Newtons 2nd Law: "The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma."

The fly will have a mass of, say, 10 grams, and is travelling at, say, 2 metres/second. The train will have a mass in the region of, say, 100 tons, and is travelling at, 30 metres/second (precisely 67.014388489208633093525179856115 mph).

Convert 100 tons into kilograms --> 100,000kg.
Convert 10 grams into kilograms --> 0.01kg

Therefore, the fly has an instantaneous Force of 0.02 Newtons, and the train has one of 3,000,000 Newtons.

Result: the fly splats on the windscreen, the new speed of the train is calculated by comparing before/after momentums.

Momentum before collision = (100,000 * 30) - (0.01 * 2) = 2,999,999.98 Newtons

Momentum after collision = (100,000.01)V Newtons (where V is the common velocity)

Momentum before collision = Momentum after collision
--> 100,000.01V = 2,999,999.98
--> V = 29.9999968000003199999680000032 metres/second
--> V = 67.014381341007909064676719431609 mph

So, to compare, the speed of the train decreases by 0.0000071482007240328784126618mph in the impact......not sure that's a change in speed I'd notice quite easily, really!







And yes, I was bored!