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Relative velocity with respect to image in the mirror

  1. Jun 20, 2013 #1
    1. The problem statement, all variables and given/known data

    As shown in the figure, a man is running towards left with
    constant velocity of 7 m/s. Also, a flat vertical mirror is moved
    to the right with constant velocity of 9 m/s. Calculate the
    relative velocity between the man and his image. Express
    your answer in meter/second.

    Figure - 5MUXD.png

    2. Relevant equations

    3. The attempt at a solution

    Since man is running to the left with velocity 7 then the image also must move right at 7 m/s. Also since the mirror is also moving , the total velocity of the image is 16 m/s towards right. So the relative velocity between the man and image should be 16+7= 23m/s . But the answer is incorrect . Please explain.
  2. jcsd
  3. Jun 20, 2013 #2
    Well, think about this. If the person stands still and the mirror moves at 9 m/s how fast does the image move. Then add the fact that the man also moves at 7 m/s, what is the net result?

    Here is another approach. Draw an initial diagram with the man say 1 meter in front of the mirror and then look at where the image is. Then move the man to the left 7 meters and the mirror to the right 9 meters and recalculate where the image is. What do you get?
    Last edited: Jun 20, 2013
  4. Jun 20, 2013 #3
    Thanks a lot ! I got it. The answer is 32 m/s.
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