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Easy gravitational problem

  1. Nov 20, 2003 #1
    i have no idea how to do this :(

    Venus is an average distance of 1.08x10^8 km from the sun. Estimate the length of the Venutian year given that the Eearth is 1.50x10^8 from the sun on the average?

    answer in years.
    one more :(

    the rings of saturn are composed of chunks of ice that orbit the planet. the inner radius of teh rings is 73,000 km, while the outer radius is 170,000 km. The mass of saturn is 5.69x10^26 kg.

    a) find period of orbiting chunk of ice at inner r
    b) find period of orbiting chunk of ice at outer r

    im thikning im going to use the formula: F = m1m2 / r^2 but im not sure how to approach it

    help would be appreciated

    thanks dudes
    Last edited: Nov 20, 2003
  2. jcsd
  3. Nov 20, 2003 #2
    For all of these problems, start with Newton's gravitational force law (which you gave, except you left out the gravitational constant G); assume that the orbits are circular, so that the gravitational force is a centripetal force. Use the centripetal force to find the orbital velocity, and with that and the radius, you can find the period.
  4. Nov 20, 2003 #3
    F = m v^2 / r is centripetal force right

    and thanks for the help
  5. Nov 20, 2003 #4
    Yes, that's the right formula for centripetal force.
  6. Nov 20, 2003 #5
    coudl you please clarify how to use the radius and orbital velocity to find the period?

    my texctbook doesn't give a good explanation?


    F = m v^2 / r

    so it would be G m m(e) / r^2 = m v^2 / r

    what would m(e) be
    Last edited: Nov 20, 2003
  7. Nov 20, 2003 #6
    If you know the distance something travels (e.g., the circumference of the orbit), and its speed, you can find out how long it takes to travel that far.

    In this context, it's the mass of the gravitating body about which the body of mass m is orbiting.
  8. Nov 20, 2003 #7
    yes thank you it is finally clear

    G ( 5.69 x 1-^26) / (170,000,000)^2 = v^2/r


    i got it :D
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