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How do you know if someone far away is walking towards you or away

  1. Sep 1, 2013 #1
    This is something trivial that has been bugging me for a while. When someone is far ahead of you on a sidewalk, you see their arms swinging and feet beating the ground. But how can you make out if they are walking towards or away from you?
    Suppose that I don't want to stand still for thirty seconds to see if they're getting closer.
  2. jcsd
  3. Sep 1, 2013 #2
    1.> Shout and ask him/her.
    2.> See which direction (s)he's facing.
    3.> Shoot two light beams at a time interval of your preference and see if the second one comes back after a shorter or longer period of time.
    4.> Run towards him/her and find out.
    5.> Can't think of any other silly suggestions... wait, so just see if the hands swing more towards you or away from you. Oh and lie down on the road and check if you can see his/her soles if yes the person's moving towards you.
  4. Sep 1, 2013 #3
    You have to rely on your eyesight. If your sight isn't good enough, you won't be able to tell.
    Or maybe you could see the direction shadows travel over the person. Back to front if they're walking away from you, and front to back if they're walking towards you.
  5. Sep 1, 2013 #4
    NOT necessarily:You can use your ears too, provided that they are extremely good. If the sound of his footsteps is getting louder or its frequency is getting higher then the person is moving towards you else he is moving away from you.

    Don't mind me -I'm on a sugar and caffeine high!
  6. Sep 1, 2013 #5


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    increasing frequency would means his speed is increasing towards you. If they are walking at a constant velocity towards you, their frequency will be constant.
  7. Sep 1, 2013 #6
    WOOPSSS!!! Had added the freq. at the last moment. Note to self: always mention what I mean and generalize...I art iDiot.
  8. Sep 1, 2013 #7


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  9. Sep 1, 2013 #8
    I think the question means in principle - how can you tell if something is moving towards or away from your position if it's too far to tell from simple geometry.

    I think really the only physical way is to use Doppler shift of some kind, be it sounds, light, or some other medium. This is the best tool astronomers have for this same problem.
  10. Sep 1, 2013 #9


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    For constant velocity, doppler shift only works if you get passed by them.
  11. Sep 1, 2013 #10
    never mind....
  12. Sep 1, 2013 #11
    Doppler shift of a reflected wave which you know the original frequency, or an emitted wave which you know the frequency of in the laboratory frame, such as an emission line. I don't think astronomers have to wait for galaxies to pass us by to measure their Doppler shift.
  13. Sep 1, 2013 #12


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    Galaxies aren't constant velocity, they change direction. That's what allows us to define red/blue shift. An object with a constant velocity never shifts (until it passes you: then it's as if its velocity reversed relative to you)
  14. Sep 1, 2013 #13


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    er, wait, I'm talking about orbiting bodies :P

    but yeah, if you know the frequency of your source, you can determine direction and velocity, agreed.
  15. Sep 1, 2013 #14


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  16. Sep 2, 2013 #15
    This does not square with SR, where there are no preferred inertial frames. They only maintain their original size in their inertial frame. In your inertial frame, they are, in fact, shrinking down to tiny little people.
  17. Sep 2, 2013 #16
    Mmmm....don't think so, the people will just suffer lorntz. contr. only in direction of motion, becoming thin not tiny...
  18. Sep 2, 2013 #17


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    The onion is a satirical newspaper...
  19. Sep 2, 2013 #18
    Don't tell me they don't become tiny. I can see with my own eyes that they become tiny.
  20. Sep 2, 2013 #19
    Okay, not telling you that they become tiny! Any way, observing them will make the quantum state collapse.
    Please don't observe people or the cat may die.
  21. Sep 2, 2013 #20
    As pointed out by others, it's not so immediate. For visual checks, aircrafts use lights of different color on the tipwings, so that one can easily determine its direction looking at them.
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