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Finding minimum angular speed

  1. Mar 17, 2009 #1
    1. The problem statement, all variables and given/known data
    The figure illustrates Michelson's setup for measuring the speed of light with the mirrors placed on Mt. San Antonio and Mt. Wilson in California, which are 35 km apart. Using a value of 3.00 x 108 m/s for the speed of light, find the minimum angular speed (in rev/s) for the rotating mirror.

    Link to figure: http://edugen.wiley.com/edugen/courses/crs1507/art/qb/qu/c24/ch24f_12.gif

    2. Relevant equations
    delta t = 2d/c

    3. The attempt at a solution
    delta t = 2(3500)/3x10^8 = 2.33x10^-4

    Now if I'm on the right track, then I'm not sure what my next step should be.. I feel like I should be dividing or multiplying by 8 since I was told the object is an octagon. Am I right??
  2. jcsd
  3. Mar 17, 2009 #2


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    35 km or 3.5 km?

    Doesn't the wheel have to advance 1/8 of a revolution by the time it bounces to see it again in the next successive reflecting surface from when it was originated?
  4. Mar 17, 2009 #3
    Oh, sorry I meant 2(35000)/3x10^8 = 2.333x10^-4

    do I then just divide that by 8 to get 2.91x10^-5 ?
  5. Mar 17, 2009 #4


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    You found Δt and that is 1/8 of a rev isn't it?

    So how long to make a rev?
  6. Mar 17, 2009 #5
    I took 2.333x10^-4 and multiplied that by 8 to get .0018664 rev/s but that answer is incorrect
  7. Mar 17, 2009 #6


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    The time for a revolution is not rev/s.
  8. Mar 17, 2009 #7
    Awesome, got it! 535.79. Thanks for the help! :]
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