Can a fly stop a train?

Discussion in 'Physics & Math' started by Atom, Aug 22, 2007.

  1. draqon Banned Banned

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    well...we can have this fly launched as a projectile and make sure the fly is within a plasma cloud which lowers the air resistance...and cancels it out. plasma stealth technology on flies.

    But if you want not laalaa-woowoo land ideas

    than here it is

    the train has a photo receptor which causes the train to stop when it gets an image of the fly. The image of the fly...the thermal imagery of the fly causes the photonic receptors to recognize this image and send a signal to microprocessor which sends a signal to stop the train to the main system of the train control.
     
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  3. James R Just this guy, you know? Staff Member

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    przyk is the only person who has got the explanation completely correct, I think.

    There is nothing wrong with this except for the last sentence. It is not true that because the fly stops, the entire train must stop. In fact, no part of the train, even down to the molecular level, has to stop.

    Every particle making up the fly must stop at some time in order to change direction. If it goes from having a positive velocity to a negative velocity (for example), then at some time it must have zero velocity.

    In fact, not all parts of the fly stop at the same time, because the fly is not a totally rigid object. But all parts stop.

    That is correct.

    It is not necessary for the surface of the pillow to stop. All it needs to do is to slow slightly (relative to the rest of the train), to change the momentum of the fly by the required amount to reverse the fly's motion.

    I'm not sure I like this argument, but it's not really important.

    I disagree that part of the train stops. All the front of the train needs to do is to exert enough force on the fly/ball bearing. That doesn't require it stopping.

    This is correct. In effect, the electrons in the molecules at the front of the train repel the molecules in the fly as it approaches the front of the train. That repulsion becomes stronger and stronger as the fly gets closer and closer to the train molecules. Thus, there is a constant repulsive force on the fly's molecules which act to slow them down to rest (for a moment) then accelerate them back in the direction of motion of the train.

    By Newton's third law, there is an equal and opposite force on the molecules in the front of the train, which causes them to decelerate and slow slightly with respect to the rest of the train (a slowing that is quickly countered by the molecules behind them pushing them forward again). They DO NOT, however, slow to rest.

    Although the fly exerts a force on the train equal to the force exerted by the train on the fly, the acceleration of the fly is much much larger than the deceleration of the train, due to the immense differences in their masses. The train slows hardly at all, while the fly slows to rest, changes direction and almost immediately is propelled to the speed of the train.
     
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  5. Pete It's not rocket surgery Moderator

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    Hi James,
    Do you remember when we had this argument over at SSSF? I think Chris was involved, too.
     
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  7. Pete It's not rocket surgery Moderator

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    Here's a post I made in another forum four years ago this week (not the same discussion as the one just mentioned). It takes a slightly different approach in that it doesn't worry about interatomic forces. It assumes perfectly amorphous, infinitely divisible bodies.

    Consider a small, perfectly elastic amorphous object (the fly) colliding with a large, perfectly elastic amorphous object (the train). The train has a much larger modulus of elasticity than the fly.
    In my chosen reference frame, the fly is moving slowly to the right, the train is moving quickly to the left.

    Consider the fly's rear in relation to the fly's front. An instant after the collision, the fly's rear will still be travelling forward, while the fly's front is travelling backward. The fly is being compressed. The compression wave progresses through the fly faster than the relative speed of the train, accelerating each component of the fly in turn until the whole fly is travelling backward very quickly.

    A similar thing happens to the train, expect that it compresses much less, and it's velocity does not reverse but only reduces slightly.

    It is clear, therefore, that neither the bulk of the train nor the bulk of the fly undergoes a discontinuous velocity change, and that the bulk of the train does not stop.

    Now consider the small chunk of fly and the small chunk of train that actually collide. It seems that at least one of these chunks must undergo a discontinuous velocity change, since immediately afterward, the velocity of the pair must match.

    But, we can apply the same reasoning - a compression wave travels through each chunk, accelerating each part in turn.

    Taking this line to its limit, it appears that only the zero mass leading 2D surfaces of the two objects undergo a discontinuous velocity change. Every other component in each object will accelerate smoothly, and no finite part of the train need ever be moving at less than the average velocity of fly and train.

    Interestingly, if the fly and train have equal densities and leading-edge shapes, the compression wave propagation is symmetrical. The symmetry doesn't break until the compression wave reaches the back of the fly. Up until that time, the leading edges are both travelling at the average velocity of the pre-collision objects. Therefore, unless the train is less dense or softer than the fly, no part of the train will ever travel at less than the average velocity of fly and train.
     
  8. Enmos Staff Member

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    Hi Chi,
    Billy wasn't real clear in his explanation. He means that the fly hit the train, but that at the moment of impact the fly changed direction because it the train hit the fly (the fly is now stuck to the train, squished).
    For the fly to change direction the fly must have been at zero velocity at one point. So if the fly was at zero velocity at one point the train was at zero velocity at that point too.
    Above is what Billy meant to say.
     
  9. Enmos Staff Member

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    Hi James R,
    Yes I know, i didn't understand the first time. The OP didn't state that the fly hit the train (or vice versa). Thats why I wrote that post.

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  10. Atom Registered Senior Member

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    It was self evident in the OP.

    The fly is neither here nor there, it could be "anything" from a table tennis ball to any light object....Enmos seems to suggest size is an issue...it isn't. As for 'squishing molecules'..again thats another case of "looking too deeply into the situation".

    There is a valuable lesson to be learnt from the answer though..I'll tell you exactly what it is later.

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  11. sniffy Banned Banned

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    Substitute Billy Chyld for the fly and you get a much more satifying result. In theory.
     
  12. przyk squishy Valued Senior Member

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    No, the same thing happens with the table tennis ball: their surface molecules repel when they're a small but nonzero distance apart, so there's nothing to imply that the train's velocity must be the same as the ball's throughout the collision. Of course you might find that some heavier objects do bring some parts of the train to rest momentarily (accompanied by some serious permanent denting or crushing), but this is far from a general conclusion applicable to all collisions.
    It's exactly what happens. In the idealised description of the situation, involving a perfectly rigid train colliding with a perfectly rigid fly which changes velocity instantaneously on impact, the fly simply doesn't have a well defined velocity at the time of the collision since its trajectory isn't differentiable at that point. Handwaving about the fly "going through all the intermediate velocities in an infinitely short time" doesn't allow you to conclude anything about the train, and in any case all you're doing is quibbling around a technicality in what's only an approximate description of a real collision anyway.
     
    Last edited: Aug 23, 2007
  13. Nasor Valued Senior Member

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    Are you saying that the correct answer doesn't fit with whatever philosophical point you are planning to make?

    Few things on internet forums annoy me more than these “I’ll try to lead you to a deep point gradually, over a series of posts” games. If you have a point to make, just explain it as concisely and clearly as possible in your opening post and let everyone read it.
     
  14. Enmos Staff Member

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    Billy, it was an issue for me, i didnt read in the OP that well. It wasnt so much your fault, it was mine. I was only explaining why i made the error.
     
  15. iceaura Valued Senior Member

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    Rather than mock and dismiss a theory of such profound consequence and implication, we should explore it.

    Since a fly can stop a train for a nanosecond, and light is made of photons traveling at much higher speed (we turn to the Koran fro the exact speed), we should be able to keep the train stopped by continuously bombarding it with trillions of photons, head on.

    Each one would only stop it for that nanosecond we have discovered in the jump discontinuity of the accelleration curve. The total bambardment would hold the train stopped as long as the flashlight batteries held out.

    So robbing trains would be easy: we stop the train with a powerful flashlight duct taped to a rail, and hold it while we kype the booty.

    But in practice this fails - every time I tried it, the train appeared to keep moving. So the photon bombardment was not effective. Clearly, then, there is something wrong with the photon theory of light.

    And remarkable thinkers of the past have solved this conundrum:

    http://www.jtkdev.com/light.html
     
  16. Enmos Staff Member

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    Last time I checked photons had zero mass..

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  17. iceaura Valued Senior Member

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    continuation of fly and train

    In response to Enmos, from the locked thread:

    The mass of the fly - or the photon - is not, I think, relevant. It's not the mass of the fly that allegedly stops the train (no mass or momentum calculation was put forth) but its reversal of direction, as we are informed. The photons also reverse direction - at least, many do - hence the train must stop for a "nanosecond". Since a sufficiently powerful flashlight will produce billions of photons, the train would be held at a stop for many billions of nanoseconds.

    That is a far superior means of stopping a train than placing large obstacles on the tracks or damaging the railway.
     
  18. James R Just this guy, you know? Staff Member

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    Moderator note: Not sure why this thread was locked. Re-opened.
     
  19. Enmos Staff Member

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    It IS relevant that the photons have no mass. No mass, no momentum. If there is no momentum there is no effect on the train.
     
  20. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    Maybe it was struck by a fly

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  21. Nasor Valued Senior Member

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    First, he wasn't arguing about stopping the train with momentum - he was arguing that it's impossible for one object (the train) to still be moving when it is in "hard contact" with another object (the fly) if the second object is not moving. The presumption is that since they are in contact, they must either both be stopped or both be moving. But I think we've pretty thoroughly beaten to death why that analysis is wrong.

    Second, although photons have no mass, they do have momentum. So you could actually stop a train by shining light on it...but you would need a heck of a lot of light, because the momentum of a photon is very small.
     
  22. Nasor Valued Senior Member

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    Yeah, but then it should only have been locked for a single, infinitely-short instant of time.
     
  23. draqon Banned Banned

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    The problem here is: how to launch a fly at such velocity that the flies momentum is greater than that of the train, whereas the fly not burning in an atmosphere before approaching with all its molecules at the train.
     

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