Naked Science Forum
General Science => General Science => Topic started by: Mark Lawton on 12/01/2009 08:55:08
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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?
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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.
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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?
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).
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If fly and train were perfectly hard, how would that change things?
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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.
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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.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.
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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.
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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.
[;D]
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"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.
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If fly and train were perfectly hard, how would that change things?
It would, but no perfectly hard materials exist so who cares?
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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.
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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?
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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?
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Jeez - what're you folks actually arguing about here?
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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.
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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'
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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!
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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 [:)]
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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.
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Geezer: NP [:)]
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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.
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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
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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.
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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]
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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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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 :-)
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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.
Sincerely I can't follow you.
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 [:)].
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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.
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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.
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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.
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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 [:D]
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?
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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... [:)]
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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.
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I rather reckon this subject has been done to death.
That's putting it mildly [:D]
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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.
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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.
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The bee must have a momentum?
Where does that dissipate?
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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?
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Ahhh, physics, what can be better than this :)
Btw: Are those British or American bees?
And, would it help to duck.
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Btw: Are those British or American bees?
Don't know where they came from, but I think they are made of depleted uranium [;D]
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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.
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But if the bee is made of kryptonite, it will have more mass than the train, so the train will bounce off the bee [:D]
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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.
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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"
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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.
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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 [:)].
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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 :)
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It will turn out to be the wrong kind of bee.
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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... [;D]
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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.
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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.
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I think this question of bees have a world wide impact. Especially considering that Stockholm's local train system is bought, if I'm not wrong, just from Italy. Very sleek and futuristic looking they are.
But, then we also have the bees?
Now, if an Italian/British/Swedish train meet an American at a uniform pace with a bee situated inside the tunnel, in a inertial system, at the exact point of penetration would that bee stop it?
I've tried to be as exact as I can here, it might help if you got the temperature and humidity too?
Warm and wet.
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Now, if an Italian/British/Swedish train meet an American at a uniform pace
Sorry Yoron (I hope you don't mind the nickname, but I can never find the underscore key) but I have detected a fatal flaw in your reasoning. The European trains will not meet the American because the European trains drive on the left while American trains drive on the right.
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Sh* another flaw in my TOE.
It hurts.
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Actually Geezer, it is because they drive on the opposite side that they will meet in the middle.
However, because of the much larger loading gauges of US trains, they'd probably get stuck in the tunnel entrance.
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Thanks for pointing that out Lee.
I'd like to see the bee that could stop one of these http://donsdepot.donrossgroup.net/gn5013.jpg
It's even got a special bee deflector mounted on the roof.
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If I recall my high-school physics correctly - in an inelastic collision, the objects stick together like gum balls, and in an elastic collision, the objects bounce off like billiard balls. (Or I maybe confusing elastic and inelastic, because the elastic in my underwear is too tight)
I would guess:
elastic collision - the bee would bounce off the train at high speed in the opposite direction, and the train would slow down a very slight amount back.
inelastic collision - the bee would stick to the train and they would both go slightly slower than the trains original speed.
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Thanks for pointing that out Lee.
I'd like to see the bee that could stop one of these http://donsdepot.donrossgroup.net/gn5013.jpg
It's even got a special bee deflector mounted on the roof.
Are you sure that's a bee deflector? It looks more like a crossbow to me, so perhaps they were planning to shoot the bees down before they hit the train.
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Thanks for pointing that out Lee.
I'd like to see the bee that could stop one of these http://donsdepot.donrossgroup.net/gn5013.jpg
It's even got a special bee deflector mounted on the roof.
Are you sure that's a bee deflector? It looks more like a crossbow to me, so perhaps they were planning to shoot the bees down before they hit the train.
(It's actually contact shoe for passing current between locomotives.)
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Hah, thank you LeeE. my Toe feels fine again.
Back on tracks so to cho choo..
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I see that this thread sort of stopped in its tracks?
It surprises me as it seems to take up one of the most intriguing and sensible question I've seen here?
And we haven't even touched the bee, train(s), and autumn leaves on the tracks either?
I can see all kind of variables here.
Consider the possibility of a bee all wrapped up in leaves doing a kamikaze toward an oncoming Italian train, slippery when wet so to speak.
What will the outcome bee here?
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Well, as I understand QM (which means I don't [:D]) from time to time, a bee might pass right through the windscreen of a train. In that event, if it was a really fast train, it might hit the train driver and cause him to lift his hand off the "dead man's handle" which would result in an emergency application of the train's brakes and bring it to a halt.
Therefore I posit that a bee can stop a train.