Veloctiy of image of a moving obect having constant veloctiy

  1. If an object moves towards us with constant velocity, its velocity is constant with respect to us. But when we view its image in stationary car rear view mirror (or any stationary mirror), its velocity is changes? Why and How?
  2. jcsd
  3. rcgldr

    rcgldr 7,408
    Homework Helper

    If the mirror doesn't change the size of the virtual image, then it only reverses the image from front to back so that a car approaching from behind appears to be approaching from in front.
  4. adjacent

    adjacent 1,533
    Gold Member

    For example,A person is coming to you,starting from 1km away(Moving at constant velocity).At first,You would be seeing him getting bigger slowly but as he approaches closer,His rate of getting bigger increases.That's why you think him getting accelerated.
    This is associated with vision.If our Field of view were parallel,the person will appear to be in the same size,No matter how far he is.
    I need a little clarification of your question.
  5. But i think its velocity is constant with respect to us.....
  6. Its due to relative velocity, that velocity appears different with respect to the car or you.
  7. sophiecentaur

    sophiecentaur 13,384
    Science Advisor
    Gold Member

    I heard of an expression "rate of looming", which is supposed to be how we judge the rate of approach of an object - this is the rate at which the angle subtended increases and will work better for large objects which subtend a large angle (understandably).
    In many circumstances we use this more than binocular vision or perspective clues - which is why the effect is used in cinematography (zooming etc.)and we 'believe' what the director wants us to.
    This effect could also be responsible, I think, for the fact that rear view car mirrors are not good at helping us to judge position and speed of overtaking vehicles.
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