# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Seany on 15/04/2011 09:23:43

Title: A Physics Question..
Post by: Seany on 15/04/2011 09:23:43
If you have a passenger inside a train, and he drops a ball.. If you neglect air resistance, would the ball be horizontally motionless to the passenger?
Title: A Physics Question..
Post by: RD on 15/04/2011 09:38:12
It will drop vertically from the passenger's viewpoint if the train is travelling at a constant speed on straight track (or stationary).

If the train is accelerating or decelerating it won't, (NB: train going round a bend counts as accelerating (http://en.wikipedia.org/wiki/Acceleration) because it's changing direction even if it's speed is constant).

Title: A Physics Question..
Post by: Seany on 15/04/2011 09:44:00
Thanks. And if it were to have air resistance. It would appear from the passenger's point of view to be going backwards.

But from an observers point of view, it would go in the direction of the train, but with a smaller velocity?
Title: A Physics Question..
Post by: RD on 15/04/2011 09:48:19
... And if it were to have air resistance. It would appear from the passenger's point of view to be going backwards.

Only if there was air flow through the train carriage, in a sealed carriage (no draughts) it would still drop vertically from the passenger's perspective if the train's speed was constant and the train tracks straight.
Title: A Physics Question..
Post by: Seany on 15/04/2011 09:53:29
Bugger [:P] I meant that he drops it outside the window
Title: A Physics Question..
Post by: yor_on on 15/04/2011 10:34:57
It would initially have the same speed as the train, and to the 'train dropper' seem to fall 'straight down' ignoring all resistance now. Same as inside the carriage in fact :)

From a stationary observer watching the train pass, it would move in a arc, now assuming that the train doesn't have a velocity allowing the ball to reach orbital speed. If you include air resistance it will also slow down relative both observers, and as you say move backwards from the train droppers POV. Assuming that the 'train dropper' defines the train as stationary he will now be in need of a strong drink to cool his nerves as everything moves the wrong way :)

I think?

:)
Title: A Physics Question..
Post by: Seany on 15/04/2011 10:56:09
[:P]
Title: A Physics Question..
Post by: lightarrow on 15/04/2011 12:00:43
If you have a passenger inside a train, and he drops a ball.. If you neglect air resistance, would the ball be horizontally motionless to the passenger?
Galilei made this and other experiments "below deck" (" sotto coverta" in his language) inside a ship and concluded that, as long as the ship is moving straight at constant velocity, you can't notice that you are moving. It's called "Galileian principle of relativity" and it's extremely important in physics.