Q68 of 118 Page 352

Two electric trains run at the same speed of 72 km/h along the same track and in the same direction with a separation of 2.4 km between them. The two trains simultaneously sound brief whistles. A person is situated at a perpendicular distance of 500 m from the track and �s equidistant from the two trains at the instant of the whistling. If both the whistles were at 500 Hz and the speed of sound in air is 340 m/s, find the frequencies heard by the person.

As the person is not sitting on the direct path of the sound wave, the person will hear a fraction of sound depending on the angle shown in the diagram. The angle will be,




Hence, the apparent frequency heard by the man from the train going away will be,






Hence, the apparent frequency heard by the man from the train coming towards will be,





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A bat emitting an ultrasonic wave of frequency 4.5 × 104 Hz flies at a speed of 6 rn/s between two parallel walls. Find the two frequencies heard by the bat and the beat frequency between the two. The speed of sound is 330 m/s.

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A bullet passes past a person at a speed of 220 m/s. Find the fractional change in the frequency of the whistling sound heard by the person as the bullet crosses the person. Speed of sound in air = 330 m/s.

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A violin player riding on a slow train plays a 440 Hz note. Another violin player standing near the track plays the same flute, When the two are close by and the train approaches the person on the ground, he hears 4.0 beats per second. The speed of sound in air = 340 m/s. (a; Calculate the speed of the train. (b) What beat frequency is heard by the player in the train?

 70

Two identical tuning forks vibrating at the same frequency 256 Hz are kept fixed at some distance apart. A listener runs between the forks at a speed of 3.0 m/s so that he approaches one tuning fork and recedes from the other (figure 16-E10). Find the beat frequency observed by the listener. Speed of sound in air = 332 m/s.