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Homework Help: Is this a trick question?

  1. Nov 25, 2005 #1
    A horizontal beam of light is reflected from a plane mirror that revolves about a vertical axis at a rate of 30 rev/min. The reflected beam sweeps across a screen that, at the point nearest the mirror, is 20 m away. With what speed does the spot of light move across the screen at the point nearest the mirror?

    Is it c? (3*10^8)
    I know I am not understanding. Can someone explain this one to me? I am not even sure where to start.
  2. jcsd
  3. Nov 25, 2005 #2
    The angular velocity of the mirror is 30*2*Pi/60 radians per second. Multiply this by 20 meters to get the velocity (in meters per second) of the endpoint of a rod which is 20 meters long and perpendicular to the mirror. The speed of the spot of light will be twice that, that is, if I understood the problem correctly. Note that when the mirror (along with the imaginary attached rod) turns by 45 degrees, the reflected ray turns by 90 degrees, i.e. 2*45 degrees.
    Last edited: Nov 25, 2005
  4. Nov 25, 2005 #3


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    Homework Helper

    Always start by drawing a diagram!
    Make sure the quantity you want (distance along screen) is on the diagram!
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