# Can a fly stop a train?

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

1. ### MuslimImmortalValued Senior Member

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If the fly stopped moving, it would fall wouldn't it? by the force of the gravity fulling it to the ground? therefor the fly would never truly stop.

3. ### EnmosStaff Member

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You are right, i forgot about that.

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lol

7. ### EnmosStaff Member

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I calculated it for you.
The train weighs 55.000 kg.
The fly weighs 40 mg.
The train travels at 100 mph.
The fly then has to travel at 137.5 * 10^9 mph, or 137.5 billion mph to stop the train.
I think its save to say that will never happen lol

8. ### CANGASRegistered Senior Member

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Can a fly stop a train?

Yes.

A fly lights upon a sensitive part of the engineers body and causes a muscular spasm throwing the engineers hand upon the brake lever.

The engineer recovers his senses after the entire train has come to a full halt.

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Agreed.

10. ### fmonroyRegistered Senior Member

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hahahahahaha.

think about the fly's wings, they need to be heavy duty to allow it to accelerate to that hyper light speed

11. ### temurman of no wordsRegistered Senior Member

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if you have a very long railtrack and the train is just rolling, without engine on, then a fly can stop it in a finite time. Well, fly should have also a lot of food and time available.

12. ### fmonroyRegistered Senior Member

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its lifespan is about 1 day, so it has only 24 hours to do the work

13. ### pencilBannedBanned

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Yes, a fly can stop a train hypothetically. If the scenario was in space, yes. If the fly can reach speeds of light, then yes (more speed = more energy and mass).

14. ### Steve100O͓͍̯̬̯̙͈̟̥̳̩͒̆̿ͬ̑̀̓̿͋ͬ ̙̳ͅ ̫̪̳͔OValued Senior Member

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Considering the possibility that the fly and train are perfectly stiff...

I believe that at the single point in time that the fly is stopped, so is the train, but as it is a single point in time nothing can be in motion anyway, so it really doesn't make any effect on the train.

I think it's like one of Zeno's paradoxes of an arrow in flight.

(these are just my random thinkings by the way, and they are probably fundamentally flawed. Please point out if so)

15. ### Steve100O͓͍̯̬̯̙͈̟̥̳̩͒̆̿ͬ̑̀̓̿͋ͬ ̙̳ͅ ̫̪̳͔OValued Senior Member

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How can you say a photon has no mass and yet has momentum? I'm pretty sure momentum = mass * velocity

16. ### fmonroyRegistered Senior Member

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I don't think it's needed because the momentum at the fly's atoms is enough to repel the entire train; it needs to crash in a key spot though

17. ### fmonroyRegistered Senior Member

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As a wave it has no mass, but it behaves like a particle too, right?

18. ### quadraphonicsBloodthirsty BarbarianValued Senior Member

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That's only for things that are moving at slow speeds (i.e., much smaller than c). Since photons move at the speed of light, that equation doesn't apply.

19. ### MuslimImmortalValued Senior Member

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kjkj

Last edited: Dec 2, 2007
20. ### psychaRegistered Member

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The logic with the original statement is inconsistent. If you had two perfectly rigid bodies, then i believe a fly would stop the train. Frankly the impact would produce and infinite force if there were contact.

But the reality is there is never contact bewteen the fly and frankly since the train and fly are flexible, different parts of them will change direction at different times as the pressure waves travel through them.

Back to my point, the fly would never touch the train, it would only approach it and the electrostatic forces would eventually overcome the fly's momemtum and reverse it's direction without contacting the train.

There is no such thing as contact and there is no such thing as infinite stiffness.

21. ### EnmosStaff Member

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All very well, but why do you think the word touch exists... ?
Maybe the everyday definition needs updating ?
Wouldn't you say the electrostatic forces interact ? Can we define this as 'to touch' ?

22. ### psychaRegistered Member

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Because in the context of the original question, the original poster is right. A fly would otherwise stop a train or the fly would experience infinite acceleration. Both are impossible which is why he stated the question in the first place.

If you consider that in reality things don't touch such as I stated then there is room for the fly to accelerate in the opposite direction without the train being affected.

23. ### EnmosStaff Member

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I agree, but to normal people that is touch.