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fly stops a train?

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fly stops a train?

Old 11th Jan 2006, 16:35
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fly stops a train?

ok this is another puzzle which i have often discussed after too many drinks:

if a train and a fly are travelling towards each other (the train at, say 60mph and the fly at 15mph) and the train hits the fly, the fly will then travel back in the exact opposite direction from where it had flown. So must the fly have at some stage stopped all together during the transition from travelling one direction to the exact oposite?

if this is the case, and if it happened whilst the train was presumably squashing the poor little bugger and was therefore in contact with the fly, does that mean that to be in contact with something which is not moving at all, the train must also have been momentarily stopped?

<donning hard hat in preparation for response>
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Old 11th Jan 2006, 16:38
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Re: fly stops a train?

Dunno... but the last thing that went through the flies mind was its arse!!
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Old 11th Jan 2006, 16:46
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Re: fly stops a train?

Imagine that the fly was on a conveyor belt....
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Old 11th Jan 2006, 16:48
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Re: fly stops a train?

I don't think so. If you consider the momentum then that of the train is HUGE and that of the fly is tiny. So fly will exert an equal and oppostie force, i.e. it will slow the train down when it hits, but not enough to make any noiticable difference.
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Old 11th Jan 2006, 16:51
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Re: fly stops a train?

Nobody mention centrifugal force or we'll be here for ten pages......

the last thing that went through the flies mind was its arse!!
But it didn't feel any pane.
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Old 11th Jan 2006, 16:55
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Re: fly stops a train?

It wouldn't have happened if the drivers were all on strike
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Old 11th Jan 2006, 16:58
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Re: fly stops a train?

Nah, the fly changed its mind when it saw the train, thats when it stopped and then headed in the other direction, sadly it didn't realise just how fast the train was going and got rammed from behind, lets just put it down to a tragic accident... so it wasn't exactly a head on collision.
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Old 11th Jan 2006, 16:59
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Re: fly stops a train?

time is a continuum and not something you can divide into discrete steps except at the peril of your reasoning. the fly is not a point either.

it was an argument of ancient greek philosophy that an arrow fired could never hit the target because you could keep halving a time interval in which case the arrow covered half the distance it flew in the previous interval... and if you summed all these distances they would never add up to the distance to the target. then again the tortoise and the hare... the hare reaches a point but alas the tortoise has moved forward; so the hare advances.. but the tortoise has moved etc. yet again the absurdness is in the claim that time measured in clock ticks is the same thing as time itself.

obviously the arrow hits the target so something is amiss and it is the reasoning of the beerholder. time is not a succession of frozen instants but a smoothly flowing continuum. equally obviously the hare catches the tortoise in reality, bums $50 off him and drinks himself to oblivion in the pub. the tortoise gets to the pub just in time to be struck on the back of the neck by an arrow fired by some bloke arguing with a fly on a passing train.
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Old 11th Jan 2006, 16:59
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Devil Re: fly stops a train?

Orac will be here before long with a theory of everything type answer....and my head will start hurting again
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Old 11th Jan 2006, 17:04
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Re: fly stops a train?

fair enough Ghostie but was the fly stopped?

btw the incident occured outside of the uk. so there were drivers available. Also, we are talking about a hardcore fly here who was willing to commit himself in a kamikaze fashion in the name of science and PPRuNe debate.

bless the little blighter.
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Old 11th Jan 2006, 17:07
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Re: fly stops a train?

So must the fly have at some stage stopped all together during the transition from travelling one direction to the exact oposite?
But shouldn't that be altogether?! Whatever, there is a lesson for all pilots here. That of the danger of being on a collision course even whilst keeping a good lookout because the apparent angle doesn't change...
...the train must also have been momentarily stopped...
I grant that the train would've experienced an infinitesimal deceleration, but probably far less than that as a result of the train driver having turned around in the cab whilst letting go an almighty fart aimed at the windscreen...
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Old 11th Jan 2006, 17:08
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Re: fly stops a train?

Yea the fly was stopped. If it changes direction along one axis then it has to stop. The train doesn't change direction so it doesn't have to stop, it just slows down.
Interestingly the speed at which the Earth rotates increases when a meteor penetrates the atmosphere.

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Old 11th Jan 2006, 17:14
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Re: fly stops a train?

"But shouldn't that be altogether?!"

Yes it should. guess i was concentrating more on what i was saying than how i said it. I don't think i would correct anyone's grammar in here for fear of having my own continually scrutinised. good spot airship.

Nice one Ghostie - didn't know that.
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Old 11th Jan 2006, 17:32
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Re: fly stops a train?

Shouldn't we be considering a fly and an airplane? Or do airplanes not hit flies? Do pilots notice when they hit a fly? Does transit time vary when flying through a fly-infested territory (such as Oz - no doubt there are worse places)?

>Interestingly the speed at which the Earth rotates increases when a meteor penetrates the atmosphere.
Then surely the rotational speed of the Earth must change as the fly reverses its direction of travel (and therefore momentum). The speed of the striking object (whether train or airplane) is relative to the rotational vector of the Earth. Are we measuring ABSOLUTE velocity or RELATIVE velocity WRT the surface of the Earth? What effect does the air velocity have?
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Old 11th Jan 2006, 17:34
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Re: fly stops a train?

We are, of course, talking about their motion relative to the Earth. The Earth itself is orbiting the Sun at about 30km/s, so from that perspective both train and fly continued in the same direction, both experiencing a slight change in speed. I suppose this is another way of looking at the definition of "stopped" or zero speed.
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Old 11th Jan 2006, 17:35
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Re: fly stops a train?

Neil, but it's not what you say, it's how you say it!
"The boy stood on the burning deck, whence all but he had fled. Twit."
Ooops, wrong M*lligan...

NB. I can behave like this because I can never know when this day will be my last.
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Old 11th Jan 2006, 17:39
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Re: fly stops a train?

Whatever part of the train it strikes has to bend momentarily to allow the fly to stop. But the rest of the train doesn't stop.
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Old 11th Jan 2006, 17:40
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Re: fly stops a train?

Surely it all depends wether the train hit the fly, or maybe the fly hit the train.
But I suggest you dont look at it from the fly's viewpoint - too painful
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Old 11th Jan 2006, 17:49
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Re: fly stops a train?

Doesn't it depend on the type of fly?

After all the wrong type of fly would stop the whole network not just one train...

PW
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Old 11th Jan 2006, 17:57
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Re: fly stops a train?

Might not the fly be blown away from the train due to the air being pushed and displaced by the train, only later to be squashed since it lost it's airspeed and fell onto the track and was run over? And, how does that affect the braking ability of the train due to the slippery track? Provided, of course, that the engineers aren't on strike?
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