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Question regarding 'center'

  1. Jun 10, 2004 #1
    hello. I was wondering if any brilliant astro-physicist can help me come to understanding regarding gravity and center in relation to orbit..

    For example, we spin around the center of earth, earth spins around center of solar system, the sun, now does this same force relate to the sun and the center of galaxy? which determines the movement of our sun inside of galaxy?

    thank you in advance..
  2. jcsd
  3. Jun 10, 2004 #2


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    The question you are asking was the one Newton answered. Gravity is universal. General Relativity is more accurate than Newton's gravity, but the principal of universality remains the same.
  4. Jun 10, 2004 #3

    I am sorry, I misphrased my question. Hmm, let me try this..

    moon orbit earth, earth orbit sun, sun (orbits?) galactic center..... is it the gravity of the galactic center that makes our sun move around the center? or is it combination with another force? Is it the same princaple in the earth around the sun? or, better yet, is earth around the sun a microcosm of the sun around galatic center?

    I ask this because I study taiji and Aikido, and we learn to find our own 'center' which moves toward the center of the earth, the center of the earth moves toward's the sun's center, and I just wanted to know if this phenomena repeats to the galactic center, i.e my center is connected to the center of the galaxy through this process..

    forgive me if my question sounds ignorant, it is just something I have been wondering about, I mean, would this not suggest that my center moves in relation to galactic center?
  5. Jun 11, 2004 #4


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    A number of bodies moving under their mutual gravitational attraction will have at any moment an "Instantaneous Center of Gravity" that can be calculated from their masses and distances apart. It's just the symmetry center where al the forces are evenly balanced. It doesn't draw the bodies itself, it's just a secondary effect.

    As the bodies move, the center moves too. Celestial Mechanics will tell yoiu the curve it moves on (in simple cases). But whether you know the curve or not, there is always a path.
  6. Jun 11, 2004 #5


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    So whether or not there is a large object at the center of the galaxy (apparently there is), stars will still orbit the center of the galaxy because of the mutual attraction of all of the matter in the galaxy.

    I doubt that has anything to do with physics. Sounds spiritual.
  7. Jun 11, 2004 #6
    wow, thank you all.

    and no, center in movement refers to an actual location in the body, our physical centers, which is around the navel area. If you look at astronauts in space, you will see them 'spin' around that same place.

    there is a mental component to center, yes, and that is where such arts do become spiritual, but it is in relation to an actual physical place that takes years of training to move from.

    moving from center is not just in Aikido or Taiji, but any martial art or dance, gymnastics, or what have you.

    so center is just where there is a balance of forces. that is interesting. in taiji, there is the old teaching that one must balance there 'chi' from the forces of earth and sky and balance them into thier 'dantien' or physical center....

    thank you all so much...I have found that arts like taiji apply and use the laws of physics in movement to an advanced degree. I dont know if anyone else here has ever studied taiji or not, but it sure seems like the intelligent human's movement
  8. Jun 11, 2004 #7


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    The point about which an object will spins is I believe its center of gravity (or "barycenter" - means the same thing). Here's some more: if you take any line through the barycenter, the forces along that line balance to zero. The ones directed one way exactly equal the ones directed the other way. And that works for each line. You can see acrobats and divers manipulate (change) their center of gravity by sticking arms and legs out in different directions, and that changes their spin and moment of inertia (resistance to spin) as well. This is all fascinating, and would be a fine study for a physics with calculus course.

    I have no doubt that the tajii arts have a perfectly normal physical content as well as a spiritual interpretation. after all, martial arts work, in the sense of bodies moving and twisting in space.
  9. Jun 11, 2004 #8
    yes, they do work. I always thought that the higher level practioners where performing high level understanding of physics in relation to their art.

    I once had a 75 year old tai ji master throw 35 of us in a line ( I was number 26) back about 10-15 feet with one motion. The force issued from a taiji 'push' feels different than being pushed with normal muscular force, it seems more like an 'unbalancing' force being sent into your center that just makes you off balance and you wind up 'throwing' yourself in find your balance.

    Also, another quick question too, now that i think about it. this relates too to taiji or aikido.

    In physics, yes, gravity is said to push or 'pull' down on us I believe about 17.5 pounds of pressure per square inch, and , I forget the name of the force, is it electro-magnetic force, I dont know, can you tell me, the force the comes from the earth to balance this out with an equal 17.5 pounds of pressure that 'props' us back up? Perhaps I have the value of the force wrong too.

    Anyway, what ever this force is called in physics, I forget it has been awhile, in taiji this is called 'earth' chi or earth energy, and the gravity is called 'sky' or heavan energy. In the arts, one learns how to find this force in others, and apply just a little exta dash in one direction or another which sends the opponent flying over. the chinese have a more poetic way of explaining things than physics, but I always felt taoist princaples of yin yang were more earth science and philosophy and not religion at all...

    Thank you all once again
  10. Jun 12, 2004 #9


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    The 17.5 pounds per square inch you are talking about is I believe the air pressure. The weight of a column of air over each square inch, from the surface up to "infinity". Since the density of the air decreases rapidly with height, the total weight has a finite integral, even though the upper limit is "infinite". Beyond some height is essentially the vacuum of space.

    The force of earth's gravity that pulls us down will vary with our mass: Newton's F = ma. So the same acceleration for all masses, about 32 feet per second per second.

    The centrifugal acceleration due to earth's spin is less than the acceleration of gravity, or we would all fly off into space! Centrifugal acceleration varies with the latitude, being greatest at the equator and zero at the poles. This is because the size of the circle you are rotating on (the latitude circle) vries with its location on the sphere.

    There is another acceleration that arises from the earth's spin, the Coriolis force. This tends to push any momentum to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It is weak at small scales, but builds up at large ones and is one of the causes of the weather, making cyclonic flows around low pressure areas. Unlike centrifugal acceleration, Coriolis is zero at the equator and increases as the sine of latitude, reaching its max at the poles.
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