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: Kepler's Third Law Problem

  1. Jun 24, 2014 #1
    1. The problem statement, all variables and given/known data
    The planet Jupiter has at least 14 satellites. One of them, Callisto, has a period of 16.75 days and a mean orbital radius of 1.883 x 109 m. From this information, calculate the mass of Jupiter.

    2. Relevant equations
    T2 = (4∏2/ GM)r3

    ∏ = pi

    3. The attempt at a solution
    I attempted to plug the values into the above formula to find the mass

    (16.75)2 = (4∏2/6.673 x 10-11M)(1.883 x 109)3

    280.5625 = (1.883 x 109)34∏2/ 6.673 x 10-11M

    280.5625 M = (1.883 x 109)34∏2/ 6.673 x 10-11

    280.5625 M = 3.95 x 1017

    M = 1.4 x 1015
    The correct answer is 1.89 x 1027 kg
  2. jcsd
  3. Jun 24, 2014 #2


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    What is the SI unit for time?

    Also, when you divided by 6.673 x 10-11, I think you must have plugged in 6.673 x 10+11 in your calculator.
  4. Jun 25, 2014 #3
    OK I figured it out. Thanks!
  5. Jun 25, 2014 #4

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    This was the source of your difficulties. I realize you have already solved this problem, but don't just "plug the values into the above formula".

    Keep the units in your expressions. G is not 6.673×10-11. It is 6.673×10-11 m3⋅kg-1⋅s-2. Change the units and you'll get a different value for G. If you had carried the units in the expression you would have realized you have a units mismatch between the period in days and the seconds used in G. Other students experience similar difficulties when distances are expressed in kilometers rather than meters. These problems go away (or at least you are alerted to them) if you keep those units in your expressions.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted