I have a question that came up in a discussion with friends. If
I throw a ball straight up in an enclosed train car moving with
constant velocity, I believe the basic physics books say it will
land in the same spot. But will it really? I think I can say
that the answer is "not in the real world". Trivially, a train
car is never enclosed. Fresh air is being allowed into the
carriage or the passengers would all die. Thus there's currents
of air that would affect the ball, agreed? If we remove the
passengers and have a trusty robot (who does not need oxygen)
throw the ball up in a carriage that really is completely air-
tight, I'm still not sure it will land in the same spot. I would
imagine that there must still be air circulation. The train had
to start from a stop. It's true the floor and the roof will drag
the air right at the boundary along with it, but just as an open
convertible car does not drag all the air in the world with it,
I assume that the air in the middle of the car will not be
dragged along at t