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What is gravity?

  1. Nov 28, 2008 #1
    I realise how stupid the question may sound, but this is a genuine question that I was just pondering about late at night. I mean, the question I really ask is why is there gravity on Earth but not in space. Is it our atmosphere that "keeps" gravity here? Or is our atmosphere just the limit of the strength of the gravity Earth possesses? Either way, I ask because I was just wondering why say there is still gravity in a vacuum (and no I do not think gravity has anything to do with "air" but still) and why we can't recreate it. Is gravity caused just by the spinning of the world?

    Anyways if the answer is too long or too simple I understand if you just write "you're an idiot". Just me wondering late at night and me envisioning futuristic anti-gravity flying vehicles. Don't go into too much effort into answering but I would like a response - even if its just a link or a reference to read something else I'll be happy.

    PS. I am not religious this isn't a satirical, rhetorical question to argue anything, I really would like a constructive answer.
  2. jcsd
  3. Nov 28, 2008 #2
    The simple answer is that gravity is a force caused by the mass of an object. Any two objects that have mass will attract each other through gravity. Thus any objects that have mass will attract each other. Gravity keeps the moon orbiting the earth, the earth orbiting the sun, the sun orbiting in the galaxy etc.

    There is also the insights of einstein, which say gravity is caused by the curvature of spacetime. But I wont overload this with too much detail at once =]
  4. Nov 28, 2008 #3


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    Hi 5thElement! :smile:

    Gravity is in space …

    the only reason that astronauts "feel" no gravity is because their rocket is free-falling …

    if you were falling straight down on the Moon (where there's no air resistance :wink:), you wouldn't feel any gravity either!

    the only difference is that the astronauts are free-falling with a high sideways velocity, so that they orbit the Earth (or Moon) … they're moving sideways so fast that they "miss" the Earth when they fall! :biggrin:

    Newton pointed out the same thing about the apple … if I throw it hard, it will travel many miles before landing, and if I throw it really hard (sideways), it will go round the Earth and never land. :smile:
  5. Nov 28, 2008 #4

    Ranger Mike

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    this question is why i signed up on this web site..i do not believe the current explanation of gravity is correct..newton even stated he did not have all the answers on it..

    objects only have MASS ifin they are in a Gravitational field..right..??? otherwise they are either Matter or Energy..i.e same stuff but different arrangement..
    now a days everything is result of the TIME SPACE fabric bein warped..
    keep digging pal..you doing ok!
    don't believe the current conventional wisdom about this and

    Last edited: Nov 28, 2008
  6. Nov 28, 2008 #5
  7. Nov 28, 2008 #6


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    Quite correct. Mass in an intrinsic property of matter that is unaffected by any environmental concerns. Weight is the apparent measurement of that mass within a gravitational field. Gravity exists throughout the entire universe, but in some places it is negligible in strength.
  8. Nov 29, 2008 #7

    Ranger Mike

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    thank you
    my education continues and i am the first to admit i need some
  9. Nov 29, 2008 #8


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    I join you in that sentiment, and we're both in the right place. :smile:
  10. Nov 29, 2008 #9


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    Actually, there is a causal relationship between gravity and the atmosphere. However, this relationship is just the reverse of the one you asked about; the gravity causes the atmosphere. Without gravity, the collection of gases that form our atmosphere would simply dissipate out into space.

    Quite right; if you pump all the air out of a container, it will have vacuum inside, but any solid object inside will behave in the normal ways, like sitting on the bottom of the container rather than floating around. An example would be old-fashioned lightbulbs (before they started filling them with inert gasses), when the filliment burned out, it fell to the bottom of the bulb.

    No but, interestingly enough, this is one of the plans for generating artificial gravity. At some point in the future, when and if humans are attempting to colonize space, one of the places we might try it is inside a large asteroid. This could be hollowed out and atmosphere pumped in to make it livable. Then, the asteroid would be set to spinning, creating a centrifugal force that would make people and other objects stick to the interior wall, much like gravity. But here on Earth, where we are sticking to the outer wall, the spinning actually acts against gravity.

  11. Nov 29, 2008 #10


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    I should have mentioned before that nobody here will do that (unless you start spouting crackpot crap). There is no such thing as a stupid question, as long as the asker is looking for a real answer. Only those who continually argue against the truth eventually get labelled as idiots. Those invariably are people who come to PF with some pet crackpot theory and refuse to accept the fact that it will not be validated. You are on good solid ground. :smile:
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