If you want to hear a train approaching from far away, why is it more convenient to put the ear to the track?

Out of solids, liquids and gases :

(a) in which medium sound travels slowest?

(b) in which medium sound travels fastest?

Which of the following is the speed of sound in copper and which in aluminium?

(a) 5100 m/s (b) 1500 m/s

(c) 3750 m/s

What is the speed of sound :

(a) in air?

(b) in water?

(c) in iron?

What name is given to those aircrafts which fly at speeds greater than the speed of sound?