Q50. Sound travels fastest in which of these?
[WBCS Mains 2021 paper, held May 2022]
- (A) Air
- (B) Water
- (C) Vacuum
- (D) Steel
Answer: (D) Steel
Explanation: Sound cannot travel through vacuum, since no elastic particles exist there. Among air, water and steel, steel has a comparatively large elastic modulus and therefore transmits sound faster under normal conditions than the gas or liquid.
Q49. In ultrasonic machining, abrasive particles move mainly in what direction relative to the workpiece?
[MPPSC Assistant Engineer, Mechanical, 3 July 2022]
- (A) Tangential
- (B) Oscillatory around the whole part
- (C) Circular
- (D) Normal to the workpiece
Answer: (D) Normal to the workpiece
Explanation: Ultrasonic machining uses a tool vibrating at ultrasonic frequencies, driving abrasive particles against the surface in the tool’s approach direction. The normal impacts remove minute fragments of hard, brittle material. Lateral motion is not the intended principal cutting action.
Q48. Which statement about the propagation of sound in air is true?
[UPSC NDA II, September 2021]
- (A) Travels in vacuum and is transverse
- (B) Cannot travel in vacuum and is longitudinal
- (C) Travels in vacuum and is longitudinal
- (D) Cannot travel in vacuum and is transverse
Answer: (B) Cannot travel in vacuum and is longitudinal
Explanation: In air, vibrating particles pass pressure changes to neighboring particles, so a material medium is essential. The molecules oscillate parallel to the direction of travel, establishing that an ordinary sound wave is longitudinal rather than transverse.
Q47. Which of the following cannot serve as a unit of sound-wave frequency?
[UPSC NDA I, April 2021]
- (A) dB
- (B) s⁻¹
- (C) Hz
- (D) min⁻¹
Answer: (A) dB
Explanation: Frequency is a count of repetitions per unit time, so hertz, inverse seconds and inverse minutes can express it. A decibel, however, is a logarithmic level or ratio, commonly associated with sound intensity rather than cycles per time.
Q46. Beats appear when two harmonic waves have frequencies that are?
[UPSC NDA I, April 2021]
- (A) Exactly equal
- (B) Very far apart
- (C) Integer multiples
- (D) Nearly equal
Answer: (D) Nearly equal
Explanation: When two waves of nearly equal frequency overlap, their relative phase changes slowly. At some moments they reinforce each other and at others partially cancel, making the loudness rise and fall. The beat frequency equals the absolute difference between frequencies.
Q45. A sound wave of frequency 4 kHz and wavelength 30 cm travels 2.4 km. How much time does it take?
[UPSC CDS II, 2021]
- (A) 2.0 s
- (B) 0.6 s
- (C) 1.0 s
- (D) 8.0 s
Answer: (A) 2.0 s
Explanation: The wavelength is 0.30 m and frequency is 4000 cycles per second. Consequently v = fλ = 1200 m/s. For a distance of 2400 m, the elapsed time is 2400/1200 = 2 seconds.
Q44. What is approximately the lowest sound frequency audible to humans?
[RRB NTPC CBT-I, 11 March 2021]
- (A) 10 Hz
- (B) 500 Hz
- (C) 50 Hz
- (D) 20 Hz
Answer: (D) 20 Hz
Explanation: Normal young human hearing begins at about 20 Hz and extends upward to roughly 20 kHz. Sounds lower than 20 Hz are classed as infrasound. Hearing limits are approximate and depend on age and individual sensitivity.
Q43. The speed of sound in air at around 20°C is approximately?
[RRB NTPC CBT-I, 27 February 2021]
- (A) 434 m/s
- (B) 343 m/s
- (C) 383 m/s
- (D) 373 m/s
Answer: (B) 343 m/s
Explanation: At roughly 20 degrees Celsius in ordinary air, the usual reference speed is 343 metres per second. Cooling the air reduces the value, while heating it increases the value. Humidity also affects speed slightly by changing air composition.
Q42. Which statement correctly defines wavelength?
[SSC CGL 2020 Tier I, held 23 August 2021]
- (A) Parallel particle oscillations define transverse waves
- (B) Frequency counts oscillations per minute by definition
- (C) Perpendicular oscillations define longitudinal waves
- (D) Shortest distance between two points in the same phase is one wavelength
Answer: (D) Shortest distance between two points in the same phase is one wavelength
Explanation: Wavelength is the smallest spatial separation between consecutive points that are in the same phase, such as adjacent compressions. Parallel medium-particle motion defines longitudinal waves, not transverse waves. Frequency is conventionally expressed as cycles per second.
Q41. Why is thunder heard after its accompanying flash of lightning?
[SSC MTS 2020 paper, held 27 October 2021]
- (A) Sound travels much slower than light
- (B) Sound travels three times faster
- (C) Sound travels twice as fast
- (D) Sound and light travel equally fast
Answer: (A) Sound travels much slower than light
Explanation: Light from lightning reaches an observer almost immediately over ordinary storm distances. Sound travels through air at only about 340 m/s, so the thunder takes noticeably longer. This difference allows a rough estimate of how far away a storm is.
