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Can a fly stop a train?

A chap drags this out from time to time concerning a fly hitting a train. A train is doing 125 mph due south. A fly is doing 5 mph due north. They collide. The fly stops travelling north and starts travelling south. To change direction the fly must stop, albeit very briefly, as it hits the train. The fly is touching the train when it stops and thus the train must be stopped also. Did the fly stop the train?
Onny from Glasgow (Age 45-54)

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One Response

  1. No it didn’t stop the train, but it did slow it down ever so slightly (though unfortunately it did not live to boast about this feat).

    As the fly and train collide, the extremely small (relative to the train) momentum of the fly is effectively transferred to a new ‘train and squashed fly’ combination. This new combination travels (other things being equal) ever so slightly less than 125mph in the same direction as the train before the collision.

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