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A bird and a train

  1. Dec 17, 2007 #1
    this questions has been bugging me for a long time now:
    Imagine you are in a train, in a compartment, sitting there. Then suddenly you see outside of the window a bird that flies exactly at the speed of the train. It gets closer and closer, until it flies into the compartment through an open window, while still maintaining the constant velocity in forward direction. What happens to that bird as it enters?
  2. jcsd
  3. Dec 17, 2007 #2


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    It accelerates forward in the train.
  4. Dec 17, 2007 #3


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    A useful thing to know if for instance you ever have to drive a mini cooper full of stolen Italian gold into the back of a converted bus!

    Anyone who has watched British TV at christmas in the last 40years could have told you this!
  5. Dec 18, 2007 #4
    yes obviously it accelerated because its flapping, but there are basically 3 outcomes i can see:

    smashes to front wall
    smashes to back wall
    suspended in air

    now i think i can rule out suspended in air... but anyway. I think the problem is that the air in the room is actually stationary, but it is moving at train's speed. The air outside is stationary, but the bird is moving through it.... I still don't get it
  6. Dec 18, 2007 #5
    If the train and the bird are in vacuum or the train is moving really slowly, then the bird will appear stationary.

    At higher speeds... erm... I need to think first.
  7. Dec 18, 2007 #6
    Think opposite way. The train is at rest, outside, the wind is very strong, say 80km/h and the bird is trying to fly head on the wind and it can only as fast as the wind, so it appears to be stationary with the train. If the bird some how is closer and closer and finally be in the train, it would hit the front wall if still flapped and just dropped down if it stopped flapping. There's no way it hit the wall behind. But I should think the tubulence of air in the window must be very complicated.
  8. Dec 18, 2007 #7
    Yeap, thinking about the air turbulence at the windows made me imagine that the bird wouldn't be able to enter the train that easily...
  9. Dec 18, 2007 #8


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    Typical physicist answer = first we assume a spherical bird flying in a vacuum !
  10. Dec 18, 2007 #9


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    I wonder if a biologist was ever studying chickens and had to make assumptions about the Earth :wink:

    Another way to think of it - if you jump upwards on a train, the path you sweep is parabolic (observed by someone not on the train, of course). When in the air, you would have a velocity equivalent to the train and assuming the train doesn't accelerate, you land exactly where you jumped up from. The bird moving at the same velocity as the train should be treated no differently once they are in the train. Any more flapping would accelerate them relative to the train.

    Doesn't matter how fast, as long as the functions describing their movement match up.
  11. Dec 18, 2007 #10
    yea, dst & pixel you make valid points. I tried to think about it like that before... I think you are right. Still, i really wish i could see it happen in real life :) It would probably be almost as funny watching the confused bird, as it is watching a cat in zero-g environment.

    Oh and... how on earth would a bird FLY in vacuum, let alone survive! haha
  12. Dec 18, 2007 #11
    Hang on a minute, I'm confused. He said the bird maintains constant velocity so surely it won't accelerate? I would have thought that the bird would be seen to flap but would appear stationary because of its relative position to the walls, ceiling and floor of the train carriage.
  13. Dec 18, 2007 #12
    you have to keep in mind how flapping works. You force air particles backwards to push yourself forwards. Hence flapping will always accelerate you forward. The only reason you are seeing birds fly at seemingly constant rate is because their acceleration forward from flapping exactly balances out the air friction they are facing
  14. Dec 18, 2007 #13


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    If the bird takes off from a perch does the weight of the plane change if it is running on a treadmill - and how long does it take for a moderator to notice?
  15. Dec 19, 2007 #14
    It doesn't matter what people assume would think in real life, it is the question that matters..

    'It maintains constant velocity'
    Since velocity is direction & speed then it will
    a) hit the other side of the train.
    b) be lucky if the other side of the trains window is open ;)

    It's just two lines crossing each other.

    Velocity already tells you the direction it will be traveling.
  16. Dec 19, 2007 #15


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  17. Dec 19, 2007 #16


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    No, you misread. It's two lines that are parallel to each other, and one goes inside the other and continues straight at some point by making a sharp turn.

    The question is whether the air inside the train will keep accelerating the bird. Assuming a spherical bird mounted with a jet engine in a vacuum, it's clear that the bird has to maintain acceleration in step with the train, as long as it's in the air. Air effects aside, there is no difference between a bird outside the train moving at the same velocity, and a bird inside the train.

    If it slows down when going into the train, it will slap into the back. Speeding up, it will slap into the front. If it moves at the exact same rate, then it'll look stationary for some period of time, and very very, very strange.
  18. Dec 19, 2007 #17
    I cannot see how they are two parallel lines.

    In velocity terms the train is traveling in its direction at a speed
    The bird is traveling towards the train at an angle (getting closer and closer) at it's speed.

    It is one of the normal reletivity questions.

    To passenger who is stationary (wrt him/herself) the bird is just moving striaght towards the train.

    Velocity does not care for air or the surroundings, if it is constant then it cannot change, otherwise it would not be constant.

    But the trick part of the question in my opinion depends on how you interperet forward.
    Since the bird continues forward with constant velocity, forward has no specific direction so I interpret it wrt to the bird since we are talking about the bird. hence it keeps going in a stright line and out the other side (if lucky)
    But if you say forward wrt the direction of the train, the bird will hit the front of the compartment.
    If it is forward wrt the passanger, then the bird is hovering, since the passenger is not moving wrt him/herself.

    Note the question mixes relativities by speeking from the passanger point of view but talking like a 3rd pesrons point of view.
    Last edited: Dec 19, 2007
  19. Dec 19, 2007 #18
    You did not say if theoretical bird that flew into the theoretiacl railroad car was anticipating it, like landing on a perch? Did he put his air brakes on?
  20. Dec 19, 2007 #19
    Think of the old Feynman trick (well, he probably didn't invent the technique, but I learned it from him) and run the scenario backwards. A bird is hovering stationary inside the carriage and suddenly goes out the open window. What happens?
  21. Dec 19, 2007 #20
    Does the open window lead to the next car or to outside environment? You must be specific to get the correct answer.
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