# A bird and a train

Hootenanny
Staff Emeritus
Gold Member
What if the train is going in the opposite direction of the bird but the magnitude of the velocity is about the same? It means the bird decelerates in the train, right
It would accelerate toward the rear of the train.
I wonder too.
I thought we'd agreed this thread was done?

Shooting Star
Homework Helper
It would accelerate toward the rear of the train.

I thought we'd agreed this thread was done?
Instead of a ground frame of reference I prefer coordinate or at least cartesian.
Not until we've settled this one, at least.

Well, lets say you have this senario:

Meassuring EVERYTHING in reference to the train.
In the train the airvelocity is zero. outside the airvelocity is negative 80 km/hour. (The speed of the train relative to the tracks).
So outside you see a bird flapping its wings to stay in air, moving with 0 km/hour relative to the train.

Bird enters window, and is now in a medium of 0 km/hour wind, and has a velocity of 0 km/hour. So either it keeps flapping , slowly accelerating again, eventually hitting the front wall in the train( or landing on a seat). Or, it gets confused by the wind change, stops flapping and crashes to the ground.

You seem to be thinking that it will suddenly be traveling with 80km/hour relative to the train because it "swiches frame of reference" which is NOT what will happen.

Shooting Star
Homework Helper
There are some birds which can cruise for long times without flapping. What if it was one of those type? Will it experience a sudden discontinuity or continue to cruise with equanimity?

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This is not that important, but first:
Birds that cruise travel at a negative velocity with respect to the train (since they are not flapping they are slowly slowing down), at the same time, they are only staying in the air, due to the friction of the wind, so if it travels at the same speed as the wind, it falls down(Same speed meaning the same direction too).

When the bird enter the train, there is no longer any wind, so they are left at exactly the same scenario as the flapping birds from before. They are at 0 km/h in a 0km/h wind field. So its left with the choice: flap or drop.

Shooting Star
Homework Helper
Oh no, they are not slowing down at all. They're using the fact that the train drags some of the air aound it, and utilising that to cruise at a steady speed, and keeping up with train. A few flaps now and then is permissible.

Oh no! A drafting blue heron.

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Shooting Star
Homework Helper
Oh no! A drafting blue heron.
It's a pleasure discussing with you. You are so perceptive.

Shooting Star
Homework Helper