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Author Topic: Does a moving train stop momentarily when it hits a bee?  (Read 23915 times)

Mark Lawton

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Mark Lawton asked the Naked Scientists:

Hi Chris and colleagues.

I know that if an object moves in one direction and then reverses it has to decelerate, stop and then accelerate the other way. How does this work when the 18.15 express train meets a bee head on?

Does the 300 ton train decelerate, stop and then accelerate again when the bee reverses its direction? How does this work with the many other things the train strikes en route?


What do you think?
« Last Edit: 04/01/2010 10:39:56 by chris »


 

Offline LeeE

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #1 on: 12/01/2009 09:25:47 »
As the bee hits there is a minute amount of elastic deformation in the windscreen and it's this that means you can hear the collision.  If the impact area didn't move at all there wouldn't be any noise from the bee-strike, so the fact that you hear the collision means that the bee has caused movement.  On a macroscopic scale, a very small part of the train is deccelerated, which then bounces back.
 

Offline lightarrow

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #2 on: 12/01/2009 12:14:58 »
Mark Lawton asked the Naked Scientists:

Hi Chris and colleagues.

I know that if an object moves in one direction and then reverses it has to
decelerate, stop and then accelerate the other way. How does this work when the 18.15 express train meets a bee head on? Does the 300 ton train decelerate, stop and then accelerate again when the bee reverses its direction?
The train, if it were moving at constant speed, decelerates during the collision to a little bit lower speed and then goes on at this new speed (probably unmeasurable difference).
 

Offline chris

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #3 on: 24/12/2009 04:01:32 »
If fly and train were perfectly hard, how would that change things?
 

Offline graham.d

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #4 on: 24/12/2009 09:27:37 »
It depends on what you mean by "hard". I guess you mean like perfect billiard balls are hard but that have elastic collisions. In this case, there will be simple conservation of momentum and no energy loss. I'm not going to do the maths here, but suffice it to say that the train will slow down a tiny amount and the bee will reverse its direction and be moving in the same direction as the train at a speed a fraction less that the sum of the train's speed and its original speed.

If you mean "hard" in another sense, I would say that the bee was being foolishly overconfident.

Merry Xmas.
 

Offline lightarrow

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #5 on: 24/12/2009 14:07:52 »
It depends on what you mean by "hard". I guess you mean like perfect billiard balls are hard but that have elastic collisions. In this case, there will be simple conservation of momentum and no energy loss. I'm not going to do the maths here, but suffice it to say that the train will slow down a tiny amount and the bee will reverse its direction and be moving in the same direction as the train at a speed a fraction less that the sum of the train's speed and its original speed.
Exactly. Essentially: the bee's speed after collision is the sum Vb + Vt = bee's speed before collision + train's speed.
Quote
If you mean "hard" in another sense, I would say that the bee was being foolishly overconfident.

Merry Xmas.
;D Very nice!
Merry Xmas to you too and to everyone else.
 

Offline Geezer

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #6 on: 24/12/2009 20:28:58 »
A related question:

Q: What's the last thing on a bugs mind when it hits the windscreen/windshield on your car?
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A: Its arse.
 

Offline lightarrow

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #7 on: 24/12/2009 21:19:55 »
A related question:

Q: What's the last thing on a bugs mind when it hits the windscreen/windshield on your car?
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.
.
.
.
.
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.
.
.
.
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A: Its arse.

;D
 

Offline yor_on

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #8 on: 25/12/2009 06:14:07 »
 "Does a train stop when it hits a bee?"

Only when it feels a a twinge of conscience.

Yep, they're conscious beings, ask Neilep.
 

Offline Bored chemist

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #9 on: 02/01/2010 12:30:19 »
If fly and train were perfectly hard, how would that change things?
It would, but no perfectly hard materials exist so who cares?
 

Offline Geezer

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #10 on: 02/01/2010 19:05:28 »
In a sense, the train actually does stop, or, at least some of it does. A few molecules of the train decelerate, stop then accelerate again to "catch up" with the rest of the train.
 
