# Reading Russell's ABC of Relativity, stuck in chapter 3 (about speed of sound)

1. Aug 1, 2009

### Wattever

This is turning out to be difficult for me, and I haven't even reached the part where he explains the theory of relativity itself :(

"Let us suppose that the shot is fired from the guard's-van, and the echo comes from a screen on the engine. We will suppose the distance from the guard's-van to the engine to be the distance that sound can travel in a second (about one-fifth of a mile), and the speed of the train to be one-twelfth of the speed of sound (about sixty miles an hour). We now have an experiment which can be performed by the people in the train. If the train were at rest, the guard would hear the echo in two seconds; as it is, it will take two and 2/143 seconds"

Isn't the motion of air molecules (inside the train) relative to the moving train no different from motion of air molecules outside the train relative to earth? Why is there a difference in speed?

This got me confused about sound propagation in general, how can the compressions and rarefactions be http://bumper.gmi.edu/~drussell/Demos/waves/wavemotion.html" the place?!

I would really appreciate any help.

Last edited by a moderator: Apr 24, 2017
2. Aug 1, 2009

### tiny-tim

Hi Wattever!

I had a look at it at pages 22 to 23 at http://books.google.com/books?id=uF...ussell&client=safari#v=onepage&q=echo&f=false

the screen is on the engine, so the sound is travelling through the air outside the train …

in other words, the air is stationary (relative to the ground), and the sound is travelling as if the train wasn't there.

(It would be different if the sound was travelling inside the train … then the time would be 2 seconds, however fast the train was moving. )
Ah, it isn't neat …

that's why sound (unlike light!) goes round corners, and is generally fuzzy!

Last edited by a moderator: Apr 24, 2017
3. Aug 1, 2009

### Wattever

Uhuh, thanks a lot! :D

4. Aug 1, 2009

### Wattever

OK I'm sorry, but why won't the same happen with light? Edit: That is the difference in time

Last edited: Aug 1, 2009
5. Aug 1, 2009

### tiny-tim

erm that's what the book's about!!

6. Aug 1, 2009

Oh, lol!

7. Aug 1, 2009

But, um...