Thread: A bird flies into a train

Hi all.

The following problem was posted by someone on another forum:

I have been having an ongoing disagreement with a friend about the outcome of a hypothetical situation involving a train and a bird. I'm hoping someone from this forum will be able to help me understand the physical laws that support my argument OR shoot me down in flames and tell me where I’ve gone wrong.

Let's imagine that there is a train travelling North at a constant speed of 50mph. Outside, there is a bird flying parallel with the train that is also moving North at a constant speed of 50mph. (with me so far?). The bird then edges closer to the train and while still facing North the bird enters the train via a window.

I propose that once inside the train, assuming the bird continues to flap at its constant rate; it will fly towards the front of the train. Someone inside the train will observe the bird moving forward through the train at 50mph.

My friend proposes that the bird will stay at the same point in the train that it entered. I.e. if it entered at the back of coach E, even though it’s still flapping like mad, it will remain at the back of coach E and will appear stationary to an observer within the train.

What worries me is how blindingly obvious it seems to me that I’m right. This feeling often coincides with me being wrong.

Who is right, the poster or their friend? I myself agree with the poster's friend that the bird will be stationary relative to the train passengers – or at any rate the bird won't be moving forwards or backwards relative to the train, but may be moving sideways relative to the train since it has to move sideways to approach the train. The poster nevertheless disagrees, claiming that it is counterintuitive to see a bird flapping its wings like mad and yet going nowhere.

So, am I (and the poster's friend) wrong? And if so, where did we go wrong?

Do you have to adjust your stride when you stpe of a travellator? If you contemplate the answer to that, as an analogy, i think you will see the correct answer.

When the bird enters the train, it will come in out of a 50 mph head wind (relative to the train's frame of reference). If it continues to flap its wings at the same rate, it will pick up speed and run into the walls of the car. If it is a flatbed car, with the wind blowing past, the bird will have to keep flapping just to keep up, or light and hold on to something that will keep him from being blown off the back.

Originally Posted by Sylvia104

Hi all.

The following problem was posted by someone on another forum:

I have been having an ongoing disagreement with a friend about the outcome of a hypothetical situation involving a train and a bird. I'm hoping someone from this forum will be able to help me understand the physical laws that support my argument OR shoot me down in flames and tell me where I’ve gone wrong.

Let's imagine that there is a train travelling North at a constant speed of 50mph. Outside, there is a bird flying parallel with the train that is also moving North at a constant speed of 50mph. (with me so far?). The bird then edges closer to the train and while still facing North the bird enters the train via a window.

I propose that once inside the train, assuming the bird continues to flap at its constant rate; it will fly towards the front of the train. Someone inside the train will observe the bird moving forward through the train at 50mph.

My friend proposes that the bird will stay at the same point in the train that it entered. I.e. if it entered at the back of coach E, even though it’s still flapping like mad, it will remain at the back of coach E and will appear stationary to an observer within the train.

What worries me is how blindingly obvious it seems to me that I’m right. This feeling often coincides with me being wrong.

Who is right, the poster or their friend? I myself agree with the poster's friend that the bird will be stationary relative to the train passengers – or at any rate the bird won't be moving forwards or backwards relative to the train, but may be moving sideways relative to the train since it has to move sideways to approach the train. The poster nevertheless disagrees, claiming that it is counterintuitive to see a bird flapping its wings like mad and yet going nowhere.

So, am I (and the poster's friend) wrong? And if so, where did we go wrong?

The bird's speed will be relative to the air he is flying through. If we assume that the air inside the train is moving with the train then the bird inside the train will fly at 50 mph relative to the train.

So if we assume sharp line between the inside air and outside air at the window, the bird will go from moving at 50 mph with respect to the air to 0 mph. Since it is his forward motion through the air that generates lift, he will start to fall until his wing flapping starts to push him forward fast enough to generate lift. So you are both right is a sense. He will start moving at the same speed as the train, but his wing flapping then accelerate him up to 50 mph with respect to the train.

In reality, there is not going to be such a sharp cut off between the inside and outside air of the train. The open window will produce some wind inside the train and turbulence outside of the window. The bird might not even maintain any stable flight and would likely just pass through the window out of control and smash into something.

Originally Posted by Janus

The bird's speed will be relative to the air he is flying through. If we assume that the air inside the train is moving with the train then the bird inside the train will fly at 50 mph relative to the train.

So if we assume sharp line between the inside air and outside air at the window, the bird will go from moving at 50 mph with respect to the air to 0 mph. Since it is his forward motion through the air that generates lift, he will start to fall until his wing flapping starts to push him forward fast enough to generate lift. So you are both right is a sense. He will start moving at the same speed as the train, but his wing flapping then accelerate him up to 50 mph with respect to the train.

In reality, there is not going to be such a sharp cut off between the inside and outside air of the train. The open window will produce some wind inside the train and turbulence outside of the window. The bird might not even maintain any stable flight and would likely just pass through the window out of control and smash into something.

That's a good point. I didn't think of that. I was considering the problem purely from kinematic considerations of relative velocity, ignoring complications due to air resistance – so instead of considering the bird's velocity relative to air, I was considering its velocity relative to the ground.

What if we ignore air resistance altogether? In fact, what if instead of a bird we consider a spaceship, and instead of a train we consider, say, a space train, and both are moving through atmosphereless outer space? Then, I'm sure, the passengers on the space train will see the spaceship burning fuel like mad but apparently going nowhere.

Originally Posted by Sylvia104

Originally Posted by Janus

The bird's speed will be relative to the air he is flying through. If we assume that the air inside the train is moving with the train then the bird inside the train will fly at 50 mph relative to the train.

So if we assume sharp line between the inside air and outside air at the window, the bird will go from moving at 50 mph with respect to the air to 0 mph. Since it is his forward motion through the air that generates lift, he will start to fall until his wing flapping starts to push him forward fast enough to generate lift. So you are both right is a sense. He will start moving at the same speed as the train, but his wing flapping then accelerate him up to 50 mph with respect to the train.

In reality, there is not going to be such a sharp cut off between the inside and outside air of the train. The open window will produce some wind inside the train and turbulence outside of the window. The bird might not even maintain any stable flight and would likely just pass through the window out of control and smash into something.

That's a good point. I didn't think of that. I was considering the problem purely from kinematic considerations of relative velocity, ignoring complications due to air resistance – so instead of considering the bird's velocity relative to air, I was considering its velocity relative to the ground.

What if we ignore air resistance altogether? In fact, what if instead of a bird we consider a spaceship, and instead of a train we consider, say, a space train, and both are moving through atmosphereless outer space? Then, I'm sure, the passengers on the space train will see the spaceship burning fuel like mad but apparently going nowhere.

If the spaceship is burning fuel, then it is not traveling at a constant speed but accelerating. In this case, in order for the space train to maintain the same speed as the space ship, it will also have to be accelerating. Thus the passengers in the train will feel a "force" pushing them towards the rear of the train. Then, from their point of view, the spaceship is indeed standing in place while firing its engines, but the firing of the engines would needed be to hold the spaceship in place against the "force" pushing everything to the rear of the train. (if the spaceship were to stop its engines, it would "fall" towards the rear of the train.