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Gravity and Uniformed Circular motion

  1. Mar 14, 2012 #1
    Is gravity a Uniformed Circular motion? I had this question for a while and i googled searched it but no results, I also did the readings which I do think it is. But in the readings , I never read the statement that says gravity is a form of uniformed circular motion. Can anyone explain why or why not it is or isn't? Thanks.
     
  2. jcsd
  3. Mar 14, 2012 #2

    Doc Al

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    Gravity is a force, not a motion.

    To have uniform circular motion, you need a net centripetal force. In certain cases, gravity can provide that force, such as a satellite in a circular orbit about a planet.
     
  4. Mar 14, 2012 #3
    Thanks for the reply. another question would be, gravity is a form of centripetal acceleration ? since they have the same units.
     
  5. Mar 14, 2012 #4

    Doc Al

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    The term 'gravity' is a bit ambiguous. Do you mean the force of gravity? Or the acceleration due to gravity? Those are different, but related, things.

    In any case, only in special cases, such as that satellite in circular orbit about a planet, would the acceleration due to gravity happen to be a centripetal acceleration.

    Anything executing uniform circular motion is undergoing a centripetal acceleration, so there must be a net centripetal force acting. It may or may not have anything to do with gravity.
     
  6. Mar 14, 2012 #5
    Thanks a lot, that was a good explaination.
     
  7. Mar 14, 2012 #6

    sophiecentaur

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    Satellite orbits: Is this what the OP is basically about?

    There is one particular speed for a satellite to travel that will cause its orbit to be exactly circular. This will be when the centripetal force is exactly equal to the gravitational force and it is moving at right angles to a radius. If you take a satellite which satisfies these conditions and give it a change in speed (boost or braking), or direction, it will no longer follow a circular orbit but an elliptical one. The gravitational force varies, depending on their orbital distance (inverse square law) and this is the only 'force law' that will lead to a stable, elliptical orbit afaik.
    Most (all) orbits of planets, moons, satellites are, in fact, elliptical and not perfect circles. Sometimes they are far away and move slower and at other times they are closer and moving faster. But they do not 'fall out' of orbit nor do they escape because their total energy (Gravitational Potential plus Kinetic) remains the same.
     
  8. Mar 16, 2012 #7
    Also, think of gravity like the picture of a negative charge. All the lines of force point inwards to a central point if the only body in the example is the body causing the gravity. At every distance, the force is the same, but decreases slowly as you move farther and farther away. This picture changes when you introduce another body. It is much like positive and negative charges in the way the force interacts.
     
  9. Mar 16, 2012 #8

    sophiecentaur

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    Is it really such a good idea to bring in a totally different force (electric) when the forces, lines and fields in gravity can be pictured in just the same way?
     
  10. Mar 16, 2012 #9
    Merely trying to explain the idea using a force simpler than gravity.
     
  11. Mar 16, 2012 #10

    sophiecentaur

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    Simpler?
    You're lucky if you find it so.
     
  12. Mar 16, 2012 #11
  13. Mar 17, 2012 #12

    sophiecentaur

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    That diagram is fine on its own but, on that link, it is followed by a whole set of diagrams, which involve numbers of opposite and like charges. Which of those would you relate to gravity and which could you not? If anything, you'd need to choose only the diagrams which involve like charges - but in those, the forces are repulsive.
    If someone looks at that link and thinks they can learn about gravity, they may emerge worse off than before. That was my worry.
     
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