Chapter 11

NCERT
Class 9
Science
Solutions
12. A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower, when is the splash heard at the top? Given g = 10 ms–2 and speed of sound = 340 ms–1.

Question:

A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower, when is the splash heard at the top? Given g = 10 ms–2 and speed of sound = 340 ms–1.

Answer:

Case-I, u = 0, s = 500 m, g = 10 m/s2

∴ s = ut +  1/2 gt2

500 = 0 + 1/2 x 10 x t12

t12 = 100

t1 = 10s

time taken by the stone to reach the surface of water = 10 seconds

Case-I, Again, s = 500 m, v = 340 ms–1

from s = vt2

t2 s/v = 500/340

= 1.47s

time taken to hear the sound at the top.

t = t1 + t2

t = 10 + 1.47

t = 11.47s

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