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If object with no gravitational attraction

  1. Apr 30, 2007 #1


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    F is an object that has no graviational attraction (i know its absurd, but its just to understand something cinetic energy/velocity)

    F also is not a feromagnetic material , and is charged neutral

    Ok now :

    if i i am outside , i have F in my hand , and i let it go
    would it suddenly fly into the sky
    (the Earth moves, object keeps its movement aquired by the cinetic energy before 'let go', but earths movement doesnt go in a straight line)

    And then as the object will be slowed down by dust particles (and air particules before away from earth), it will slowly come to a point where it shouldnt move , right ?

    And maybe there we could talk of almost true position, unless if there exists other forces

    Is it true or wrong ?
  2. jcsd
  3. Apr 30, 2007 #2


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    There is an awful lot wrong with that idea. For starters, if you had an object that was weightless and you dropped it, it wouldn't shoot off into space, it would just sit there moving at the same velocity it was before you dropped it. It would require a force to make it move away from you. Next, if dust particles in space were at rest, we could just use them to find the universal rest frame, couldn't we? They aren't in any universal rest frame - such a thing does not exist.
  4. May 1, 2007 #3
    Actually, I think it would shoot off.

    If it doesn't experience gravity, it can't orbit the sun. It'd continue travelling with the instantaneous velocity the Earth had at the instant it was released. However, because earth is orbiting the sun, its velocity changes whereas the object F will not.

    Actually, if we're going to stretch this analogy, the object wouldn't experience the gravitational attraction of the centre of mass of the galaxy, either...
  5. May 1, 2007 #4
    Well, if the object had no mass but did occupy space (had volume) then it would at least be boyant in air and rise like a H2 baloon. But what would it do when it ran out of atmosphere is the question. I think it would continue floating randomly until it found a spot in space furthest from any gravity source. This would not be a unerversal rest frame, however.
  6. May 1, 2007 #5
    I think what the OP is trying to understand is the concept of inertia - the tendency for an object to remain in constant velocity. Just remember that velocity has direction and speed. Think about spinning around in circles holding a bucket of water in one hand. If you suddenly let go, it will continue in the straight direction in was headed in the instant you let go.
  7. May 1, 2007 #6
    But an object with no mass has no inertia. It would act like it had negative mass until it ran out of boyant particles in space.
  8. May 1, 2007 #7


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    It is difficult to predict from a counter-factual assumption. You need to be more specific on what you mean by "no gravitational force". In Newtonian gravitation all objects fall at the same rate regardless of their mass. In the limit as you let the mass go to zero the object would behave as any other object and fall to the ground (in a vacuum chamber).

    In Einsteinean gravity the same will occur as the object must follow a geodesic curve through space-time. However when the object becomes exactly massless it cannot be moving at any speed less than speed c. But its path will still be bent by gravitation in the form of curved space-time.

    Mass is the "charge" to which gravitation couples and at the same time is the inertial mass of the object so the kinematics of objects in a gravitational field is independent of the value of their mass.

    But to answer your question as I think you ment to ask it. Suppose that we simulate gravitation with some other magic force. We build a hallow massless "earth" and use the magic force to hold ourselves down. We then release an object which isn't affected by the magic force, it will begin accelerating upward relative to us at a rate of:
    where [tex] \omega = 2\pi / (24\cdot 3600)[/tex] radians/second
    and where R is the radius of the Earth (or more precisely the distance to the axis of Earth's rotation).

    This comes to a bit over 33 cm/sec^2 at the equator. Thats about 3.5% the acceleration due to gravity. If you are on one of the poles it won't move. (Unless you further magically simulate the earth's orbit about the sun). Somewhere in between the equator and poles it will accelerate away from the Earth's axis at the above number times the cosine of your lattitude. (90deg at poles?).

    Finally as it flies upward it will begin drifting West following a straight line as we move around in a circle.
  9. May 1, 2007 #8


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    i was thinking , if like u could 'magically' remove gravitons from mass
    even if i think its mostly impossible


    but lets just say the object is a vacum, and that theres no atmosphere on the planet (not even dust particles)
    and that we could magically track the position of this vacum
  10. May 3, 2007 #9


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    Yea, the thing is... well first "gravitons" are just a way to resolve (a hypothesized) quantum gravitational interaction. Your question is still in the classical domain so we needn't invoke "gravitons" per se. Just look at the current theories of gravity.

    Once you "turn off" the coupling between mass and gravitation you are effectively stepping out of the established theory so no prediction is possible unless and until you suggest another theory. This is what I ment by the difficulty with counterfactual assumptions. You can pick whatever theory you want now and predict whatever you wish. Since you'll never see an example any prediction is meaningless. Nonetheless physicists often make "counterfactual assumptions" but always (if done correctly) in the context of a perturbative expansion i.e. ignoring effects below some scale. Since the effect of gravity is independent of the value of the mass, i.e. since inertial mass equals gravitational mass, you can't just look at the limit as the mass goes to zero.

    I guess the way to answer question is to allow inertial mass to stay the same but let gravitational mass "go to zero" in which case my "magic simulation" is exactly what would result.

    James Baugh
  11. May 4, 2007 #10
    it would prolly(if it weighed 0) just stay where it was dropped.unless u used some force such as the wind to move it.It cant float like a hellium ballon becase helium is waaaay lighter than air.
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