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### Author Topic: Does a moving train stop momentarily when it hits a bee?  (Read 23909 times)

#### graham.d

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #25 on: 04/01/2010 12:56:16 »
Speculation on a collision between two inelastic perfectly hard rigid objects is like trying to work out the sound of one hand clapping. There is no real physics that can solve it.

By definition an inelastic collision does not conserve kinetic energy and will dissipate the energy as heat; this means moving molecules about faster. This is not consistant with perfect rigidity as the front molecules have to move, initially, before the back ones. The best would be to propagate a movement backwards at lightspeed. In this case you could (maybe) assume the train would be stopping for a time associated with the time for the shockwave to reach the back of the train, but what happens then I am unsure. The wave may come back given that no energy can be dissipated and continue back and forth in the train forever.

If you get rid of the real physics and do imagine a rigid rod (train) impacting a smaller rigid object (bee), and also have the energy just disappearing (inelastic collision) plus no lightspeed limit to propagation (perfectly rigid) then I think both bee and train have infinite decelerations to result in the resulting net velocity as given by conservation of momentum. The train does not stop but just slows a little. The bee will reverse direction and both will be stuck at the new forward train-speed.

When you consider how many bees must hit trains the importance of this research can not be underestimated :-)

#### lightarrow

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #26 on: 04/01/2010 13:28:19 »
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 traveling 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.
Put a frame of reference on the train's centre of mass.
When the train's atom collides with the bee's atom, that is, when they begin to interact, the repulsive forces slow down the bee's atom and at the same time accelerate the train's atom. After a little time interval, theyr speed will be the same; in the hypotesys that the intermolecular forces inside the bee are the same as those inside the train's windscreen, the train's atom will not accelerate further, if it has an equal or greater mass ; but since the bee's atom has slowed down a little, the train's atom will go to this speed, that is less than the bee's initial speed. If, as it should be the case, the intermolecular forces inside the windscreen are greater than those inside the bee, the previous reasoning is even truer.
About atom's masses: if a windscreen is made of SiO2, the atom is Si or O, and they both are heavier or at least have the same mass of C, H, N, O atoms of which the bee is made  .
« Last Edit: 04/01/2010 13:40:15 by lightarrow »

#### yor_on

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##### Re: Does a moving train stop momentarily when it hits a bee?
« Reply #27 on: 04/01/2010 16:53:05 »
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.

Sean, he sounds real bloodthirsty that guy, starting with bins filled with gravel, then upgrading to cars and trucks you say?

Don't let him near me.

#### Geezer

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #28 on: 04/01/2010 18:09:58 »
After a little time interval, theyr speed will be the same;

Lightarrow, here's how I see it. As you say, they will both achieve the same velocity. But we know that the bee molecule actually changed direction. It cannot instantaneously change direction, so, for a brief, but finite interval, its velocity was zero i.e., it was "stopped". Now, if both molecules are in contact and they have the same velocity, the train molecule must also have zero velocity at that time, so it too "stopped".

The question is whether or not the molecules really did achieve the same velocity. The train molecule must have undergone some deformation, but if it happened to have a much greater mass than the bee molecule, as you suggest, the deformation might only result in some deceleration of its mass. On the other hand, if the molecules have similar masses, I think they would both stop very briefly.

Of course, the whole discussion is slightly ridiculous because we are paying no attention to the effects of the air that is being compressed by the front of the train. If the train is a highly streamlined TGV going at full speed, I would not be surprised to learn that the bee cannot actually make contact with the train's windshield.

« Last Edit: 05/01/2010 05:25:19 by Geezer »

#### Mike G

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #29 on: 05/01/2010 00:22:52 »
Geezer is considering the real world :D  The incompressible train does not stop, none of it.  I agree with graham.d  This is not a real physics experiment because no ideal hard object exists, it is a mind experiment.  So no need to use limitations like molecules or the speed of light.  There is no propagation of waves down a perfectly hard train. However I can't see why kinetic energy is not conserved, I see no problem with that.

I think the logic to a fly instantaneously stopping a train would go as follows:
Either 1) Assuming a perfect non-compressible fly, the change in the velocity/direction is instantaneous.  Therefore the acceleration is infinite.  The fly has a finite mass.  By F=ma, this means the force is infinite.  Since every force has an equal and opposite reaction, the train receives an infinite force.  Therefore it stops for an instant (or even bounces backwards away from the fly ;^)
Or 2) The fly changes direction so at some point it is stationary.  The train is in contact.  Therefore the train is stationary.

