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Some quesitons about Sun and Earh

  1. Oct 3, 2009 #1
    1) What keeps the Earth orbiting around the Sun?
    I know that gravity gives the force of the Earth towards the center of the Sun but what about the tangential force? What gives the Earth the tangential force?

    2) What force makes the Earth spin?
  2. jcsd
  3. Oct 3, 2009 #2


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    1) There is no tangential force (assume the orbit to be circular), just the force of gravity. What makes you think there would be a tangential force? No such force is required for an object to orbit. Perhaps you should read up on Newton's cannon?

    2) Similarly, there is no force. The earth has always been spinning, and as a result of conservation of momentum, will continue spinning. You don't need a force to keep an object in a state of constant motion, which is newton's 1st law.
  4. Oct 3, 2009 #3
    Well i get the part about conservation of momentum now that make sense.

    For the first question i guess i have to read about orbits. Is there any simple explanation of that? I can only image 2 bodies moving opposite x-directions but are located in different y-directions. And so when they cross paths, the gravity try to pull each other so gravity will play role of centrifugal force while their initial velocity will play roles of tangential velocity. But that is the case where the both masses are orbiting around empty space. How do i create the scenario for something orbiting around a mass?
  5. Oct 3, 2009 #4


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    There is only one scenario in what you just described...
  6. Oct 3, 2009 #5


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    Well, two bodies of equivalent mass will seem to orbit around space, but they're really orbiting around each other. The center of gravity of the combined bodies is exactly right between them (since they have equal mass).

    As you begin to make one mass bigger, the center of mass moves towards it... until it's so huge that the center of mass (of the two bodies) is practically right in the center of the mass of the larger body.

    An example of the last scenario (one extremely massive body and a not-somassive body) could be the Sun and any of the planets around it (say... Earth). So the center of gravity for both bodies (the Sun and the Earth) is right about where the center of gravity is for the Sun.

    In reality, it's a little bit off from the Sun's center of gravity, so the sun does actually wobble a bit.
  7. Oct 3, 2009 #6
    Now it makes sense. I forgot about adding the center of gravity situation. Thanks for clarifying everything for me.
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