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: Astro help please

  1. Oct 31, 2007 #1
    1. The problem statement, all variables and given/known data

    What is the ratio of the earth's orbital period about the sun to the earth's period of rotation about its own axis

    2. Relevant equations

    none, theory

    3. The attempt at a solution

    Its theory

    Two 335 kg masses (744 lb) are separated by a distance of 5 m. Using Newton's law of gravitation, find the magnitude of the gravitational force exerted by one mass on the other.

    F = Gm1m2
    r (squared)

    God... I'm putting it in, but it says its wrong... help....

    Joe has a weight of 750 N (about 170 lb) when he is standing on the surface of the earth. What would his weight (the gravitational force due to the earth) be if he tripled his distance from the center of the earth by flying in a spacecraft?

    The formula should be...

    w x a = f

    help please!

    Thank you members of the community
    Last edited: Oct 31, 2007
  2. jcsd
  3. Oct 31, 2007 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The first question should be fairly easy. I assume you've been given the information about both periods of rotation.

    For the two masses what answer are you getting? That might help in diagnosing whats going wrong.

    Weight is defined as W=mg, where:

    [tex] g= G\frac{M_E}{r^2} [/tex]

    What will his weight be if r is tripled?
  4. Nov 1, 2007 #3


    User Avatar
    Staff Emeritus
    Science Advisor

    What is the definition of a year timewise? What is significant about the length of a day?

    For the second problem, please show the values used, e.g. gravitational constant, G, in the formula.
  5. Nov 1, 2007 #4
    Umm it doesnt say...

    and grav. should be 9.81
  6. Nov 1, 2007 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    What do you think are the periods for the period of rotation of the Earth and the period of the Earth round the sun. Its fairly obvious.

    No, g = 9.81 ms-2 and G = 6.67x10-11 m3kg-1 s-2
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook