A girl is standing in front of a large plane mirror, contemplating her image. If she moves toward the mirror at speed v, we have to calculate the speed with which her image moves toward her; (a) in her own reference frame and (b) in the reference frame of the room in which the mirror is at rest.
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Solution: Click For PDF Version (b)The distances of an object and its image from a mirror are equal. Therefore, in the reference of the room in which the mirror is at rest, as the girl is moving toward the mirror with speed v, her image will also move toward the mirror with speed v but in opposite direction. (a)
In her rest
frame she will observer that her image is moving toward her with
speed
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which by velocity addition theorem is
.