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Thread: Basic Questions: perpetual motion, universe and gravitation

  1. #1 Basic Questions: perpetual motion, universe and gravitation 
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    Hi everybody!

    I have recently begun to study physics just because it's fun. well, I am at the very begining of my studies and so I have some questions that bother me (in fact some of them drive me crazy ) well, here are my questions

    a) in space, the sun exerts gravitation on the earth and vice versa. if they would both have the same mass the gravition of both would be the same and would cancel each other out. but the sun has a much higher mass than the earth thus its gravitation should be higher as well, right? so, if the earth gets too close it will crash into the sun and it is too far it will leave the orbit of the sun and fly out into space. but how can there be an equilibrium (meaning that the earth can keep turning round the sun for ever)? would'nt the contrary be much more likely, so that we would observe planets leaving the orbits or crash into each other?

    b) i learned that a perpetual motion machine is not possible. this comes from the law of energy conservation. when the earth is turning round the sun it is in motion because of the gravitational energy of the sun (just like a stone that somebody picks up and drop it). but somebody has to pick up the stone which "costs" energy that is transformed into potential gravitational energy that you "can get back" when you drop the stone. what is the analog of the earth? what energy is lost when the earth is turning around the sun continously? will it stop someday?

    c) if you would put two tennis balls in space (just like the earth and the sun) they would attract each other. at the right distance they would move around each other. why is that not a perpetual motion machine?

    d) energy is conserved which means it is transformed into some kind of other energy. the overall amount stays the same. does that mean that the whole energy in the universe is constant? it has ever been the same amount and will never change?

    e) but how can the universe expand then? would'nt it be impossible for it to extend for ever? because in order to extend you need energy which come from somewhere. just like if you play pool and you hit one ball with the white ball. the force of the white ball will set the other ball into motion but the other ball will not move forever?

    well, i know the answer to those questions must be simple for somebody who has an advanced understanding of physics. to me, there are just some big questions marks and i can't wait until i will have learned more about the fundamental laws of physics.
    so thank's for your help


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  3. #2  
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    a) No, there can be an equilibrium at different orbital radii, which is how all the planets can be stable at their various orbits, and how earth's satellites can orbit at different altitudes.

    b) No energy is required to keep the earth in orbit. It just stays there by the momentum that was imparted to it when the solar system was formed. It will stop some day just due to the minute amount of friction from collisions with space dust and such, or maybe when the sun blows up.

    c) It's not a perpetual motion machine because it will eventually run down due to friction. When people refer to perpetual motion machines, they usually mean a machine that you can extract useful energy from while it continues to operate.

    d) Yes, it is believed that the energy in the universe is constant, but we can't say it's forever. Nobody really knows what the laws of the universe were before the big bang.

    e) The universe can continue to expand without energy being added, but forever? I don't know.


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  4. #3  
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    Thank's Harold.

    a) it just seems so unlikely to me that the planets are arranged in such a way that the whole system is stable. maybe, it has something to do with the creation of the planets. the matter too close to the sun was already attracted by the sun and the matter sufficiently far away was used to create the earth. do we have evidence that somewhere in the universe a planet has already crashed into a sun, or has just left its orbit?

    e) in my naive thinking i would say that, if the only thing that can stop the earth turning round the sun is the friction from collision with dust and so on, then the universe could expand forever as there is no such thing as collision when it is expanding because expanding means that there was nothing before.
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  5. #4  
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    Marc;

    a) Of everything still remaining in our solar system, 99.95% is the sun.
    A star, binary or multiple stars form, with in Giant Molecular Clouds or Molecular Clouds, usually found around galaxy. Gravity becomes unstable (the natural cloud status) allowing a process where gravity pulls matter into a ball of plasma, which over time ignites into a star. The debris, whether from the formation or the actions during ignition becomes all else, which in time can form planets or objects in that system. No doubt, during formation, while the system remains in that cloud there is probably many more objects, which stay with the cloud or move along with the newly formed star, which is probably heading for a galaxy of some sort.

    e) Orbits are determined by velocity of the object around its host. The stars also has a small orbit, determined by the stuff orbiting. As the sun settled down and planets cooled, most debris moving to slow to maintain an orbit probably did fall into the sun and anything moving to fast to be effected by the suns gravity could have left the system or become parts of a debris belt, thought to orbits outside the solar system.

    Gravity caused orbit, is explained as FREE FALL to the effected object. We are in free fall to the sun, just as a man made orbiting objects is to earth. Ironically the moon, which is much lighter than earth is in free fall to the sun, while orbiting both the sun and earth. Each year that earth orbit expands a couple inches and I suspect the earth gets a little closer to the sun each year as well. If correct and to give some meaning, the original moon orbit was said to be about 24k miles, now about 240k miles.

    Universal expansion, is part of the Big Bang Theory, which has nothing to do gravitation effects on matter. The expansion is to space itself which matter is in. Gravitation between objects would become less as the space between them increases, if the objects were stationary. They are not, most in some form of gravitational pull with other objects. Solar systems, Galaxy or what have you.

    I don't know if any of this helps and offer only as my understanding. You can google *Molecular Clouds*, *Planet Formation* and form your opinions...
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