ChatterBank9 mins ago
Can a fly stop a train?
105 Answers
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)
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)
Answers
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...
12:30 Thu 25th Feb 2010
-- answer removed --