Railway Science (Physics-Sound) (Part-II)

Total Questions: 42

31. If the wave completes 20 vibrations in 2.5 s, its frequency will be: [RRB Group D 27/09/2018 (Afternoon)]

Correct Answer: (b) 8 Hz
Solution:

As given, Time = 2.5 seconds, Vibrations = 20
so, frequency = π‘‰π‘–π‘π‘Ÿπ‘Žπ‘‘π‘–π‘œπ‘›s/π‘‡π‘–π‘še = 20/2.5
= 8 Hz.

32. The speed of sound waves is 700 ms⁻¹ . How much time will sound waves take to travel 7 km? [RRB Group D 04/10/2018 (Afternoon)]

Correct Answer: (a) 10 s
Solution:

Given that : Speed of sound waves (v) = 700 m/s,
Distance to be traveled (d) = 7 km = 7000 m
Using the formula:
Time, t = π·π‘–π‘ π‘‘π‘Žπ‘›π‘π‘’/𝑆𝑝𝑒𝑒d = 𝑑/v = 7000/700 = 10 s.

33. A wave traveling with a speed of 500 m/s has a wavelength of 20 m. The time interval will be _________ . [RRB Group D 05/10/2018 (Afternoon)]

Correct Answer: (d) 0.04 s
Solution:

34. The time period of a vibrating body is 0.04s, then the frequency of the wave is [RRB Group D 11/10/2018 (Morning)]

Correct Answer: (a) 25 Hz
Solution:

Given, Time = 0.04 s
As we know, Frequency =Β  1/π‘‡π‘–π‘še = 1/0.04 = = 25 Hz.

35. A sound wave travels at a speed of 340 m𝑠⁻¹ . If its wavelength is 2 cm, what is the frequency of the wave ? [RRB Group D 12/10/2018 (Evening)]

Correct Answer: (b) 17000 Hz
Solution:

Given that, velocity (v) = 340 m/s, wavelength (Ξ») = 2 cm = 0.02 m, frequency (f) = ? We know that, Velocity = Frequency Γ— Wavelength.
β‡’ 340 = f Γ— 0.02 β‡’ f = 340/0.02 = 17000Β  Hz.

36. The sound wave travels at a speed of 339 ms⁻¹ . If its wavelength is 1.5 cm, what will be the frequency of the wave? [RRB Group D 26/10/2018 (Evening)]

Correct Answer: (d) 22600 Hz
Solution:

Given : Speed (v)= 339 ms⁻¹ , Wavelength (λ) = 1.5 cm = 0.015 m.
Speed (v) = Frequency(f)Γ—wavelength (Ξ») 339 m/s = f Γ— 0.015 m
f = 339 π‘š/s/0.015 m = 339/0.015 Hz β‰ˆ 22600 Hz.

37. The speed of a wave in a medium is 600 m/s. If 600 waves are passing through a point in the medium in 1 minute, the wavelength is: [RRB Group D 1/11/2018 (Evening)]

Correct Answer: (b) 60 m
Solution:

Given that : Speed of a wave (v) = 600 m/s, n = 600 waves, Time (t) = 1 Γ— 60 sec = 60 sec.
∡ Frequency (f) = 𝑛/π‘‘π‘–π‘še = 600/60 = 10 Hz.
The relation between Frequency (f),
Wavelength (Ξ»), Speed (v) is given by,
v = Ξ»f
Ξ» = v/f = 600/10 = 60 m.

38. For an echo to be heard there should be a distance of about _______ between the source and the reflector. [RRB Group D 07/12/2018 (Morning)]

Correct Answer: (b) 17 m
Solution:

Echoes are heard due to the phenomenon of Reflection of sound waves. To hear the echo clearly, the reflecting object must be more than 17.2 meters from the sound source for the echo to be heard by a person standing at the source.

39. A sound wave has a frequency of 4 kHz and a wavelength of 40 cm. The time taken by the sound wave to travel a distance of 3.2 km is: [RRB ALP Tier - I (13/08/2018) Afternoon]

Correct Answer: (a) 2.0 s
Solution:

Any point on the sound wave covers a distance equal to Ξ» (wavelength) in time T (time period).
Velocity of sound = (Wavelength Γ— Frequency) = [(40 Γ— 10⁻²) (4 Γ— 10Β³ )] = 1600 m/s.
Time = Distance/Velocity = ( 3.2 Γ— 10Β³/1600) = 2.0 s.

40. An echo is returned in 3 s. What is the distance of the reflecting surface from the source, considering the speed of sound as 342 ms⁻¹? [RRB ALP Tier - I (21/08/2018) Evening]

Correct Answer: (c) 513 m
Solution:

Echo - The repetition of a sound by reflection of sound waves from a surface. Given: An echo is returned after time (t) = 3s
Speed of the sound (v) = 342 m/s
Now , Distance travelled (d) = v Γ—Β t
= 342 Γ— 3 = 1026m
New distance (D) = 2d.
2d = 1026 β‡’ d = 513 m