 

Offline lightarrow

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #11 on: 03/01/2010 20:52:53 »
In a sense, the train actually does stop, or, at least some of it does. A few molecules of the train decelerate, stop then accelerate again to "catch up" with the rest of the train.
 
Can you prove it?
 

Offline Geezer

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #12 on: 03/01/2010 21:08:45 »
In a sense, the train actually does stop, or, at least some of it does. A few molecules of the train decelerate, stop then accelerate again to "catch up" with the rest of the train.
 
Can you prove it?
Probably not :D

I think it's reasonable to assume that there is some molecular interaction between the two bodies.

Would you think the molecules of the train would be unaffected by the collision?
 

Offline LeeE

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #13 on: 03/01/2010 21:29:22 »
Jeez -  what're you folks actually arguing about here?
 

Offline Geezer

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #14 on: 03/01/2010 21:40:07 »
Jeez -  what're you folks actually arguing about here?


Er, well, my, ahem, "theory" suggests that a teensy weensy part of the train actually does stop during the collision. Lightarrow seems to disagree.

We will then move on to debate the number of angels that can fit on the head of a pin.
 

Offline LeeE

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #15 on: 03/01/2010 22:06:09 »
Hmm...  I refer you to the first response in this thread...

... if any part of the bee stops and reverses its direction as a consequence of hitting something moving in the opposite direction, then whatever it has hit, to bring about that reversal of direction, must have stopped too.

The only other alternative is that part of the bee, at least, has reversed direction without stopping, which has some very serious and awkward implications for physics as we know it.

Edited: correct typo 'as' -> 'has'
« Last Edit: 03/01/2010 22:39:31 by LeeE »
 

Offline litespeed

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #16 on: 03/01/2010 22:16:20 »
Such collisions occur at the atomic level where the repulsive forces of equal and negatively charged electrons repulse each other. In this frontal encounter, the train has LOTS of electron back-up to prevent the bee from making a break through. In other words, the train windshield electrons momentarily slow their forward speed only enough for an equal and opposite reaction to transpire.

Its all a matter of massive objects meeting at speed. Bees are much less massive then the ballistic barriers they encounter, and have no additional matter and very little speed behind them to help out.  On the other hand, a 2x4 piece of lumber flung by a tornado might actually exceed the speed of the train and accelerate the windshield in the opposite direction at a negative speed relative to the train until it encounterred enough mass to reverse the process.

A bee traveling near the speed of light would not stop the train either. It would, however, both stop and reverse direction of many small pieces of the train!
 

Offline Geezer

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #17 on: 03/01/2010 22:20:01 »
Hmm...  I refer you to the first response in this thread...

... if any part of the bee stops and reverses its direction as a consequence of hitting something moving in the opposite direction, then whatever it as hit, to bring about that reversal of direction, must have stopped too.

The only other alternative is that part of the bee, at least, has reversed direction without stopping, which has some very serious and awkward implications for physics as we know it.

I see what you mean. My post was highly redundant. Sorry :)

 

Offline Geezer

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #18 on: 03/01/2010 22:36:24 »
Such collisions occur at the atomic level where the repulsive forces of equal and negatively charged electrons repulse each other. In this frontal encounter, the train has LOTS of electron back-up to prevent the bee from making a break through. In other words, the train windshield electrons momentarily slow their forward speed only enough for an equal and opposite reaction to transpire.


That seems to suggest that the molecules that make up the front end of the bee accelerate to the speed of the train in zero time. Is that possible? Even though they don't have a lot of mass, surely those molecules must take time to reverse direction and accelerate to the speed of the train.
« Last Edit: 04/01/2010 05:51:48 by Geezer »
 

Offline LeeE

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #19 on: 03/01/2010 22:38:35 »
Geezer: NP :)
 

Offline Geezer

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #20 on: 04/01/2010 07:26:14 »
In a sense, the train actually does stop, or, at least some of it does. A few molecules of the train decelerate, stop then accelerate again to "catch up" with the rest of the train.
 
