1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Gravitational Fields and Density.

  1. Jan 29, 2012 #1
    1. The problem statement, all variables and given/known data
    If a planet has a diameter of 6*103km and, one of it's satellites is in a circular orbit of radius 9.0*103km with a time period of 8.0 hours.
    What is the density of the planet?

    2. Relevant equations
    ρ=m/v - density v=4/3 *∏*r2
    GMm/r2 = mw2r

    3. The attempt at a solution
    Firstly convert the Km into M,
    Diameter of planet = 6*106 → 3*103
    Radius of Satellite = 9.3*106
    Time period = 28800s

    Where G = 6.7*10-11

    Therefore I used

    GMm/r2 = mw2r

    And subbing in the values;


    So I cancelled out the little m and rearranged the equation for M,

    Giving M an answer as 1.87*1033

    I then subbed it into the equation for Density,

    1.87*1033/4/3*∏(3*106) which gives me an answer of 4.96*1019,

    However the given answer is 4.6*103, Any guidance would be appreciated ;D.
  2. jcsd
  3. Jan 29, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    (1) Why did you add the radii?
    (2) ω is the angular frequency, not the period.
    (3) On the left side, that radius must be squared.
  4. Jan 29, 2012 #3
    volume is (4/3)πr^3 you have written (4/3)πr^2 and you have not cubed the value in your calculation
    Also ω = 2π/T and you have used T as pointed out by Doc Al
    Last edited: Jan 29, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook