Michael Barr wrote:
It makes no more sense to ask what happens in the limit than it does to ask which way the fly was flying when it was crushed between the two locomotives.
What does "when it was crushed" mean? Assuming an infinitely small fly, i.e. a point, and modeling locomotives as line segments (since they're much bigger than flies), if the locomotives are open at the front (the better to absorb the shock of hitting a cow) then "when it was crushed" cannot be when the frontiers of the locomotives met since there was room for the fly then. But at every other candidate for "when it was crushed" we can ask in which direction it was flying. The argument looks fine with at least one locomotive closed (cows be damned). With exactly one the fly is crushed before the locomotives collide, while if both are closed the fly is crushed when they collide. In either case you're right. Vaughan