Can a fly stop a train?

Answer 1

it depends: if the fly is stopping a train the size of a period or if the fly is

trying to stop a 10-ton train.

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Actually it would seem that a fly traveling exactly opposite to the

direction of a moving 10-ton train and colliding with it would have the

effect of stopping the train dead still...at least for a micro-milli-second.

Consider the velocity of the fly relative to the track. It is intuitive that upon

impact, the fly's velocity accelerates to zero and then accelerates in the

opposite direction to match the velocity of the train.

But if it is at the moment of impact of the fly and the train when the fly's

velocity relative to the track is zero...and since the fly is in contact with the

train at that instant of impact...then the velocity of the train relative to the

track must also be zero. Right?

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I don't think so, if we're talking about a train moving at speeds we'd

consider normal:

The train stops the fly for a micro second before the fly reverses direction,

that's true, and perhaps the fly's momentum slows a portion of the train

directly in contact with the fly in an elastic-like collision (forming a

molecular-sized temporary indentation). But the bulk of the forces of the

collision are borne by the fly, who's body is deformed and splattered by the

rapid change in direction.

However, a fly can stop a train, if the fly's momentum is equal to or greater

than that of the train: e.g. if the train's velocity is so small that it's momentum

is less than that of the fly flying directly at it. This would be a train moving so

slowly it would be imperceptible to us.

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Relatively-Useless Contribution #4:

If the train is under power, then the answer is a big fat undebatable 'no',

because the train's engines deliver more power than the fly can ever match.

But if the train is coasting, then the fly simply needs to apply a constant force

against the front of the locomotive, and keep applying it until the product of

(the force applied) x (the distance the train rolls while the force is applied)

is equal to the kinetic energy the train had when the fly arrived. Nothing to it.