Question

a ringing mobile phone get slipped by hand of a boy standing at balcony at height 100m, the freq of sound heard by the person standing at balcony of height 55m from ground just before mobile phone passes him will be (actual frequency of ringing sound of mobile ia 200hz and velocity of sound is 330m/s)​

Answer 1

Given: Ball falling from height 100m. Person standing at height 55m. Actual frequency of phone ringing is 200 Hz and velocity of sound(v) is 330m/s.

To find: Frequency of sound heard by person just before mobile phone passes him

Explanation: Distance travelled by phone=100-55

= 45 m

Velocity (v1) of ball just before passing=

$\sqrt{2gh}$

=

$\sqrt{2 \times 10 \times 45}$

= 30 m/s

Using Doppler effect,

Apparent frequency when source travels=

$\frac{v}{v - v1} \times \: frequency$

=

$\frac{330}{330 - 30} \times 200$

= 330*200 / 300

= 220 Hz

Therefore the frequency of phone ringing heard by person is 220 Hz.