When sound waves are propagated through a medium, the physical quantity/quantities transmitted is/are
- (a)matter only
- (b)energy only
- (c)energy and matter only
- (d)energy, momentum and matter
Correct — B, energy only. A sound wave is a disturbance travelling through a material medium, and the essential thing to understand about any wave is that the disturbance travels while the medium stays put. When you speak, the layers of air near your mouth are alternately squeezed together and pulled apart, and each layer passes that squeeze on to the next; every individual air molecule merely oscillates back and forth about a fixed mean position and ends up roughly where it began. Nothing material makes the journey from the speaker's mouth to the listener's ear — if it did, a shout would deliver a puff of the speaker's breath across a room. What does make the journey is the energy of the vibration, which is why sound can rattle a windowpane at a distance and why a loud enough sound can be felt as well as heard. One honest refinement for a student who wants to go further: a wave does also carry momentum, and acoustic radiation pressure is a genuine, measurable effect. But the only option that mentions momentum bundles it with matter, and matter is definitely not transmitted, so option (b) is the best available statement and is the key's answer.
- (a)matter only — Exactly backwards. Matter is the one thing a wave does not transport. The particles of the medium are needed to pass the disturbance along, but they oscillate in place rather than travelling with it.
- (c)energy and matter only — Correct about energy but wrong to add matter. Reasoning that air must move because sound needs air is a natural mistake, but a moving mass of air is a wind, not a sound wave, and the two are quite different things.
- (d)energy, momentum and matter — The most sophisticated-looking option, and it does contain a truth that the others miss, since waves genuinely carry momentum as well as energy. It fails all the same because it also asserts that matter is transmitted, which is precisely what a wave does not do.
A wave is a travelling disturbance that carries energy through a medium without carrying the medium along with it. Sound is a longitudinal mechanical wave: the particles of the medium vibrate back and forth along the same line in which the wave advances, producing regions of compression where they are crowded together and rarefaction where they are spread apart. Because it needs particles to pass the disturbance on, sound cannot cross a vacuum.
The discrimination the examiner wants is between the transport of energy and the transport of matter, and it is worth having a concrete image ready. Picture a row of people passing a shove down a line: the shove reaches the far end, but nobody has walked there. That is a longitudinal wave. Option (d) is included to reward and then punish a candidate who knows about wave momentum, because the word 'matter' quietly spoils it — always read an option to its final word before marking it. NDA itself has keyed elsewhere that waves carry energy and exert pressure, so the momentum idea is not being denied here; it is only being kept out of a statement that also claims matter moves.
- Sound is a longitudinal mechanical wave, travelling as compressions and rarefactions of the medium.
- The particles of the medium oscillate about fixed mean positions and are not carried along with the wave.
- The wave transports energy, which is why a distant sound can be felt as a vibration.
- Sound requires a material medium and cannot travel through a vacuum.
- The speed of sound depends on both the elasticity and the density of the medium, and is greatest in solids and least in gases.
The disturbance advances while the medium stays in place, which is why only energy is transmitted.
- Assuming that because sound needs air, air must travel with the sound.
- Marking the longest and most technical option without reading it to its final word.
Sound supplies two or three NDA GAT items in most sessions, usually one on what a wave is and one on pitch or loudness, so keep the definition of a wave as clear as the definitions of its properties.
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
Pushes the same idea further — sound in a solid travels as elastic waves, and not only longitudinal ones, since a solid can also carry transverse waves that a gas cannot.
Which of the following statements about electromagnetic waves, sound waves and water waves is/are correct ? 1. They exhibit reflection 2. They carry energy 3. They exert pressure 4. They can travel in vacuum Select the correct answer using the code given below :
- (a) 1, 2 and 3
- (b) 2 and 4
- (c) 1 and 3 only
- (d) 1 only
Answer(a) 1, 2 and 3
NDA's own key confirming both halves of the point made here — waves carry energy and even exert pressure, but only electromagnetic waves can cross a vacuum, and none of them carries matter along.
- practice — not a real PYQ
Sound waves in air are
- (a)transverse waves
- (b)longitudinal waves
- (c)electromagnetic waves
- (d)stationary waves
Answer(b) longitudinal waves — the air particles vibrate along the same line in which the wave travels.
- practice — not a real PYQ
Sound cannot travel through a vacuum because
- (a)a vacuum absorbs all the energy of the wave
- (b)there are no particles to pass the disturbance along
- (c)the speed of sound becomes infinite there
- (d)sound is an electromagnetic wave
Answer(b) there are no particles to pass the disturbance along — a mechanical wave needs a material medium.