# A bird and a train

## Main Question or Discussion Point

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?

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russ_watters
Mentor
It accelerates forward in the train.

mgb_phys
Homework Helper
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!

It accelerates forward in the train.
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

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.

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.

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...

mgb_phys
Homework Helper
If the train and the bird are in vacuum
Typical physicist answer = first we assume a spherical bird flying in a vacuum !

dst
Typical physicist answer = first we assume a spherical bird flying in a vacuum !
I wonder if a biologist was ever studying chickens and had to make assumptions about the Earth

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.

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

It accelerates forward in the train.
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.

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.
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

mgb_phys
Homework Helper
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?

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.

Hootenanny
Staff Emeritus
Gold Member
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?
:rofl::rofl::rofl:

dst
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.

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.

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.

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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?

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?

Does the open window lead to the next car or to outside environment? You must be specific to get the correct answer.

dst
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.

Note the question mixes relativities by speeking from the passanger point of view but talking like a 3rd pesrons point of view.

Ah, two different approaches.

How I was thinking of it, is that the bird flies parallel to the train, and then swoops inwards when it lines up with a window, or a strong side wind blows it in. It's going to be a different scenario if it's going diagonally of course.

i was thinking of it originally as dst describes: the bird flies as fast as the train, and then swoops in through the window. Although, its a very interesting idea to consider what happens if it flies in perpendicularly... I have no idea. This seems like it should be an EASY problem but yet I just cant wrap my head around it!

Why is this on page 2?

Why does the bird flap its wings when outside? -DRAG

When the bird flies inside the train, is there any forward air causing drag on the bird? NO The air in the train is moving at the same speed as the bird. Relative velocity of air to bird is now zero.

As russ said, it accelerates forward inside the train.

-The End.

Not true, the window is open so you'll either have air pulled out of the train or air blowing in to the train (depending on the speed of the train) hence the air is not stationary. That and the air conditioning is going crazy as you should have the windows closed for them to work properly...

... a bird flaps it wings for many reasons... To counter the effects of gravity (and hence make flight much easier) using the air and the wind to propel itself, hover, turn and cool itself down. Also to poo on people.

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Why is this on page 2?

Why does the bird flap its wings when outside? -DRAG

When the bird flies inside the train, is there any forward air causing drag on the bird? NO The air in the train is moving at the same speed as the bird. Relative velocity of air to bird is now zero.

As russ said, it accelerates forward inside the train.

-The End.
:) Yes Cyrus that's correct. Its a pretty good explanation too :) Oh and when the bird flies perpendicularly inside the window, I think it will just hit the back wall because the bird has no velocity in forward direction like the train does.