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Waves

Multiple Choice Questions

81.

A source of frequency 240 Hz is moving towards an observer with a velocity of 20 m/s. The observer is now moving towards the source with a velocity of 20 m/s. Apparent frequency heard by observer, if velocity of sound is 340 m/s, is

268 Hz
270 Hz
360 Hz
240 Hz

82.

Assertion: Soldiers are asked to break steps while crossing the bridge.

Reason: The frequency of thick marching may be equal to the natural frequency of bridge and may lead to resonance which can break the bridge.

If both assertion and reason are true and reason is the correct explanation of assertion.
If both assertion and reason are true but reason is not the correct explanation of assertion.
If assertion is true but reason is false.
If assertion is false but reason is true.

83.
The observed wavelength of light coming from a distant galaxy is found to be increased by 0.5% as compared with that coming from a
terrestrial source. The galaxy is

stationary with respect to the earth

approaching the earth with velocity of light

receding from the earth with velocity of light

receding from the earth with a velocity
equal to 1.5 10⁶ m/s.

receding from the earth with a velocity
equal to 1.5 10⁶ m/s.

As the wavelength is increasing it is redshift of light due to Doppler's effect and the star is moving away from earth.

Now, $\frac{Δλ}{λ} = \frac{v}{c}$

$\frac{Δλ}{λ} = \frac{0.5}{100}$

$v = \frac{0.5}{100} \times c$

( $∵$ c=3 $\times 10^{8}$ )

v=1.5 $\times 10^{6}$

84.

When a certain volume of water is subjected to 100 kPa pressure, the volume of water decreases by 0.005%. The speed of sound in water must be

140 m/s
300 m/s
1400 m/s
5000 m/s

85.

A man stands in a narrow, steep-sided valley. When he shouts he hears two echoes, one after 1s and other after 2s.If the velocity of sound in air is 330 m/s, the width of the valley is

330 m
495 m
660 m
990 m

86.

A cylindrical tube, open at both ends emits a fundamental frequency f in air. The tube is dipped vertically in water, so that half of it is in water. The fundamental frequency of air column is now

f/2
3f/4
f
2f

87.

The end correction of a resonance column is 1.0 cm. If the shortest length resonating with the tuning fork is 15.0 cm, the next resonating length will be

31 cm
45 cm
46 cm
47 cm

88.

A tuning fork A produces 4 beats/s with another tuning fork B of frequency 320 Hz. On filling one of the prongs of A, 4 beats/s are again heard when sounded with the same fork B. Then, the frequency of the fork A before the filling is

328 Hz
316 Hz
324 Hz
320 Hz

89.

When the length of the vibrating segment of a sonometer wire is increased by 1%, the percentage change in its frequency is

100/101
99/100
1
2

90.

The equation of a simple harmonic wave is given by $y = 5 \sin \frac{π}{2} (100 t - x)$ where x and y are in metre and time is in second. The period of the wave in second will be

0.04
0.01
1
5