In western movies you often see a person putting his ear against ...

[lindiwe]

In western movies you often see a person putting his ear against the rails in the train tracks to find out if a train is approaching.
 
  Why would this give him advance knowledge of the approach of a train?

Question 2

You are on an airplane traveling with a constant velocity at an altitude of 20,000 m. What is the acceleration of gravity at that altitude? The radius of Earth is 6.37  106 m.
 
  A)
 
  9.81 m/s2
 
  B)
 
  9.78 m/s2
 
  C)
 
  9.75 m/s2
 
  D)
 
  9.72 m/s2
 
  E)
 
  9.69 m/s2

[Mholman93]

Answer to Question 1

The speed of sound in metals is considerably faster than the speed of sound in air.

Answer to Question 2

C

[lindiwe]

Gracias!

[kjohnson]

Wow, this really help