Can a fly stop a train?

[Barnacle_Bill]

Probably an old chestnut, but it's been worrying me since schooldays - and that's a long time.

A train is heading down the track and a fly is travelling up it. The two collide. The fly's velocity has changed from positive to negative (or vice versa!) so at some instant in time it must have been zero. At that instant it was in contact with the train so the train's velocity must also have been zero. So for that instant the fly stopped the train. I don't think so, but where's the flaw in the logic?

The only thing I can think of is that the fly's velocity has INSTANTLY changed without going through zero, but that doesn't sound very satisfactory. Can anyone finally put me out of my misery? (I hope calculus is not involved)

[vascop]

Tim
NO part of the train stops, even for an instant.

[factor-fiction]

Yes vascop- I am completely unconvinced by the claim that in the microsecond when the items are touching they must have zero velocity. Isn't this case about conservation of combined momentum, from which it can be shown that the effect on the train's velocity will be negligible?

[vascop]

doc
That's another old chestnut.
Just because something is somewhere at every instant doesn't mean its stationary. If I had a special camera I could take a series of stills of your ball and each one would show the ball at a different place but it wouldn't be stationary, it would be on its way to the next position. Think - don't just repeat some dumb stuff you've read somewhere.

[vascop]

factor
yes, it is about conservation of momentum and the rest of your post is also correct. I can put the maths on AB if anyone is interested. It's not difficult to understand.

[factor-fiction]

I thought docspock was joking at first but now I'm not sure.
Err, so is it stop-start-stop-start? To go from a moving state to a stationery there would need to have been deceleration. And then to move again there must be acceleration. Where does all the acceleration and deceleration come from, docspock?

[vascop]

doc
Anytime! Don't mention it. You know what your saying is bunkum. Think about it - that's what your brain is for.

[Tim123]

vascop, sorry, you are wrong. A tiny part of the train deforms, and in so doing the point of contact really does stop moving relative to a stationary observer. I'm not suggesting that the whole train stops, only the point of contact with the fly. The amount of the deformation and its duration may well be too small to measure, but it does happen. That's how the mechanics of collisions work.

[vascop]

That's nonsense. Give us a reasoned argument to prove it if you really believe it. I agree with you that a small deformation takes place, as in all collisions, but to someone standing on the ground ( presumably that's what you mean by a stationary observer) no part of the train stops.

[Tim123]

Yes it does. Imagine that instead of a train hitting a fly, a marble was hit by a baloon. The marble would push into the baloon as it decelerated, and eventually the marble and the part of the baloon with which it was in contact would stop moving relative to a stationary observer. During this process, of course, the marble and the relevant part of the baloon would be moving backwards relative to the motion of the baloon as a whole. Next, the baloon would return to its normal shape, the deformed part travelling in the same direction as the baloon as a whole, but with a slightly greater velocity. As the rigidity of the object involved increases, the magnitude and duration of the deformation decreases, but as no object can be totally rigid, the effect never disappears completely.

[vascop]

No that's not right. When the marble moves into the balloon, the marble eventually stops relative to the balloon, but the balloon does not stop. You are trying to argue that because the marble stops the balloon must stop - but this is a nonsense. The balloon is still moving forward at the instant the marble stops.

[tamborine]

wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong wrong

Fly gets into the driver's eye.....crash - stop; disaster!

[McMouse]

My fly was open and it stopped a police car.

[Tim123]

It is right. At some point, the marble will stop relative to the centre of mass of the baloon, but at a previous point, the marble will stop moving relative to a stationary observer. At this point, the point on the skin of the baloon in contact with the marble will also be stationary relative to a stationary observer. It must be, it's in contact with a stationary object. However, this change in velocity of PART of the baloon is totally down to deformation of the baloon. The centre of mass of the baloon, or the baloon AS A WHOLE, to put it another way, never stops moving at any stage.

[vascop]

No, as I pointed out in an earlier post, you can be in contact with a stationary object and still be moving - like the billiard balls.

[vascop]

No, because I am right.