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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 todecelerate, 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?
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.
A related question:Q: What's the last thing on a bugs mind when it hits the windscreen/windshield on your car?.............A: Its arse.
If fly and train were perfectly hard, how would that change things?
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.
Quote from: Geezer on 02/01/2010 19:05:28In 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?
Jeez - what're you folks actually arguing about here?
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.
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.
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
Quote from: lightarrow on 03/01/2010 20:52:53Quote from: Geezer on 02/01/2010 19:05:28In 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.
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.
After a little time interval, theyr speed will be the same;
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.
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.
I rather reckon this subject has been done to death.
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.
Btw: Are those British or American bees?
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?
Quote from: lightarrow on 05/01/2010 18:28:23You 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.
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.
Quote from: LeeE on 10/01/2010 00:55:27Actually, 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
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