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: Gravitation Please Help

  1. Mar 11, 2006 #1
    Q:
    Suppose that a binary star system consists of two stars of equal mass. They are observed to be separated by 340 million kilometers and take 5.0 Earth years to orbit about a point midway between them. What is the mass of each?
    I figured out that:
    mass=4pi^2(radius)^2/Gravitational Force(#of years)*(distance)2
    m= [4(3.14)^2(3.3x10^29)^3]/[(6.67x10^-11){(8.0 years)(3.4x10^7}^2] =3.33x10^29 then (3.33x10^29)/2 = 1.7x10^29

    I don't know what I'm doing wrong here. Any ideas?
     
  2. jcsd
  3. Mar 11, 2006 #2

    tony873004

    User Avatar
    Science Advisor
    Gold Member

    Maybe an algebra mistake in getting to this point. Should be:
    mass=4pi^2(radius)^3/(Gravitational Force(#of years*seconds per year)^2)

    How did you know your answer was wrong? It's very close to correct. Does the back of the book give the answer?
     
    Last edited: Mar 11, 2006
  4. Mar 11, 2006 #3
    so if i reply this equation it comes out as:
    m=4pi^2(3.3x10^29)^3/6.67e-11(5yearsx 3.155815296E7 sec per yr)^2

    is that right? or am i using the wrong radius? isnt the radius half the distance between the two stars? if thats true than the radius id 170 million, isnt it? im so confused!!
     
    Last edited: Mar 11, 2006
  5. Mar 11, 2006 #4

    tony873004

    User Avatar
    Science Advisor
    Gold Member

    Where did you get 3.3 x 10^29. They give you the distance of 340,000,000 million kilometers. This becomes your a or radius (3.4 x 10^11 meters)

    It's not half the distance between the 2 stars since. Pretend 1 star is still, and the other orbits it. It will trace an orbit whose diameter is twice the distance between the 2 stars. Therefore, the distance between the 2 stars becomes the radius, or semi-major axis in this problem.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook