a fly can stop a train?

Freddie2008
Posers and Puzzles 21 Jul '05 22:15
1. 21 Jul '05 22:15
This is a paradox that has been puzzling me for a while now. A fly is flying in the opposite direction to a moving train. The fly hits the train head-on. As the fly strikes the front of the train, it's direction direction of movement changes through 180 degrees., because it hits the windscreen and continues as an amorphous blob of fly-goo on the front of the train.

At the instant it changes direction, the fly must be stationary and because, at that instant it is also stuck on to the front of the train, the train must also be stationary. Thus a fly can stop a train.

The poser is: where is the logical inconsistancy with this. Or does it explain something about British Rail ðŸ˜‰

Fred
2. 21 Jul '05 22:46
Ermmmm. ðŸ˜•
3. ark13
Enola Straight
21 Jul '05 23:35
Originally posted by Freddie2004
This is a paradox that has been puzzling me for a while now. A fly is flying in the opposite direction to a moving train. The fly hits the train head-on. As the fly strikes the front of the train, it's direction direction of movement changes through 180 degrees., because it hits the windscreen and continues as an amorphous blob of fly-goo on the front ...[text shortened]... the logical inconsistancy with this. Or does it explain something about British Rail ðŸ˜‰

Fred
I think the logical inconsistency is in the fact that at the point where the fly is completely stationary, it's only partly on the windshield. Once the fly is completely on the windshield, it is going at the same speed as the train.

Since it's speed before hitting the train is completely reversed, you are right in asserting that it must be stationary at some point. But we also know that once it's on the windshield, it's speed is the same as the train's, which is ever so slightly less than before it hit the fly. So, the fly's speed must change continuously between the time it's flying along slowly, and the time it's on the windshield traveling quickly the opposite way. Common sense tells us that there clearly must be an intermediate stage between flying and on windshield in which the fly slows down, comes to a stop, and speeds up again. Of course, this happens very quickly, but that stage is still there.

We must remember that crashes and kinetic energy transfer don't occur instantaniously. There is almost always some sort of give in one of the two objects that forces a slower transfer of energy. When you kick a soccer ball, it doesn't come off your foot immediately. If it did, then why would you be told to "follow through" after you kick. It's to maximize the time of impact.
4. Bowmann
Non-Subscriber
21 Jul '05 23:59
The direction of the train is unchanged.
5. 22 Jul '05 01:41
All I can think of, is that a slight bend in whatever part of the train that the fly hits, in the opposite direction, accomodates this change of direction. ie the wind shield bends back a tad, only a little for a very small amount of time. This would explain the paradox.
6. ark13
Enola Straight
22 Jul '05 01:48
Originally posted by jimslyp69
All I can think of, is that a slight bend in whatever part of the train that the fly hits, in the opposite direction, accomodates this change of direction. ie the wind shield bends back a tad, only a little for a very small amount of time. This would explain the paradox.
Yes, I agree. I think the fly bends, and the windshield bends, and the whole fabric of space time bends creating a curvature too great to be sustained by the laws of physics, which results in the collapse of the universe as we know it, and as we're being pulled down into nothingness, there's only one thought that goes through each and every human mind. "Why can't we just agree on a pronunciation for the word 'tomato?'"
7. 22 Jul '05 01:56
Originally posted by ark13
Yes, I agree. I think the fly bends, and the windshield bends, and the whole fabric of space time bends creating a curvature too great to be sustained by the laws of physics, which results in the collapse of the universe as we know it, and as we're being pulled down into nothingness, there's only one thought that goes through each and every human mind. "Why can't we just agree on a pronunciation for the word 'tomato?'"
I do concur wholesomely. If we can both say potato, what are are we all fighting about. Nations of the world unite and say it together...................

POTATO!!!

ðŸ˜µ
8. Bowmann
Non-Subscriber
22 Jul '05 03:02
Originally posted by jimslyp69
I do concur wholesomely. If we can both say potato, what are are we all fighting about. Nations of the world unite and say it together...................

POTATO!!!

ðŸ˜µ
I think you'd prefer POTATION!
9. sonhouse
Fast and Curious
22 Jul '05 03:34
Originally posted by Freddie2004
This is a paradox that has been puzzling me for a while now. A fly is flying in the opposite direction to a moving train. The fly hits the train head-on. As the fly strikes the front of the train, it's direction direction of movement changes through 180 degrees., because it hits the windscreen and continues as an amorphous blob of fly-goo on the front ...[text shortened]... the logical inconsistancy with this. Or does it explain something about British Rail ðŸ˜‰

