1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Homework Helper

    Always start by drawing a diagram!
    Make sure the quantity you want (distance along screen) is on the diagram!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Is this a trick question?
  1. Trick Questions? (Replies: 22)

  2. Trick question? (Replies: 2)

  3. Trick question (Replies: 6)