# Thread: How a fly can stop a train

1. ## How a fly can stop a train

Ok, a fly and a train are travelling towards each other. Ultimately they will collide. As they do, the overpowering mass of the train will force the fly to travel in the opposite direction. To do this the fly must first stop.

So, in that millisecond when the fly stops to change direction it is in contact with the train, therefore the train must be stopped too...

I must lie down now, quantum physics was never my thing

2. equal and opposite reactive forces...the precise moment the fly hits the train both stop, only the momentum of the train due to its superior mass over the fly (by an order of several million probably) pushes it through that short moment as though nothing happened.

3. Nope, the fly does stop... but the train does not.

The fliy's relative motion will be towards the train, then for a tiny fraction of a second it will ahve stopped, then the fly's direction of travel will be reversed and it will move in the same direction as the train.

The train, however, will (if you wanted to be REALLY picky), have slowed a tiny, tiny, tiny, amount but will carry on in it's original direction. The train will only be slowed by a tiny amount, but not stopped.

4. no one likes a smart *rse...

5. Schrödingers Cat anyone?

6. It may be logical but its not correct. What you are essentially saying is that in an extremely tiny time frame the train does not move but you cannot describe time in that way. Following the same logic you can say that an arrow can never catch a tortoise and nothing should be able to move.
A nice man called Xeno first noticed it and wrote lots of examples (Xenos paradoxes)
Mr Newton solved the problem with Integration & Differentiation

7. Originally Posted by rj45ethernut
So, in that millisecond when the fly stops to change direction it is in contact with the train, therefore the train must be stopped too...

I must lie down now, quantum physics was never my thing
No, it's nothing to do with quantum physics. As Deckard said, the train doesn't stop, although it will be slowed down by a tiny and probably immeasurable amount. What you have to remember is that nothing is totally solid and undeflectable. In fact, flies are pretty squishy, so what happens is that when the fly's head hits the train it almost immediately changes direction and starts moving backwards. Meanwhile though, the fly's body is still moving forwards. There is a moment when overall the fly has zero momentum but at that moment some of its component parts are moving backwards while others move forwards.

If the fly was replaced by something hard like a stone, then that still happens. However, it will also deform the front of the train a little more. If the metal at the front of the train is deformed beyong its elastic limit then a dent will be left and some of the energy of the stone will be absorbed, and the stone won't ping backwards at the same speed; in any case a little energy is always dissipated in the form of heat and noise (splat!).

This is a bit of a hard concept to grasp TBH, so much so that I once had to point out to my A-Level Physics teacher that a question about two vans crashing into each other was fairly nonsensical because they would apparently end up squashed into a .5m space.

Rich :¬)

8. Originally Posted by Rave
here is a moment when overall the fly has zero momentum but at that moment some of its component parts are moving backwards while others move forwards.

Rich :¬)
So the last thing to go through it's mind IS it's a**e

Sorry, couldn't resist

*edit* And I aint mocking your comments guys. Felt I had to throw this one in cos it's always a good'n. I know the squishability of flies, but just love the theoretical side

9. That sounds like the right explanation Rave

It reminds me of my brother and his school physics teacher. My brother and a friend of his would be constantly coming up with interesting questions and arguments which, of course, the techaer had trouble answering . Considering that my brother is pretty interested in physics and knows a bit about quantum physics, aerodynamics etc you can imagine some of the rather complicated discussions that came up.

10. The explanation doesn't work though rave. Take the same scenario but with an incompressible object. Are you saying that the train would be 'stopped' by that?

11. Originally Posted by GreenPiggy
The explanation doesn't work though rave. Take the same scenario but with an incompressible object. Are you saying that the train would be 'stopped' by that?
No object is incompressible.

Rich :¬)

12. quarks are, its not the point of the question.

13. Originally Posted by GreenPiggy
quarks are, its not the point of the question.
Well, if the question is based on a supposition that is basically untrue outside of one extreme example, then it's pretty nonsensical surely?

If a Quark hit the front of the train I imagine it would react pretty quickly with whatever other subatomic particle it happened to bump into. It's not going to happen though so the point is moot.

Rich :¬)

14. Originally Posted by GreenPiggy
The explanation doesn't work though rave. Take the same scenario but with an incompressible object. Are you saying that the train would be 'stopped' by that?
I think hes saying the softer side will absorb the impact more, but not necessarily receive a larger impact than the other...

Additionally, I think hes saying that there is no such thing as "an incompressible object", just varying decrees of compression.

Seems to make perfect sense

EDIT: you lot are talking faster than IRC!!
EDIT: I think I will take a back-seat from here on in

15. Part of a train almost never moves anyway. The train can travel hundreds of miles without this part (actually a very essential part of the train) from moving so much as an inch. It's the part of the wheel which is in contact with the track.

16. Originally Posted by TeePee
Part of a train almost never moves anyway. The train can travel hundreds of miles without this part (actually a very essential part of the train) from moving so much as an inch. It's the part of the wheel which is in contact with the track.
You lost me there!

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