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Homework Help: Light Question

  1. Dec 12, 2007 #1
    1.Dubbie is moving directly along a normal line toward a plain mirror at a speed of 3.5 m/s, what is the speed of the image relative to Dubbie?

    2. What is the speed of the image, relative to Dubbie, if Dubbie walk away from the mirror surface at 3.5 m/s at an angle of 300 to the mirror surface?

    First answer is 3.5m/s*2 which is 7m/s right?...

    but next question I just don't get it... what kind of formula do I have to use?...

    can some one please help me T.T
  2. jcsd
  3. Dec 12, 2007 #2


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    Homework Helper

    Draw the diagram of the motion of Dubbie and image starting from the mirror. You can see that at every instant the Dubbie, image and starting point form an equilateral triangle. Hence the speed of the image, relative to Dubbie is 3.5m/s.
  4. Dec 12, 2007 #3
    I don't understand it still, what do you mean by hence the speed of the image?...
  5. Dec 12, 2007 #4
    For the first question, is Dubbie's speed relative to the stationary mirror? If that is so, then, the speed of the image relative to Dubbie is 7 m/s.

    For the second question, just find the component of the speed towards the mirror ie, 3.5 cos 60. Then multiply that by 2 just like the first question.
  6. Dec 12, 2007 #5
    ok, thankyou~~
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