early detection:- in the nineteenth century, people put their ears to a railroad track to get an early warning of an approaching train. why did this work?
Answers
Answe1. The speed of sound in metal is something like 5000 m/s, compared with 340 m/s in air.
2. In air, any sound spreads out in 3 dimensions, so the intensity reduces as (1/distance squared) from the source. In a metal bar, the sound is traveling along the length of the bar not spreading out into space, so the intensity reduces more much slowly with distance. In fact, according to the simplest theory of sound transmission the intensity would not reduce at all however long the rail was, but obviously that is only approximately true.
This also explains why you have to physically touch the rail (or at least get very close to it) hear anything. If the bar was radiating a lot of energy into the air so you could easily hear the sound, the vibrations in the bar would not travel so far.r: