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Homework Help: Rotating binary star system & three star system

  1. Apr 18, 2008 #1
    1. The problem statement, all variables and given/known data
    1) Astronomers discover a binary star system that has period of 90 days. The binary star system consists of two equal mass stars each with a mass twice the sun's. They rotate about the center of mass at the midpoint between them. How far apart are the two stars?

    2) Three stars each with a mass of our sun, form an equilateral triangle with sides 1* 10^12 m long. The triangle has to rotate because otherwise the stars would collide. What is the period of rotation?

    2. Relevant equations

    3. The attempt at a solution

    For #1,

    There doesn't seem to me to be any clear pivot or mass around which the stars orbit, and that's what baffles me. I thought about pretending they were connected by a massless string and calculating the moment of inertia, and then using rotational kinetic energy, but I'm not sure how to start

    #2 I think I'll have a better grasp on this after figuring out #1
  2. jcsd
  3. Apr 18, 2008 #2


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    Homework Helper

    Hi bcjochimo07,

    Draw a force diagram for one of the stars. What is the only force that acts on it? The stars are moving in a circle, so you also know a formula for the acceleration.

    Be careful when you assign distances--the distance that is used in the force formula is not the distance that is used in the acceleration formula.
  4. Apr 18, 2008 #3
    Thanks! I got the first problem right. Now for the second one.... I would have to choose one of the stars and sum the force components. Next I would use trigonometry to find the radius & then I would solve for angular velocity & then the period.
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