This can be seen to be wrong because the momentum argument has the train continuing in the forward direction.  If the train was ever stationary, there is no force to continue its forward operation.  Momentum will not 'pick up again' in an incompressible train, it takes compressibility for that (the back half of the car continues after the front hits the wall).

So I think the answer to this thought experiment is that the incompressible train hits the incompressible fly for an infinitely small quanta of time, essentially zero.  So the train appears stationary because in that freeze frame of time, it does not move.  The momentum makes a step change but the train was never actually stationary.

#### Geezer

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #30 on: 05/01/2010 00:47:58 »
There is no propagation of waves down a perfectly hard train. However I can't see why kinetic energy is not conserved, I see no problem with that.

Blimey! Here we go again

OK, so please explain how the kinetic energy is conserved during an inelastic collision. Was there an instantaneous energy transfer between two bodies by some means that we cannot describe?

#### lightarrow

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #31 on: 05/01/2010 11:52:10 »

So I think the answer to this thought experiment is that the incompressible train hits the incompressible fly for an infinitely small quanta of time, essentially zero.  So the train appears stationary because in that freeze frame of time, it does not move.
It's better if you re-study physics at school...

#### graham.d

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #32 on: 05/01/2010 11:59:34 »
I think that by definition, an inelastic collision does not conserve kinetic energy. It is normally asumed to be dissipated as heat but with perfectly rigid objects this is also not possible.

I rather reckon this subject has been done to death.

#### Geezer

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #33 on: 05/01/2010 16:42:12 »
I rather reckon this subject has been done to death.

That's putting it mildly

#### Mike G

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #34 on: 05/01/2010 16:56:24 »
Well the bee's certainly a gonner, and I'm repeating myself so I will too.

On the energy front, doh!  Slip of the keyboard, I read KE and thought mv.  Thanks for pointing out that error.

Lightarrow, if you have an objection, you are not explaining it very well.  Which bit of distance = speed x time should I study?  In the inelastic collision momentum is transferred instantaneously.  So a train which has a velocity hits a fly for zero time, during which it travels no distance.  Fly changes direction.  No need for a stationary train.  No puzzle.  Or approach it from the elastic direction: part of the train bends.  Then as you make the train stiffer, less and less of it bends, until none of it does.  Either way, no stationary train required.

#### lightarrow

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #35 on: 05/01/2010 18:28:23 »
Well the bee's certainly a gonner, and I'm repeating myself so I will too.

On the energy front, doh!  Slip of the keyboard, I read KE and thought mv.  Thanks for pointing out that error.

Lightarrow, if you have an objection, you are not explaining it very well.  Which bit of distance = speed x time should I study?  In the inelastic collision momentum is transferred instantaneously.  So a train which has a velocity hits a fly for zero time, during which it travels no distance.  Fly changes direction.  No need for a stationary train.  No puzzle.  Or approach it from the elastic direction: part of the train bends.  Then as you make the train stiffer, less and less of it bends, until none of it does.  Either way, no stationary train required.
You wrote:
<<...So the train appears stationary because in that freeze frame of time, it does not move. >>

But then the same could be applied to every moving object in the universe. So, should every moving body in the universe appear as stationary?
I don't understand the meaning of that phrase.

#### yor_on

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #36 on: 07/01/2010 09:59:36 »
The bee must have a momentum?
Where does that dissipate?

#### yor_on

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #37 on: 08/01/2010 12:05:07 »
Ahhh, okay?

The train is shaking itself to smithereens at the same time as our indestructible bee bores its way through the same train at a ever increasing pace of revenge?

Would that cover it?

--------

Ahhh, physics, what can be better than this :)
Btw: Are those British or American bees?
And, would it help to duck.
« Last Edit: 08/01/2010 12:10:49 by yor_on »

#### Geezer

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #38 on: 08/01/2010 19:40:26 »
Btw: Are those British or American bees?