Can you prove it?

On second thoughts, perhaps I can prove it.

Let's assume that at least one molecule of the bee and one molecule of the train collide.

Ultimately, we know that the bee molecule must undergo a dramatic change in kinetic energy i.e., a reversal of direction in a very short time.

Now, the molecule of the train with which the molecule of the bee collided experienced a similar dramatic change in its kinetic energy. The atomic forces of the colliding molecules are too great to allow the molecules to coalesce, so, for an instant in time, they were both travelling at the same velocity before they reversed their directions. We know the bee molecule must have stopped and changed direction. Therefore the train molecule also had to stop for a very brief interval. But stop it did, nonetheless.
 

Offline chris

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #21 on: 04/01/2010 07:50:00 »
Ok, that's clear; but now what happens when you extrapolate that to the whole train? On this scale does the train nonetheless stop, albeit for a very short period of time?

Chris
 

Offline SeanB

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #22 on: 04/01/2010 08:25:24 »
Not really, the elastic deformation experienced by the small area of the train that experiences the force of the bee impact, plus the larger plastic deformation that the bee undergoes, ensures the rest of the train does not experience any force at all. If the bee was truly incompressible, along with the train, then you would have a very easy form of nuclear fusion, and a very ablated train after a few minutes.

I know of one train driver that had the experience of hitting a lot of bee equivalents ( around a dozen bins filled with gravel, an attempt to derail the train) and, aside from a loud bang from the impact of hitting them, nothing happened aside from the flat steel bins wrapped around the bumper. The same with the cars and trucks he hit on unguarded level crossings. Of course all train drivers can tell about the very large "bees" that wander onto the tracks at times, and the train has no steering ability or short stopping time, just a loud horn to warn with. Train always wins. Always. Gloves and a big galvanised bin are pretty much standard equipment.
« Last Edit: 04/01/2010 08:29:30 by SeanB »
 

Offline Geezer

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #23 on: 04/01/2010 09:13:21 »
Ok, that's clear; but now what happens when you extrapolate that to the whole train? On this scale does the train nonetheless stop, albeit for a very short period of time?

Chris

An entire train would stop for a possibly unmeasurable amount of time, if it was truly inelastic. But an entire train is actually rather elastic. In fact, there are springs (buffers) between the various coaches/wagons in the train, so no, the whole train will not stop, although the impact of the bee will reduce the train's kinetic energy to a small extent, as Lightarrow pointed out. Therefore we can probably focus on the effect on the first vehicle in the train. However, even that vehicle is not entirely inelastic.

If we assume the bee hits the windscreen of the locomotive at the front of the train, the windscreen of the locomotive will actually deform (slightly) as a result of the impact. I believe some molecules of the windscreen had to stop in the process.

There may be some confusion here regarding so called "inelastic" collisions. Inelastic collisions are an approximation used to determine the actions of rather inelastic materials. However, no materials are completely inelastic. Even a collision between two diamonds will involve a small amount of elasticity of the diamonds.

I suspect a truly "inelastic" collision would require the instantaneous transfer of an infinite amount of energy for a short interval. This would seem to be improbable, if not impossible.

EDIT:

A truly inelastic collision requires at least one of the objects to alter its velocity in zero time. A change of velocity in zero time constitutes infinite acceleration.

F=ma, therefore the force required to produce an infinite acceleration of any mass is also infinite. Sounds slightly suspect to me!

Admittedly, this is a rather old fashioned view of molecular interaction, so I may need to learn a thing or two here  :D
« Last Edit: 04/01/2010 09:32:46 by Geezer »
 

Offline chris

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #24 on: 04/01/2010 10:38:50 »
This is why I raised the theoretical question about the train and bee "beeing" perfectly hard. This would make the collision inelastic and so, with sufficient time resolution, would we not see the train stop for a (very brief) moment in time?
 
 

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Re: Does a moving train stop momentarily when it hits a bee?
« Reply #24 on: 04/01/2010 10:38:50 »

 

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