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Homework Help: Gravitational and electric fields

  1. Oct 20, 2009 #1
    1. The problem statement, all variables and given/known data
    By considering the centripetal force which acts on a planet in a circular orbit, show that T^2 is proportional to R~^3 where T is the time taken for one orbit around the sun and R is the radius of the orbit

    2. Relevant equations
    I'm thinking keplers third law?

    3. The attempt at a solution

    P=2piR/ sqrt*GMm/r

    rearranged to give P = 2pi sqrt*R^3/GM

    Lost now..
  2. jcsd
  3. Oct 20, 2009 #2

    Where did that expression come from?

    Find an expression for the radial acceleration in terms of the orbital speed and R.

    Then use Newton's second law.
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