If a train is going South, at 100 miles per hour...
and...
A fly is flying North,towards the train, at 5 miles per hour
and...
The fly splatters on the windshield of the train
At what point is the train reduced to zero miles per hour?
Never, you say? Well consider this.
In order for the Northbound fly to switch from northbound motion to southbound motion, it would have to come to a complete stop before it can begin motion in the opposite direction. Correct?
Well...
If the fly is then going zero miles per hour, and it is in contact with the trains windshield... then by definition, wouldn't the train also be going zero miles per hour, if even for a tiny fraction of a second?
I'm a nerd... I know...