Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child.

Let the distance between the two children be d

Also let the time taken by sound to reach other child via air be t1 and via aluminium rod be t2.

Velocity of sound in air va=340m/s

t1 = d/va =d/340 —–(1)

Velocity of sound in aluminium rod vr = 6420m/s

t2 = d/vr = d/6420 ——-(2)

Divide 1 and 2 equations

t1/t2 = 6420/340 = 18.88