A bird flies into a train

Sylvia104 · 2011-10-12 01:40:06

This is a problem proposed by someone from 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 Ive 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 its 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 Im right. This feeling often coincides with me being wrong.

So who is right, the poster or their friend? I made some replies on that forum but it seems they were not being taken kindly to. I'd like to link you to the original discussion in the other forum but it seems I can't post links on this forum.

bobbym · 2011-10-12 01:48:53

Hi Sylvia104;

I'd like to link you to the original discussion in the other forum but it seems I can't post links on this forum.

Why not? Can I help? What forum?

This feeling often coincides with me being wrong

I hate partially agreeing with your friend, my feeling is that you are not wrong very much.
I think your friend is right if the bird suddenly appeared in the train. But how can that occur? To enter through a window the bird must abandon its parallel trajectory. It must accelerate to enter through the window.

Sylvia104 · 2011-10-12 02:47:46

When I try to post a link, I get this message:

Sorry. In an effort to stop automated spam only established members can post links. Please describe where instead.

www.qi.com/talk/viewtopic.php?t=22094&start=0

BTW, neither the poster nor the poster's friend is my friend. They're just members of that forum (of which I'm also a member). My conclusion is that the bird is stationary with respect to the train passengers, but the poster (Melonhead) doesn't agree, claiming that it's counterintuitive to see a bird flapping its wings like mad and still going nowhere.

bobbym · 2011-10-12 02:56:51

Before you get to 10 posts you will be unable. It should be possible now.

I do not suppose you ever jumped a train to hitch a ride. You run alongside the boxcar until you match its speed. Then to duck in you must accelerate. Your motion will now have a vertical and horizontal component. By addition of vectors your velocity has increased. When you leap into the boxcar you will roll forward.

I would think the only way the bird would be stationary is if it could magically appear in the train. In the real world that is not possible.

Sylvia104 · 2011-10-12 03:00:07

Yes, but when you leap into the boxcar, you land on the train. The bird doesn't land on the train at all.

bobbym · 2011-10-12 03:04:00

But even before you land, your motion will have a small additional velocity over the trains. Otherwise you would not have entered the train. Relative to the train once in the car you are travelling forward. That is why you roll forward.

I have placed the link you want in post #3.

By the way, I wished you have told me his name is Melonhead.

Sylvia104 · 2011-10-12 03:41:50

bobbym wrote:

But even before you land, your motion will have a small additional velocity over the trains. Otherwise you would not have entered the train. Relative to the train once in the car you are travelling forward. That is why you roll forward.

What if you don't land? What if you can fly like the bird and you enter the train without making contact with any part of it?

bobbym wrote:

I have placed the link you want in post #3.

Thanks!

bobbym wrote:

By the way, I wished you have told me his name is Melonhead.

Me too.

bobbym · 2011-10-12 03:46:20

Same thing. Essentially your explanation over there is correct. But I think you are forgetting that the angle the bird enters the trains window will have a forward component relative to the train. Remember, the bird must accelerate to catch up to the window. So its velocity will be higher than the trains. Landing does not matter. Also their explanation about air is false. The exact same thing would occur in a perfect vacuum. Except of course the bird could not fly.

Did you know that at one time physicists did some calculations proving that trains could not exceed 25 mph?

Sylvia104 · 2011-10-12 04:02:11

I still don't understand why the bird has to accelerate. Can't it just fly in such a way that its forward speed is the same as the train's so it can just enter the train by slipping in sideways? Its overall velocity will of course be greater than 50mph, but the forward component of its velocity is 50mph. The train passengers will see it move sideways, but it will be stationary to them in the forwardsbackwards direction.

bobbym · 2011-10-12 04:06:30

Hi;

The bird must accelerate because when no longer travelling parallel to the train it must travel a greater distance. To stay even with the window will require more speed. The fact that there is a forward component means we add it to the trains. Say that the bird had to achieve 56 mph to cover the extra distance. When it enters the train the observers there will see moving forward at 56 - 50 = 6 mph.

The train passengers will see it move sideways

I think sideways and slightly forward.

bobbym · 2011-10-12 04:46:32

Hi Sylvia;

Please check the drawing out. If the bird at E flies in on the line shown won't it hit the wall of the train at F. Just as if the train was motionless and the bird came straight in on that line. Because since there forward components are equal they are relatively motionless. Doesn't F appear to the observers at A and D as forward motion?

Bob · 2011-10-12 06:22:47

hi Silvia and bobbym

I think there is an air factor.

Forget the bird for the moment and think about the air.

If the train window is shut, and you are in the train, all the air travels along at the same speed as the train.

Now open the window. Hear that rushing noise? That's the air outside. From your inside_the_train point of view it's rushing backwards at the speed of the train. You could stick your head out to test this BUT IT'S NOT RECOMMENDED AS YOU MIGHT GET IT KNOCKED OFF BY A PASSING TRAIN / TREE BRANCH ETC.

So we'll imagine sticking our head out. The air is pushing hard against your imaginary head at whatever speed the train is going. Notice what happens to hairstyles if you go off in an open top car?

So let's bring the bird back in. Wait! Too fast! I didn't mean inside the train. Just into the analysis.

If this magnificent bird is keeping pace with the train it must be battling against all that air. It would be like a wind tunnel for the little creature. Then, and never mind whether it goes at right angles or on a slant, it gets into still air but still, according to the story, battling like mad against the now non-existent wind. So sure it rockets forward at a great rate of knots.

Many years ago I was travelling with someone (who will remain anonymous to protect them from embarrassment). They had a small child with them who was being 'potty trained'. Hopefully that phrase translates across the pond. In case not I'll be more graphic. It'll help build up the picture. As a child changes from wearing nappies (diapers) to going to the toilet like adults, there's a time when you have to lug around a pot so they can go in that. So my friend now had a pot full of wee. What to do with it? He had heard that in the old days train toilets used to empty under the train onto the track. OK then. Wind down the window. Here comes some more for the track. Being a mathematician with some knowledge of relative velocities I yelled a warning! But too late! He had done it.

Now if the bird solution is that the bird hovers in the train not going anywhere, the same is going to be true in reverse. The wee can be dumped straight out the window and it neatly lands on the track below.

I have to report this did not happen.

The pot contents splatted along all the windows of the train, aft of our position. All the passengers downwind wondered why it had suddenly started to rain ... strange yellow rain as well. Sadly one couple had seen the cause of this. Which still he might have got away with but for the fact that their window was open. ....... ........

I'm sure you appreciate that, in the interests of driving forward the frontiers of human knowledge, my friend has done the experiment for getting to the bottom (no pun intended) of this problem.

LATER CORRECTION: Sorry, Sylvia104, I think you are incorrect!

Bob

Last edited by Bob (2011-10-12 19:33:02)

bobbym · 2011-10-12 07:24:06

Hi bob;

I think you have verified the wrong people.

Sylvia wrote:

My conclusion is that the bird is stationary with respect to the train passengers,

bob bundy wrote:

Now if the bird solution is that the bird hovers in the train not going anywhere, the same is going to be true in reverse. The wee can be dumped straight out the window and it neatly lands on the track below.
I have to report this did not happen.

I am contending along with MelonHead whose very name inspires confidence that the bird flies towards the front of the train, although not at 100 mph. The wee wee story just adds more creedence to Me and Melonhead. You see I was on that train when that urine tosser was.

Incidentally though all the people here at home are agreeing with Silvia. She may very well be right. Still my confidence in Melonheads analysis remains.

Bob · 2011-10-12 08:03:14

hi bobbym,

Arrhh, I see where I went wrong. I thought in post #1 that it was Sylvia who was 'speaking'. I see now it wasn't. Whoops!

The rest I still hold to. Sorry you were on that train. But what were you doing in Kent in 1967 ?

Meanwhile I have another bird problem.

A truck driver has a closed truck with lots of wooden perches. He comes to a bridge that says "Warning. Maximum load on this bridge: 5 tons." His truck including the perches and his own weight is just 5 tons. So all would be well if he had no other load.

But he is transporting a large flock of canaries which are quietly roosting on the perches. They are conserving their strength because their destination is the train testing track where they will be taking part in experiments later. Although canaries are light-weight (maybe feather-weight even?) there's a lot of them. It looks like the extra load will break the bridge.

So he bangs hard on the metal wall between the cab and the canary-compartment. The birds take fright and fly up into the air. He reasons thus: "Now their weight is no longer on the perches but 'in the air' the total weight is back down to 5 tons. If I hurry across before they start to settle, I can cross without breaking the bridge."

So, does he get away with it?

Bob

bobbym · 2011-10-12 08:17:58

Hi bob bundy;

Kent? That happened in Brooklyn in 49.

I think that guys analysis is for the birds. Of course the closed system weighs exactly the same.

Bob · 2011-10-12 19:42:00

OMG! So it's happened twice! I know my friend has never been to Brooklyn.

Maybe the rail companies should be having a major poster campaign, warning people of the dangers of throwing stuff out the window. The picture on the poster would be interesting.

So you're saying that an open truck would be different? Obviously the canaries might fly away, but let's say they are well trained and stay just above the truck.

Bob

bobbym · 2011-10-12 23:10:54

Hi;

In the closed truck the weight would stay the same. A famous chemistry experiment seems to validate this.

If I have an an open shoe box with a 5 kg weight in it and place the whole thing on a scale it will weigh 5 kg ( neglecting the small weight if the box ). If I then reach in and lift the weight and hold it 2 cm above the bottom the scale will no longer register the weight.

But this is not the same as the birds hovering within the confines of the truck that just has no sides or no top. To hover they are using air to exert a downward force on the bed of the truck. This should be equal to their weight. Looks like the trucks weight would stay about the same.

Bob · 2011-10-12 23:46:46

hi

To hover they are using air to exert a downward force on the bed of the truck.

Ah ha! You have fallen into my trap! he he he!

So when I used to live near Heathrow airport, where you could practically see the colour of the pilot's socks they were so close, how come I'm not crushed by the weight of the jet as it passes by overhead?

Bob

bobbym · 2011-10-12 23:55:16

Hi Bob;

That is not a trap at all. The planes downward force is distributed over a very large area. That is the reason it is in flight. Standing under landing planes as they get low to the ground you would not be crushed but you would buffeted around tremendously.

Bob · 2011-10-13 01:23:14

Oh darn it. Foiled again.

Back to the drawing board.

Bob

Math Is Fun Forum

#1 2011-10-12 01:40:06

A bird flies into a train

#2 2011-10-12 01:48:53

Re: A bird flies into a train

#3 2011-10-12 02:47:46

Re: A bird flies into a train

#4 2011-10-12 02:56:51

Re: A bird flies into a train

#5 2011-10-12 03:00:07

Re: A bird flies into a train

#6 2011-10-12 03:04:00

Re: A bird flies into a train

#7 2011-10-12 03:41:50

Re: A bird flies into a train

#8 2011-10-12 03:46:20

Re: A bird flies into a train

#9 2011-10-12 04:02:11

Re: A bird flies into a train

#10 2011-10-12 04:06:30

Re: A bird flies into a train

#11 2011-10-12 04:46:32

Re: A bird flies into a train

#12 2011-10-12 06:22:47

Re: A bird flies into a train

#13 2011-10-12 07:24:06

Re: A bird flies into a train

#14 2011-10-12 08:03:14

Re: A bird flies into a train

#15 2011-10-12 08:17:58

Re: A bird flies into a train

#16 2011-10-12 19:42:00

Re: A bird flies into a train

#17 2011-10-12 23:10:54

Re: A bird flies into a train

#18 2011-10-12 23:46:46

Re: A bird flies into a train

#19 2011-10-12 23:55:16

Re: A bird flies into a train

#20 2011-10-13 01:23:14

Re: A bird flies into a train

Board footer