# Transparent train

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Hello! I've got a question for you guys. A friend of mine told me this: If someone is travelling in a transparent train and drops a coin (standing on his feet, whatever this has to do with anything), for him, the coin would go straight down, but for someone standing outside the train as it goes by, the coin wouldn't fall only straight down, but also in the trains moving direction making the coins movement look diagonal.

I think this is wrong as the coin still relates to the guy dropping it, and therefore it will look like the coin is going down in a straight line or else it would land some feet in front of the dude inside the train, which makes no sense at all.

Well, who's right and who's wrong? I'm not very knowledeable with physics so I'd like to have someone explain who's right and why.

Thanks in advance and sorry for my bad english (it's not easy for a kid living in Sweden with swedish as his native language instead of "la lingua pura" ).

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Relative to anything inside the train or the train itself, the coin falls straight down.

Relative to anything outside of the train, the coin is moving at whatever speed the train is moving and that velocity must be considered in addition to the acceleration caused by gravity. The direction that the coin falls is not straight down (as would be perceived by the man dropping it) because the man dropping it does not perceive himself moving.

Try to consider also what would happen if the man were walking down an aisle in the train when he dropped it. Or maybe consider that hardly anything is ever truly at rest - while the train is chugging along on the earth, the earth is spinning on its axis and rotating around the sun. There may even be some motion of the solar system in our galaxy, or some motion of our galaxy in the universe.

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Both perspectives are considered equally right.

Motion is dependent on the observer.

Different observers will see different types of movement from the coin.

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But wouldn't it still look like the coin is falling straight down for the observer outside the train? I mean, it's kinda the same perspective as for the guy inside the train because he's there and the coin will fall straight down in relation to the dude.

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But wouldn't it still look like the coin is falling straight down for the observer outside the train? I mean, it's kinda the same perspective as for the guy inside the train because he's there and the coin will fall straight down in relation to the dude.

"The dude" isn't stationary relative to the observer on the ground. You're mixing coordinate systems. The trajectory will be a parabola (not a diagonal) using the stationary coordinate system.

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Giving my question a little more thought I've come to realize the question itself is rather stupid. It's all so obvious. Sorry for wasting your time guys, I was wrong.

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It's not such a stupid question after all though. If you consider this: further the thought experiment to light, then you get into interesting relativity. We know that light in a vacuum always travels at the same speed of light, called 'c'.

But if the man on the train sees it go up, straight, but we see it go diagonal down then we see it move further (a diagonal from ceiling to floor is longer than a straight line)... but how can the light move further, in the same time (from top to botton) when we both measure the speed of the light as the same. How can it go further, when it's travelling at the same speed?

The answer gets into a bit of basic relativistic mathematics, although the simple answer is that because the train is moving the train's (from our, on platform point of view) time is moving slower. This is known as time dilation.

A stationary observer (you on platform) sees a moving observer's (a train going past) time going slower. But the man on the train thinks that he is stationary. If he looked outside he would think the man is moving past, and he is still.

This is how relativity is sometimes introduced to university students for the first time. Well, near the very beginning anyhow! If you're interested, and willing to possibly get a bit confused(!), then I'd sugges reading into a bit of basic relativity. It is kinda interesting!

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It sounds REALLY interesting acctually! Confusion is always fun, hehe. My only problem will be the language. As you might have noticed by now I don't master english as well as I'd like to, and I'm sure relativity is a whole lot better explained in english than it is in swedish, as english is the international language of science.

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You can probably find good translations of books about relativity in Swedish.

And don't worry, your English isn't bad at all. Compared to a lot of people on the Internet you're doing very well.

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Thanks mate, though I'm not really into translations, especially not when it comes to advanced stuff. A text is always best in its original shape

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I found relativity an extremely confusing topic when we studied it last year (I'm in High School).

We studied another scenario involving a mirror on the ceiling of the train and a light beam on the floor that is directed towards the mirror. For the person on the train, the light travels straight up and down. For a "stationary" observer on the platform, the light ray appears to travel in an upside-down "V" shape.

Thanks 5614, your explanation was concise, uncomplicated and easy-to-understand. =)

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