# fly and the train!!!

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Here we go...

A fly is flying about, minding its own business. However he does not realise he is on a direct collision course towards ano on coming train.

Now neglecting air resistance, when he hits the train he becomes a part of it right? At that very same moment he must change his direction. However in order to change direction he must STOP, now as he is part of the train the train must also STOP for that brief second....

Therefore a fly can stop a train, now where is the flaw in this argument....?

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Here we go...

A fly is flying about' date=' minding its own business. However he does not realise he is on a direct collision course towards ano on coming train.

[b']Now neglecting air resistance, when he hits the train he becomes a part of it right? At that very same moment he must change his direction. However in order to change direction he must STOP, now as he is part of the train the train must also STOP for that brief second....

Therefore a fly can stop a train, [/b] now where is the flaw in this argument....?

The bolded part!

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Ah come on, J.C.MacSwell, you can do better then that...

As I said neglect air resistance, pretend it happens in vacuum, (yes I am aware that a fly could not fly in a vacuum).

you still haven't pointed out the flaw in the argument.... You can't just highlight what I said and say it's wrong, why is it wrong???

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It's sort of related to related to Xeno's paradox.

The train never stops. v=ds/dt and the fly stops for an infinitesimal length of time, but that doesn't mean the train does.

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Interesting.

According to the train, it sees a fly stop relative to it. According to the fly, it sees the train stop relative to him.

The problem is for the outside observer, when he sees the fly come in contact with the train. Perhaps a shock wave is sent through the train accomodating this infinitesimal stopage!

Man it's terrible when Zeno (Xeno) raises his ugly head, he did the same in my flashing light bulb problem on the maths thread:-)

Thank god the real world stops these mad situations happening....

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I wouldn't have thought the fly would stop, it merely changes direction and the sudden deceleration on its current vector equates to an equally sudden acceleration on the vector of the train. But, I ain't no physicist.

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now as he is part of the train the train must also STOP for that brief second....

Therefore a fly can stop a train' date=' now where is the flaw in this argument....? [/quote']

eh?? since when can a tiny fly have an effect on a train? the flaws i see here are pretty obvious.

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I wouldn't have thought the fly would stop, it merely changes direction and the sudden deceleration on its current vector equates to an equally sudden acceleration on the vector of the train. But, I ain't no physicist.

he does indeed stop for an instant. the train does not, its just slowed down a little bit. the only time the fly and the train are going the same speed is when the fly has fully accelerated to the speed of the train, AFTER he stops.

pretty big and obvious flaw if you ask me : P

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• 2 months later...

Nothing - neither the fly nor the train - is prefectly rigid. When the fly hits the train, it microscopically deforms the surface of the train - even if only temporarily. If one wanted to calcualte the momentum of the train (the train *alone*, not including the fly) before- and just after- impact, one would see that the momentum of the train was infinitesimally smaller. Because a very small piece of the train has slowed down, the train's *average* speed is less.

This puzzle is only a paradox is a universe where objects are perfectly rigid.

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• 2 weeks later...

I agree with DaveC426913, but don't see what you mean by it being a paradox is the objects are perfectly rigid. Since momentum is conserved in all situations, the train will, in any case, decrease an infinitesimally small amount.

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This puzzle is only a paradox is a universe where objects are perfectly rigid.

Even then, the "infinitesimals" part of the argument still holds. The train has a small dv when ds —> 0, but in that limit, ds/dt is nonzero. It never stops, even in the rigid body approximation.

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