ICSE Class 10 Physics 2020 Solved Paper

Question : 46

Total: 67

A man standing in front of a vertical cliff fires a gun. He hears the echo after 3.5 seconds. On moving closer to the cliff by 84m, he hears the echo after 3 seconds. Calculate the distance of the cliff from the initial position of the man.

Solution:

Let the distance of cliff from the initial position of man be ' d ' metres and speed of sound in air be ' V ' m∕ s.
For the first echo,
‌t1=3.5 s
‌V=‌

∴‌‌V=‌

3⋅5

......(i)
On moving closer to the cliff by 84m, the distance of cliff from the new position becomes (d−84)m, then
for second echo, t2=3 s.
∵‌V‌=‌

2(d−84)

∴‌V‌=‌

2(d−84)

.......(ii)
From (i) and (ii), we get
‌‌

2(d−84)

‌=‌

3⋅5

⇒3⋅5(d−84)‌=3d
⇒3⋅5d−294‌=3d
⇒3⋅5d−3d‌=294
⇒‌0.5d‌=294
⇒‌d‌=‌

294

0.5

=588m.

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