with that, and you may get some nasty surprises quiet quickly. Or you're lucky and improve your locale climate, it's a gamble because it is extremely difficult to understand which human decision may be connected to which effect, because of the chaos theory and the lack of thousands of copies of the Earth for statistically relevant experiments.
[quote="Guest"]I guess... In much the same way that physics teachers often explain about blocks on inclined planes without accounting for the curvature of the earth, or explain the parabolic motion of a ball without using relativistic equations.[/quote]
Well, but the teachers usually do the experiment and verify that their planes are not actually affected by the curvature of earth, and the do throw their balls slow enough so that the non-relativistic equations remain sufficiently accurate.
The teacher in the comic on the other hand talks compares the small gravity wells of the moons while ignoring the fact that they are themselves at different positions of the larger gravity wells of their planets, which in turn are also at different positions of the even larger gravity well of the sun. In this case, the moons are actually the negligible part of the problem, not the important one. Its like saying it takes more work to get from a 10m deep hole in the Netherlands to the peak of Mt. Everest than it takes from a 20cm deep ditch in the North Base Camp, because the hole is deeper. Its just very much not true, unless you make some assumption like "You only need to get out of the hole yourself, the rest of the way is done by helicopter". Yes, such assumptions exist and can even be realistic, but you can not just leave them out of your teaching.
[quote="Guest"]
Also, interestingly, if you say, take an asteroid from approximately Mars orbit and move it in to approximately earth orbit, you'd be adding energy to the rest of the solar system, not reducing it, as the system is conservative and you've reduced the energy of the asteroid. Probably, you would increase the orbital energy of the earth and decrease that of Mars, as the asteroid would be dragging Mars inward and the earth outward. It's difficult to say in such simple terms, but at any rate, no energy would have to be lost by the system until you exert a non-conservative force such as a rocket engine.[/quote]
Fair enough, though reducing and increasing are rather irrelevant terms regarding the energy of a system, as the 0 is a completely arbitrary definition (Do you have 0 kinetic Energy because you just sit in your chair, or do you have a lot because you travel quickly around the sun?). Also as long as you change the sign on all your definitions, the physics remains exactly the same.
But granted, the general definition will state that the Asteroid now has less energy because it is on a lower orbit around the sun. So depending on where and how you have done your swing-bys, either Mars or Earth or both will now have a larger orbit about the sun with more energy, or maybe the Earth-Moon Distance is now larger. Dragging Mars inward would not help you, because that would push the asteroids out, not pull them in.
But where you are incorrect here is by calling the rocket fuel "non-conservative". It is not, if you use the rocket engine to push the asteroid in, you push the rocket fuel into a higher solar orbit. So the entire solar system, including the now interplanetary fuel cloud, would still keep the same energy. Some of it will be transferred from chemical potential energy to heat due to the way effective rockets work, but there is still no overall loss of energy. There never is.
Unless you push earth a tiny bit to far out. Our solar system is "dynamically full". If the distances between the planets get smaller, they start to interact with each other (much like a swing-by), and there is a high chance that one of the two planets will eventually get pushed on an increasingly eccentric orbit, and finally get removed entirely from the solar system. I believe I read that Mercury is destined to be kicked out in a few billion years, as its still a tiny bit too close to Venus for its own good.
[quote="guest2"]The same could be said about Solar Power. It's not free energy; instead of heat that would have radiated to the atmosphere, it is instead converted to electrical energy and then used elsewhere. Eventually we will create a global weather shift due to Solar Power... Of course, as you pointed out, the "eventually" is very, very far out. Also (assuming we don't use it for rockets) the system we are in is closed for the most part.[/quote]
Absolutely true. Firstly though, rocket fuel is also solar power, just photochemical rather than photo-voltaic. Secondly, I would not necessarily agree that Solar Power will be very slow to change the weather. When you remove heat from a sufficiently large area, you create artificial low pressure centers. Our complete weather system is created by nothing but local differences in ground absorption, water evaporation and wind speed resistance. The sun itself would only quiet homogeneously heat up half the planet at a time, so all the weather effects are created by local differences in cooling and convection. Play around with that, and you may get some nasty surprises quiet quickly. Or you're lucky and improve your locale climate, it's a gamble because it is extremely difficult to understand which human decision may be connected to which effect, because of the chaos theory and the lack of thousands of copies of the Earth for statistically relevant experiments.