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At which temperature the speed of sound in hydrogen will be the same as that of the speed of sound in oxygen at 100°C :

– 148°C

– 212.5°C

– 317.5°C

– 249.7°C

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Explanation

Speed of sound in gases is $v = \sqrt{\frac{γ R T}{M}}$ ⇒ $T \propto M$

(Because v, γ-constant). Hence $\frac{T_{H_{2}}}{T_{O_{2}}} = \frac{M_{H_{2}}}{M_{O_{2}}}$

⇒ $\frac{T_{H_{2}}}{(273 + 100)} = \frac{2}{32}$ ⇒ $T_{H_{2}} = 23 .2 K = - 249 .7 ° C$

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The phase difference between two points separated by 1m in a wave of frequency 120 Hz is 90°. The wave velocity is :

180 m/s

240 m/s

480 m/s

720 m/s

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Explanation

Path difference $Δ = \frac{λ}{2 π} \times ϕ$ ⇒ $1 = \frac{λ}{2 π} \times \frac{π}{2}$ ⇒ λ = 4m

Hence $v = n λ$ = $120 \times 4 = 480 m / s$

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The echo of a gunshot is heard 8 sec. after the gun is fired. How far from him is the surface that reflects the sound? (velocity of sound in air = 350 m/s) :

1400 m

2800 m

700 m

350 m

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A man sets his watch by the sound of a siren placed at a distance 1 km away. If the velocity of sound is 330 m/s

His watch is set 3 sec. faster

His watch is set 3 sec. slower

His watch is set correctly

None of the above

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Explanation

$Time = \frac{Distance}{Velocity} = \frac{1000}{330} = 3 .03 s e c$ .

Sound will be heard after 3.03 sec. So his watch is set 3sec, slower.

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Velocity of sound in air is

Faster in dry air than in moist air

Directly proportional to pressure

Directly proportional to temperature

Independent of pressure of air

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Explanation

$v = \sqrt{\frac{γ P}{ρ}}$ ; as P changes, ρ also changes. Hence $\frac{P}{ρ}$ remains constant so speed remains constant.

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The frequency of a sound wave is n and its velocity is v. If the frequency is increased to 4n, the velocity of the wave will be

v/4

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Explanation

Velocity of sound is independent of frequency. Therefore it is same (v) for frequency n and 4n.

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The speed of a wave in a certain medium is 960 m/s. If 3600 waves pass over a certain point of the medium in 1 minute, the wavelength is :

2 metres

4 metres

8 metres

16 metres

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Explanation

$n = \frac{3600}{60} = 60$ Hz ⇒ $λ = \frac{v}{n} = \frac{960}{60} = 16 m$

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Speed of sound at constant temperature depends on :

Pressure

Density of gas

Above both

None of the above

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Explanation

Speed do sound, doesn’t depend up on pressure and density medium.

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Sound velocity is maximum in :

H₂

N₂

O₂

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Explanation

$v = \sqrt{\frac{γ R T}{M}}$ ⇒ $v \propto \frac{1}{\sqrt{M}}$ .

Since M is minimum for H₂ so sound velocity is maximum in H₂.

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The minimum distance of reflector surface from the source for listening the echo of sound is :

28 m

18 m

19 m

16.5 m

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Explanation

$2 d = v \times t$ , where v = velocity of sound = 332 m/s

t = Persistence of hearing = $\frac{1}{10} \sec$ .

⇒ $d = \frac{v \times t}{2} = \frac{332 \times \frac{1}{10}}{2} = 16 .5 m$

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