NCERT Physics Class 11 Exemplar Ch 15 Waves Part 4

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Short Answer

25. A steel wire has a length of 12 m and a mass of 2.10 kg. What will be the speed of a transverse wave on this wire when a tension of is applied?

Ans: .

26. A pipe 20 cm long is closed at one end. Which harmonic mode of the pipe is resonantly excited by a source of ? (Sound velocity in air )

Ans: 3nd harmonic

27. A train standing at the outer signal of a railway station blows a whistle of frequency 400 Hz still air. The train begins to move with a speed of 10 m towards the platform. What is the frequency of the sound for an observer standing on the platform? (Sound velocity in air )

Ans:

28. The wave pattern on a stretched string is shown in Fig. Interpret what kind of wave this is and find its wavelength.

Stationary waves

Stationary Waves

Stationary waves

Ans: Stationary waves; 20cm

29. The pattern of standing waves formed on a stretched string at two instants of time are shown in Fig. The velocity of two waves superimposing to form stationary waves is and their frequencies are 256 Hz.

Tuning fork over reservoir

Standing Waves

Tuning fork over reservoir

(a) Calculate the time at which the second curve is plotted.

(b) Mark nodes and antinodes on the curve.

(c) Calculate the distance between A′ and C′.

Ans:

(a) .

(b) Nodes-A, B, C, D, E. Antinode s , .

(c)

30. A tuning fork vibrating with a frequency of 512Hz is kept close to the open end of a tube filled with water (Fig.). The water level in the tube is gradually lowered. When the water level is 17cm below the open end, maximum intensity of sound is heard. If the room temperature is , calculate

Tital: Reservoir

Tuning Fork over Reservoir

Tital: Reservoir

(a) Speed of sound in air at room temperature

(b) Speed of sound in air at

(c) If the water in the tube is replaced with mercury, will there be any difference in your observations?

Ans:

(a)

(b)

(c) Resonance will be observed at 17cm length of air column, only intensity of sound heard may be greater due to more complete reflection of the sound waves at the mercury surface.

31. Show that when a string fixed at its two ends vibrates in 1 loop, 2 loops, 3 loops and 4 loops, the frequencies are in the ratio 1:2:3:4.

Ans:

From the relation, , the result follows.

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