Gravity account

Replies

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  Saandeep Sreerambatla

  This may be a stupid question! bvut I dare to ask.
  
  If something is out of the gravitational forceof the earth it will not fall onto the surface of the earth.
  
  We know that object rotates in space.
  
  What happens to the object while the earth is rotating in elliptical orbits , the force increases as it comes near which causes a displacement in the object even it can eventually fall?
  
  One more thing is when starts are very far from earth they are not effected by the Earth's gravitational force. If some collision happens between some stars why the star dust fell on the ground?
  
  What force is pulling them down?/
• 

  skipper

  Hmm.
  A gravitational force exists because of a field. The earth's field is distorted by the moon's.
  Far away from the earth-moon the field is a lot smoother, but to get really smooth you need to be in intergalactic space - preferably a nice big void about 10m lightyears across.
  
  But since you can think about a smooth field that isn't distorted you can use the local field to estimate quite accurately what a small volume of such a field is like - it's called a tensor field in EFE, the equations in Einstein's relativity theories.
  
  Note that: Newton's gravitational constant G is independent of g the local field-generator, g is due to the mass of earth, which is inertial. Inertia is one of the simplest things to observe, but hardest to explain or really get, erm, a grip on. Understanding why any mass also has inertia (because quarks do) is part of the process, which Einstein's equations are an approximation of - we don't know the "right" formula for mass yet...
  
  Conjecturally, mass and quarks are also "field distortions", the field that "gives" mass to quarks is the Higgs field. If the LHC or Fermilabs can't find it it's back to the drawing board for particle physics. If the Higgs doesn't exist, something really weird is going on...
• 

  vik001ind

  Newtonian Gravitational law is surpassed by Einstein's Space time laws (theory of relativity).
  Newton knew that there is some force pulling the objects but he can't clearly explain it.
  Why does an object fall to the earth? Is something pulling it, thats not much expressive. Infact the object is pushed by curved space around us. Anything in space having mass creates a space time curvature around it. Likewise moon revolves around the earth in that curved area, thats the reason why the orbital paths are elliptical.
  If we follow strict laws of Newton, abiding to his well defined formulas, we will always get a circular orbital paths.
• 

  skipper

  Well, in Newton's time they were still searching for the "essence" of metals - the Philosopher's Stone - and using the forerunner of modern chemistry, or alchemy.
  Newton declares in the Principia that the inertial mass of a body depends on the amount of material, and not its nature, in keeping with Galilean principles. In short mass is a density that depends on volume, not shape or other characteristics.
  
  This form of inertial energy, and description of mass survived for over 200 years. Newton recognized that the important thing was finding an inertial "independence" for which the local acceleration would be equivalent if say, the earth was made of fluid, or a much larger sphere filled with a gas. That is to say that inertial energy of mass is independent of the form mass takes, or volume/density is constant.
  
  Einstein applied Galilean principles, then Lorentzain ones to find the connection between small material particles in an inertial frame, and large material bodies in non-inertial frames (such as the earth-moon system). He found that, far from any large distortions of the field (i.e. other planets, stars or galaxies) a material particle is "at rest" if it has constant velocity, that is, if it's "weightless" which means isn't subject to acceleration from a gravitational field. This is why G has units of m/s and any (singular) g has units of m/s^2. This is the question Einstein's theories answers: what is inertial motion? It's acceleration, since a small enough body with constant velocity is not distorting the field, and not subject to accelerations. The smallest body with mass he considered (in Lorentz frames) was the electron - this has such a small mass, it's presumed to not distort a smooth field significantly, but space gets distorted because charged particles in motion generate "free modes" of exchange of energy, called radiation (photons).
  
  This is only approximately true on a very small region of a gravitational surface that looks locally flat, and only for small intervals of time. It's only universally true if the small material particle in an inertial gravitational field is traveling through a fixed volume. This is not true - the universe is expanding into a larger volume, which is why the cosmological constant is required, to account for the change in Hubble volume over large distances (of travel). The CC "closes" the theory of GR, but we don't know why the theory requires an expanding universe. We only know that a static model is unstable.
  
  Why is the universe not static? Is it because motion (of material particles and so inertial mass) requires an expanding space, or it wouldn't exist - there would be only material particles at a constant "velocity equilibrium", no acceleration and a universe full of "black bodies" with infinite mass?
• 

  skipper

  You see, tensor calculus lets you do stuff like imagine what a tank of water, or its pressure gradient is like at g = 9,81, then "boost" the tank by removing g, or placing the tank in an inertial frame.
  What happens if the tank isn't filled? What if it is?
  
  Newton had a crack at some liquid and gravity thought experiments too, since gravity appears to act in a fluid way itself. Kepler was into the musical scales and pitches of the "harmonics" in planetary motion.
  
  In say, a cylindrical tank which is half filled and vertical at g, boosted to an inertial frame so g vanishes, now in empty space and at constant momentum. Does the surface tension of the liquid change the shape of the surface, if the tank isn't filled? The column of liquid/fluid has the greatest pressure at the lowest point when vertical and g is acting on the tank+fluid.
  
  When the gradient vanishes (immediately) how would a fluid recover? Under g each small layer of fluid bears the downwards "weight component" of force from all the layers above, and transfers it in turn to a lower layer. In that sense g acts to "polarize pressure" or form a constant density pressure-gradient.
  If instead the tank was filled with a dense gas, under g a density gradient would form, in the "matter field".
  
  Here we have a relation between density and pressure that alternates between phases, in the field (of material particles).
  
  What you do by making g = 0, is called a global phase change: the fluid starts at the vanishing point, compressed at the lowest constant density, it will expand or react, just like a gas will when compressed. In a gas-filled cylinder, the density gradient will vanish as the molecules diffuse, or "find" a maximum separation from each other - what's called a mean free-path volume. Both reactions to suddenly being free of gravity are examples of elastic surfaces that were stretched, recovering their shape.