Fred
train yourself and able to leap out of the way at the last minute.
You have a glass of water in your hand and a soda straw,
at the last possible minute you let loose with the water, spaying
a 1/4 inch column of water on the train. What happens then
is the water doesn't stop, it just changes direction, gets acel in
a sideways direction so there is no moment when the water is not
moving. The same thing happens to the fly, the front of the fly
starts acel sideways but the back of the fly for that few milliseconds
did not stop, it is joining its front half microsecond by microsecond
and if you had a million frame per second camera recording it thats
what you would see, the front getting squashed and accelerating
sideways while the back of the fly is meeting the front but it never
actually stops, just spats out sideways, eventually coming to a halt
in a tiny pile of goo someone has to clean off the windshield.
See you think if you count picosecond by picosecond you would
find a place where both are temporarily stopped but not so.
10. 22 Jul '05 09:48
Originally posted by sonhouse
train yourself and able to leap out of the way at the last minute.
You have a glass of water in your hand and a soda straw,
at the last possible minute you let loose with the water, spaying
a 1/4 inch column of water on the train. What happens then
is the water doesn't stop, it just ...[text shortened]... icosecond by picosecond you would
find a place where both are temporarily stopped but not so.
I disagree. If object A is moving in a direction, then Force F, acts on it at 180 degrees to its original direction, the acceleration of the force slows the velocity of the object down until it is still then accelerates it back in the other direction. So at some point the object must be still. Imagine a ball being thrown up in the air. It will slowly be accelerated in the opposite direction until at some point it stops then falls back to Earth.
11. 22 Jul '05 10:26
Using the tracks as a reference frame:

The train deaccelerates an infinitesimal amount due to the equal and opposite force of the fly, but at no point is it stationary. Your error is assuming that at the instant the fly is stationary, it has the same speed as the train - it doesn't. At this point, the fly has finished deaccelerating to zero, and is starting a very rapid accerlation to the speed of the train.

The same happens if you run towards a trailer and jump on it. You experience a large force in the opposite direction in order to come to the trailers speed.
12. 22 Jul '05 15:14
Your issue is assuming that the fly must be stationary. If we forget squishes and such, the fly never has to have a zero velocity. We have that when a projectile is thrown, because it decelerates due to gravity over time. A parabolic graph.

In this situation, we have more of an absolute value graph (only not semetrical). It is not differentiable at the point of contact. The point where the fly INSTANTLY goes from a positive velocity to a negative one. The derivitive at that point would be undefined. The fly is never at a zero velocity.
13. sonhouse
Fast and Curious
22 Jul '05 17:29
Originally posted by jimslyp69
I disagree. If object A is moving in a direction, then Force F, acts on it at 180 degrees to its original direction, the acceleration of the force slows the velocity of the object down until it is still then accelerates it back in the other direction. So at some point the object must be still. Imagine a ball being thrown up in the air. It will slowly be accelerated in the opposite direction until at some point it stops then falls back to Earth.
Thats why I made my example liquid. A liquid will follow the force
exactly and in the case of a narrow column of water hitting head-on
into the windshield of a train, the front molecules don't communicate
with the back molecules in the same way as a solid. A solid
will send its reververations back up its own self and effect the back
end but a liquid will just go on its merry way splashing this way and
that, just following the vectors given it by the colliding surface.
If the surface slants left, then thats where the water ends up going.
It doesn't just stop momentarily and hang a left from a dead stop,
its an ongoing process of vector forces acting on it. The individual
water molecules never stop even for an attosecond as the result
of colliding with a windshield. They get deflected, big differance.
14. ark13
Enola Straight
22 Jul '05 17:53
Originally posted by Coconut
Your issue is assuming that the fly must be stationary. If we forget squishes and such, the fly never has to have a zero velocity. We have that when a projectile is thrown, because it decelerates due to gravity over time. A parabolic graph.

In this situation, we have more of an absolute value graph (only not semetrical). It is not differentiable at the po ...[text shortened]... ative one. The derivitive at that point would be undefined. The fly is never at a zero velocity.
No, that's incorrect. Since the fly's velocity is changing 180 degrees, there's a time in which the fly is completely stationary. It's a very short time, but we know it's there, because of the knowledge that accelerations don't happen instantaniously.
15. 22 Jul '05 18:24
Originally posted by ark13
No, that's incorrect. Since the fly's velocity is changing 180 degrees, there's a time in which the fly is completely stationary. It's a very short time, but we know it's there, because of the knowledge that accelerations don't happen instantaniously.
I don't believe you.

Consider a position graph in the parabolic, and a graph in the absolute value. To get the velocity at any point, we must find the slope of the tangent to that graph at that given point. Parabolic? Easy, the velocity (slope) will slowly become less positive, zero for an instant, then more negative. As if a ball were thrown into the air.

An absolute value type graph is made of two lines that meet at a point. Before this point, the slope is constant (constant velocity) after it, it is a negative constant, in this example at least. Yes there is a clear point where the slope changes, but at that point, the slope/velocity isn't zero, it's undefined. What's that mean? Not completely sure, but I think it means the velocity went from positive to negative instantly.

Think of it in another way. You can zoom in on a parabolic enough to see a flat line at the vertex (zero velocity) You can zoom in on this absolute value graph all you want, and you'll still just see a point. Like this: ^

I will consult other, smarter people on this one as well.