Which one of the following does not apply to sound waves in fluids?
- (a)They transport energy
- (b)They need a medium to travel
- (c)They are transverse
- (d)They travel faster in liquids than in gases
Correct — C, they are transverse. In a fluid, sound travels as a longitudinal wave — the particles vibrate back and forth along the direction of travel, creating compressions and rarefactions. Fluids cannot support the sideways (shear) motion that a transverse wave needs, so a sound wave in a gas or liquid is never transverse. Since the question asks which statement does NOT apply, the odd one out is (c).
- (a)They transport energy — This does apply. Like all waves, sound carries energy from source to receiver without any net transport of the medium itself, so this statement is true and is not the answer.
- (b)They need a medium to travel — This does apply. Sound is a mechanical wave: it requires a material medium (solid, liquid or gas) and cannot travel through a vacuum, so this statement is true.
- (d)They travel faster in liquids than in gases — This does apply. Sound moves faster where the medium is stiffer relative to its inertia; liquids have a far higher bulk modulus than gases, so sound is much faster in water (~1480 m/s) than in air (~343 m/s). The statement is true.
Sound is a mechanical wave produced by vibrating matter. In fluids it travels only as a longitudinal wave — the medium is alternately compressed and rarefied along the direction of propagation. Its speed depends on the medium's elasticity (bulk modulus) and density: v = sqrt(B/rho).
The question tests the one property that separates fluids from solids. A transverse wave needs the medium to resist shear (sideways) deformation, which only solids can do; that is why earthquake S-waves travel through rock but stop at the liquid outer core. In gases and liquids there is no shear rigidity, so sound is purely longitudinal — making 'they are transverse' the statement that does not apply.
- Sound in fluids (gases and liquids) is a longitudinal wave — vibration is along the direction of travel.
- Transverse mechanical waves need shear rigidity, which only solids possess; so sound in fluids is never transverse.
- Sound is mechanical and cannot travel through a vacuum — it needs a medium.
- Speed of sound: about 343 m/s in air, ~1480 m/s in water, and faster still in most solids.
Three statements apply; only 'they are transverse' does not — that is the answer.
- Forgetting that sound can be transverse in solids but only longitudinal in fluids.
- Assuming sound is always slowest or fastest in the same medium — it is fastest in solids, slowest in gases.
Asked as 'sound is a ______ wave', 'sound cannot travel in ______', or by comparing the speed of sound across media.
Consider the following statements: 1. A flute of smaller length gives waves of lower frequency. 2. Sound travels in rocks in the form of longitudinal elastic waves only. Which of the statements given above is/are correct?
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer(d) Neither 1 nor 2
Same concept — the nature of sound waves. UPSC's statement 2 (sound in rocks is longitudinal only) is actually false because solids also carry transverse S-waves, a neat contrast to fluids, where sound really is longitudinal only.
- practice — not a real PYQ
Sound waves cannot travel through:
- (a)Water
- (b)Steel
- (c)Vacuum
- (d)Air
Answer(c) Vacuum — sound is a mechanical wave and needs a material medium.
- practice — not a real PYQ
In which medium does sound generally travel fastest?
- (a)Vacuum
- (b)Air
- (c)Water
- (d)Steel
Answer(d) Steel — sound travels fastest in solids because of their high elasticity (stiffness).