Don't know where they came from, but I think they are made of depleted uranium

#### Mike G

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #39 on: 08/01/2010 20:01:41 »
You wrote:
<<...So the train appears stationary because in that freeze frame of time, it does not move. >>
But then the same could be applied to every moving object in the universe. So, should every moving body in the universe appear as stationary?
Exactly so.  distance = speed x time.  If time=0, distance=0 regardless of speed.  Even photons.  This is a philosophy problem, not a physics problem. The bee is made of kryptonite, which is incompressible as we all know.  Forces are infinite, acceleration is infinite, momentum change is instanstaneous.  We just need a cogent story ie: the train bounces the bee and not the other way round.
I don't know the make up of the duck though.

#### Geezer

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #40 on: 08/01/2010 20:16:15 »
But if the bee is made of kryptonite, it will have more mass than the train, so the train will bounce off the bee

#### lightarrow

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #41 on: 08/01/2010 20:21:20 »
You wrote:
<<...So the train appears stationary because in that freeze frame of time, it does not move. >>
But then the same could be applied to every moving object in the universe. So, should every moving body in the universe appear as stationary?
Exactly so.  distance = speed x time.  If time=0, distance=0 regardless of speed.  Even photons.  This is a philosophy problem, not a physics problem. The bee is made of kryptonite, which is incompressible as we all know.  Forces are infinite, acceleration is infinite, momentum change is instanstaneous.  We just need a cogent story ie: the train bounces the bee and not the other way round.
I don't know the make up of the duck though.
Someone in this thread considered this case of an infinitely rigid bee and train, but this is not what the OP asked, he asked about a bee and train collision. The answer is that the train doesn't stop, neither ideally nor in reality.

#### yor_on

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #42 on: 10/01/2010 00:45:03 »
So? Just how big does this bee need to be to make itself noticeable?

"Friend of the truth, the sole truth and nothing but the truth"

#### LeeE

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #43 on: 10/01/2010 00:55:27 »
Actually, if it was a British train then it would have probably already stopped because of leaves on the track, or the wrong type of snow, or whatever, and would have therefore been stationary when the bee flew into it.

#### lightarrow

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #44 on: 10/01/2010 02:30:41 »
Actually, if it was a British train then it would have probably already stopped because of leaves on the track, or the wrong type of snow, or whatever, and would have therefore been stationary when the bee flew into it.
You say so because you don't know italian trains  .

#### yor_on

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #45 on: 10/01/2010 17:26:56 »
Actually, if it was a British train then it would have probably already stopped because of leaves on the track, or the wrong type of snow, or whatever, and would have therefore been stationary when the bee flew into it.

I see LeeE, you are introducing a whole new level of difficulty here, right?

You don't have to explain it to me (I'm so infernally clever) but for those of you missing the Q. I will write it out.
As a service to mankind.

Q. If we have a stationary British, or as Lightarrow points out, even better an Italian train being hit by a bee.

Will it recoil?

The train I mean, not the bee :)

#### Bored chemist

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #46 on: 10/01/2010 18:47:28 »
It will turn out to be the wrong kind of bee.

#### lightarrow

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #47 on: 10/01/2010 18:47:40 »
Actually, if it was a British train then it would have probably already stopped because of leaves on the track, or the wrong type of snow, or whatever, and would have therefore been stationary when the bee flew into it.

I see LeeE, you are introducing a whole new level of difficulty here, right?

You don't have to explain it to me (I'm so infernally clever) but for those of you missing the Q. I will write it out.
As a service to mankind.

Q. If we have a stationary British, or as Lightarrow points out, even better an Italian train being hit by a bee.

Will it recoil?

The train I mean, not the bee :)
For the second kind of train (italian) it' probable that the train get damaged from the air displacement of the flying bee...

#### Geezer

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #48 on: 10/01/2010 22:45:16 »
But there is no difference between a British train and an Italian train http://en.wikipedia.org/wiki/Pendolino

So, any difference would have to be a function of the nationality of the bee. I think a Swiss bee would be best, because they are neutral.

#### LeeE

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##### Does a moving train stop momentarily when it hits a bee?
« Reply #49 on: 11/01/2010 00:18:00 »
Q. If we have a stationary British, or as Lightarrow points out, even better an Italian train being hit by a bee.

Will it recoil?

The train I mean, not the bee :)

If the train is scared of bees, or mistakes it for a wasp or a hornet, then yes, it will probably recoil.

#### The Naked Scientists Forum

##### Does a moving train stop momentarily when it hits a bee?
« Reply #49 on: 11/01/2010 00:18:00 »