Question

A man jumps from a building with a source of sound. When he reaches halfway down, he starts the siren of frequency 100Hz. A person standing below him on the ground hears the first sound to be of frequency 120Hz. What is the height of the building? Speed of sound = 330m/s. Take g=9.81m/s^2.

(a) 154m

(b) 308m

(c) 77m

(d) 500m

I got this question by my school principal while I was bunking the class.

Asked question is from Waves in portion Waves of Physics – Class 11

Answer 1

The correct choice is (b) 308m

The best explanation: We can find the speed corresponding to the apparent frequency and then use that to find the height of the building.

120 = 100(330)/(330-v)                                                                                                                                                                        v = 330 – 3300/12 = 55m/s.

Using, v^2 = 2gh, we can find the height covered by the man.                                                                                       The double of this value will be the height of the building as he is mid-way.

55^2 = 2*9.81*h                                                                                                                                                                                                   h = 154m                                                                                                                                                                                                   Height of building = 2h = 308m.