Waves

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A stone dropped from the top of a tower of height 300 m splashes into the water of a pond near the base of the tower. When is the splash heard at the top? Given that the speed of sound in air is 340 m s

^{-1}

? (g = 9.8 m s

^{-2}

)

Solution:

Here, h = 300 m, g = 9.8 m s

^{-2}

, v = 340 m s

^{-1}

t_1 =

time taken by stone to strike the surface of water in the pond, then from

s=u t+\frac{1}{2} a t^{2} ;

300=0+\frac{1}{2}\times 9.8 t_{1}^{2}

t_{1}=\sqrt{\frac{300}{4.9}}=7.82\text{ s}

Time taken by sound to reach the top of tower

t_{2}=\frac{h}{v}=\frac{300}{340}=0.88\text{ s}

Total time after which splash of sound is heard

= t_1 + t_2

= 7.82 + 0.88 = 8.70\text{ s}.

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