You are walking along, throwing a ball straight up into the air. When it comes back down, will it be slightly behind you, or will it drop back into your hand? One of my 16 year old son's teachers says the "drop back into hand" scenario is correct, I can't see how.

To me it'seems to be the same principle as when you jump in the air in a moving train - do you "move backwards". I haven't tried this, because I don't think commuters on the Melbourne train network would appreciate the display.

Also, is it the same thing as when some unfortunate soul who isn't wearing a seatbelt in a car gets "thrown" through the windscreen? S/he doesn't really get thrown, but continues moving after the car has stopped or slowed down (or is momentum something different again?).

Very confused.

I am absolutely certain that if you try to experiment throwing a ball while walking you'll come to the conclusion that the teacher was wrong. Do try.

However, in principle, the teacher was right. Imagine not walking but travelling on an open train platform. Under ideal circumstances, i.e. in the absence of air resistence and still earth, a ball thrown vertically relative to the moving car will travel and fall back vertically relative to the car.

Why do you need a moving platform? Because, being human, you'll affect the experiment. In all likelihood, you'll slow down just before throwing the wall and speed up, even if a little, immediately after.

The thrown up ball participates in two motions: due to the gravity after being thrown up and due to the inertia acquired through the association with the moving object (a human or a car.) These two motions add up. To the thrower who continues walking or stays on the platform, the ball appear just to go up and down. To an observer at rest on the ground, the ball will appear moving up and simultaneously forward.

The ball held in a hand of a moving person has a velocity of that person. If just dropped, relative to the fellow it will fell vertically, but to an observer on the ground, it will appear that the ball continue to move forward while simultanneous approaching the floor of the car.

>To me it'seems to be the same principle as when you jump in
>the air in a moving train - do you "move backwards".

No, you move forward along with the train. You need to jump backwards in order to land on your feet and be able to roll back. You would break your legs jumping forward. In the best scenario, if you jump forward, you'll continue moving forward even after you feet touch the ground. If your legs hold up, chances are you'll rotate around the feet and fall flat on the stomach and face.

>I haven't tried this, because I don't think commuters on the
>Melbourne train network would appreciate the display.

It is very wise for a variety of reasons. You must be applauded for making the right decision. Very responsible attitude, yes.

>Also, is it the same thing as when some unfortunate soul who
>isn't wearing a seatbelt in a car gets "thrown" through the
>windscreen? S/he doesn't really get thrown, but continues
>moving after the car has stopped or slowed down (or is
>momentum something different again?).

Yes. The fellow of the car travels at the speed of the car. If unfettered he/she will continue in that motion if the car stops abruptly. If it just swerves, the fellow will be thrown aside or pressed to a window.

>Very confused.

If the memory serves, it's Newton's First Law of Motion.