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: Short Grade 12 Gravitational Problem

  1. Jan 22, 2012 #1
    The Question
    The Planet V has a year lasting 360 days. In the same star system is the Planet X. The Planet X has half the mass and triple the radius. A planet X year would be approximately _____ days

    The attempt at a solution

    Tv= 360days
    Tx= ?
    mv = mv
    mx = 1/2mv
    rv = rv
    rx = 3rv

    I derived a formula that incorporates time.
    From which v = sqrt ( GM/r )
    since T = 2pir / v

    sub v into that formula and you get:

    T = sqrt (4 pi^2 r^ 3 / GM )

    with that I had two sets of values, Vulcan and X:
    I plugged them into the formula and arranged for a common values such as Mv.
    Then I subbed Mv from one formula into another.

    This is what results:

    8pi^23rv^3 / GTx^2 = 4pi^2rv^3/ GTv^2

    through cancellations I ended up getting:

    Tx = sqrt ( (2)(3)^3(360)^2 )
    which was that amount of days.

    Not sure if this method would be the correct way to go with this question or not.
  2. jcsd
  3. Jan 22, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Do you mean that the radius of the orbit of planet X is triple that of planet V ?

    Does the mass of the planet matter? Is the M in the equation, v = sqrt ( GM/r ), the mass of the planet, or the mass of some other object?

    Use the subscript, X2, & superscript, X2, buttons above the "Go Advanced" message box.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook