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Homework Help: GRE Subject questions

  1. Sep 8, 2007 #1
    My background is not strong enough to solve it on my own. Sometimes I'm clueless which area I could find related stuff that I could solve it on my own, so I come here, and if you could tell me hich area it falls on, it'd be great then I could find it out and go through it myself.

    I'd be appreciated if anyone could help me. I list some below. Thank you !

    1.a satelite of mass m orbits a planet of mass M in a circular orbit of radius R. The time required for one revolution is ?

    2. 2 stars r separated by an angle of 3x10^-5 radians. What is the diameter of the smallest telescope that can resolve the 2 stars using visible light (600 nanometers) (ignore any effects due to earth's atmosphere.)

    3. an 8-cm-diameter by 8-cm-long NaI(T1) detector detects gamma rays of a specific energy from a point source of radioactivity. When the source is placed just next to the detector at the center of the ciucular face, 50 percent of all emitted gamma rays at that enerfy are detected. If the detector is moved to 1 m away, the fraction of detected gamma rays drops to ?
  2. jcsd
  3. Sep 8, 2007 #2


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    Homework Helper

    1. Involves Newton's law of gravitation and centripetal force/acceleration...

    2. Angular resolution... specifically angular resolution of a telescope.

    3. This involves "solid angle"... radiation travels as an expanding sphere from the point source... you need the "solid angle" that is subtended by the detecter... this is closely related to a circle, chord, and the angle subtended by a chord in a circle.

    Hope this helps.
  4. Sep 8, 2007 #3


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    1. orbital mechanics - Kepler's third law - see also Newton's law of graviation

    2. optics - resolution of a telescope

    3. radiation detection/detector or counting efficiency
  5. Sep 8, 2007 #4
    Thanks a lot for these info. :)
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