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Homework Help: Universal Gravitation and X-ray pulses

  1. Dec 11, 2003 #1
    I'm having a little bit of a problem with this one. Here's the question:

    X-ray pulses from Cygnus X-1, a celestial x-ray source, have been recorded during high-altitude rocket flights. The signals can be interpreted as originating when a blob of ionized matter orbits a black hole with a period of 4.7 ms. If the blob were in a circular orbit about a black hole whose mass is 18 * MSun, what is the orbit radius?

    Here's my train of thought. I used one of Kepler's laws:

    T^2 = \frac{4\pi^2r^3}{GM_{blackhole}}\\
    r = (\frac{T^2GM_{blackhole}}{4\pi^2})^(\frac{1}{3})\\

    I changed the milliseconds to seconds, and I got a answer of 110196.285 meters. I'm letting the mass of the sun be 1.991 x 1030 kg.

    I enter this into webassign, but I doesn't like it. What am I doing wrong?
  2. jcsd
  3. Dec 11, 2003 #2
    I don't see anything wrong in what you did (other than keeping too many significant digits). Try rounding the answer to 110 km.
  4. Dec 11, 2003 #3


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    Staff Emeritus
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    Kepler's laws won't apply when you're this close to a black hole. I'll have to think about this one a bit. Although the title of this thread suggests you're supposed to be using Newtonian mechanics...

    - Warren
  5. Dec 11, 2003 #4
    These questions were in the "Universal Gravitation" chapter. It's got to have something to do with either Kepler's laws, or the law of universal gravitation.
  6. Dec 11, 2003 #5
    Heh, it was a decimial problem. Thanks for pointing that out.

    If only all of these problems would be so simple!
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