Waves - Live Session - NEET 2020Contact Number: 9667591930 / 8527521718

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1.

A tuning fork is used to produce resonance in a glass tube. The length of the air column in this tube can be adjusted by a variable piston. At room temperature of 27ºC two successive resonances are produced at 20cm and 73 cm of column length. If the frequency of the tuning fork is 320 Hz, the velocity of sound in air at 27ºC is

1. 330 m/s

2. 339 m/s

3. 300 m/s

4. 350 m/s

2.

The fundamental frequency in an open organ pipe is equal to the third harmonic of a closed organ pipe. If the length of the closed organ pipe is 20 cm, the length of the open organ pipe is

1. 13.2 cm

2. 8 cm

3. 16 cm

4. 12.5 cm

3.

Two cars moving in opposite directions approach each other with speed of 22 m/s and 16.5 m/s respectively. The driver of the first car blows a horn having a frequency 400 Hz. The frequency heard by the driver of the second car is [velocity of sound 340 m/s]

1. 350 Hz

2. 361 Hz

3. 411 Hz

4. 448 Hz

4.

The two nearest harmonics of a tube closed at one end and open at other end are 220 Hz and 260 Hz. What is the fundamental frequency of the system?

1. 10 Hz

2. 20 Hz

3. 30 Hz

4. 40 Hz

5.

The second overtone of an open organ pipe has the same frequency as the first overtone of a closed pipe L metre long. The length of the open pipe will be

1. L

2. 2L

3. $\frac{\mathrm{L}}{2}$

4. 4L

6.

Three sound waves of equal amplitudes have frequencies (n – 1), n, (n + 1). They superimpose to give beats. The number of beats produced per second will be

1. 1

2. 4

3. 3

4. 2

7.

A siren emitting a sound of frequency 800 Hz moves away from an observer towards a cliff at a speed of 15 ms^{–1}. Then, the frequency of sound that the observer hears in the echo reflected from the cliff is (Take velocity of sound in air = 330 ms–1) [NEET-2016]

1. 885 Hz

2. 765 Hz

3. 800 Hz

4. 838 Hz

8.

An air column, closed at one end and open at the other, resonates with a tuning fork when the smallest length of the column is 50 cm. The next larger length of the column resonating with the same tuning fork is

1. 200 cm

2. 66.7 cm

3. 100 cm

4. 150 cm

9.

A uniform rope of length L and mass m_{1} hangs vertically from a rigid support. A block of mass m_{2} is attached to the free end of the rope. A transverse pulse of wavelength $\mathrm{\lambda}$_{1} is produced at the lower end of the rope. The wavelength of the pulse when it reaches the top of the rope is $\mathrm{\lambda}$_{2}. The ratio $\mathrm{\lambda}$_{2}/$\mathrm{\lambda}$_{1} is

1. $\sqrt{\frac{{\mathrm{m}}_{1}+{\mathrm{m}}_{2}}{{\mathrm{m}}_{1}}}$

2. $\sqrt{\frac{{\mathrm{m}}_{1}}{{\mathrm{m}}_{2}}}$

3. $\sqrt{\frac{{\mathrm{m}}_{1}+{\mathrm{m}}_{2}}{{\mathrm{m}}_{2}}}$

4. $\sqrt{\frac{{\mathrm{m}}_{2}}{{\mathrm{m}}_{1}}}$

10.

A string is stretched between fixed points separated by 75.0 cm. It is observed to have resonant frequencies of 420 Hz and 315 Hz. There are no other resonant frequencies between these two. The lowest resonant frequency for this string is

1. 105 Hz

2. 155 Hz

3. 205 Hz

4. 10.5 Hz

11.

A source of sound S emitting waves of frequency 100 Hz and an observer O are located at some distance from each other. The source is moving with a speed of 19.4 ms^{–1} at an angle of 60° with the source observer line as shown in the figure. The observer is at rest. The apparent frequency observed by the observer (velocity of sound in air 330 ms^{–1}), is

1. 97 Hz

2. 100 Hz

3. 103 Hz

4. 106 Hz

12.

The fundamental frequency of a closed organ pipe of length 20 cm is equal to the second overtone of an organ pipe open at both the ends. The length of organ pipe open at both the ends is

1. 140 cm

2. 80 cm

3. 100 cm

4. 120 cm

13.

If n_{1}, n_{2} and n_{3} are the fundamental frequencies of three segments into which a string is divided, then the original fundamental frequency n of the string is given by

1. $\frac{1}{\mathrm{n}}=\frac{1}{{\mathrm{n}}_{1}}+\frac{1}{{\mathrm{n}}_{2}}+\frac{1}{{\mathrm{n}}_{3}}$

2. $\frac{1}{\sqrt{\mathrm{n}}}=\frac{1}{\sqrt{{\mathrm{n}}_{1}}}+\frac{1}{\sqrt{{\mathrm{n}}_{2}}}+\frac{1}{\sqrt{{\mathrm{n}}_{3}}}$

3. $\sqrt{\mathrm{n}}=\sqrt{{\mathrm{n}}_{1}}+\sqrt{{\mathrm{n}}_{2}}+\sqrt{{\mathrm{n}}_{3}}$

4. $\mathrm{n}={\mathrm{n}}_{1}+{\mathrm{n}}_{2}+{\mathrm{n}}_{3}$

14.

The number of possible natural oscillations of air column in a pipe closed at one end of length 85 cm whose frequencies lie below 1250 Hz are (velocity of sound = 340 ms^{–1})

1. 4

2. 5

3. 7

4. 6

15.

A speeding motorcyclist sees traffic jam ahead of him. He slows down to 36 km/hour. He finds that traffic has eased and a car moving ahead of him at 18 km/hour is honking at a frequency of 1392 Hz. If the speed of sound is 343 m/s, the frequency of the honk as heard by him will be

1. 1332 Hz

2. 1372 Hz

3. 1412 Hz

4. 1454 Hz

16.

A : The propagation of sound in air should be an isothermal process.

R : As air is bad conductor of heat, its temperature does not change by compression or expansion.

17.

A : Velocity of sound in air increases with increase in humidity.

R : Velocity of sound doesn’t depend upon medium.

18.

A : Longitudinal waves do not exhibit the phenomenon of polarisation.

R : In longitudinal waves medium particle vibrate in direction normal to the wave propagation.

19.

A : If a wave moving in a rarer medium, gets reflected at the boundary of a denser medium, then it encounters a sudden change in phase of $\mathrm{\pi}$.

R : If a wave propagating in a denser medium, gets reflected from rarer medium, then there will be no abrupt

phase change

20.

A : Speed of sound in moist air is more than its speed in dry air.

R : Dry air is denser than moist air at atmospheric pressure.

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