NDA (II) 2018 General Ability Test · Question 52 of 146

Correct — A, Both the statements are individually true and Statement II is the correct explanation of Statement I. Statement I is true: pump the air out of a jar containing a ringing electric bell and the sound dies away to nothing even though the hammer can still be seen striking. Statement II is true as well: sound is a mechanical wave, a travelling train of compressions and rarefactions in which each layer of the medium squeezes the next and springs back, so it needs matter that has both elasticity and inertia. And here the second test succeeds. A vacuum contains no particles, so there is nothing to compress and nothing to spring back; the absence of a medium is exactly why the wave cannot get across. Statement II is not a second fact standing beside Statement I, it is the reason for it, and the code is (a).

• (b)Both the statements are individually true but Statement II is not the correct explanation of Statement I — This would be right if the two statements were unrelated truths. They are not — the elastic, medium-dependent nature of sound is precisely what rules out propagation through a vacuum, so the explanation test succeeds rather than fails.
• (c)Statement I is true but Statement II is false — Statement II is a standard and correct description of sound. Its speed in a medium is governed by the medium's elastic and inertial properties, which is why sound moves at roughly 343 m/s in air, about 1500 m/s in water and about 5000 m/s in steel.
• (d)Statement I is false but Statement II is true — Statement I is not false. No experiment has ever carried sound across a vacuum, and the bell-jar demonstration is the standard school proof of it.

Waves fall into two families. Mechanical waves — sound, water waves, waves on a string, seismic waves — are disturbances passed from particle to particle, so they exist only inside matter. Electromagnetic waves — light, radio, X-rays — are oscillating electric and magnetic fields that sustain each other and need no matter at all. Sound belongs to the first family and is longitudinal: the particles of the medium oscillate back and forth along the direction the wave travels, never leaving their average positions.

Work every item of this six-item block in two separate steps, never one. Step one, test each statement on its own for truth and ignore the other completely. Step two, and only if both survived step one, ask whether Statement II states the reason Statement I is true. Two true statements that merely sit side by side give (b), not (a). This item is the block's clearest (a), because the second statement is the textbook reason for the first rather than a companion fact. The contrast the examiner is really after is with light: an astronaut outside a spacecraft can see another astronaut but cannot hear him without a radio link, because light crosses the vacuum and sound cannot.

• Sound is a longitudinal mechanical wave and requires a material medium — solid, liquid or gas.
• Its speed depends on the elasticity and the density of the medium, and is greatest in solids and least in gases.
• In dry air at about 20 °C sound travels at roughly 343 m/s; in water about 1500 m/s; in steel about 5000 m/s.
• The wave transports energy through the medium; the particles themselves only vibrate about fixed positions and are not carried along.
• Light and other electromagnetic waves do travel through a vacuum, which is the standard contrast this question sets up.

• Assuming that because both statements are true the code must be (a) — here it happens to be, but only because the explanation test genuinely succeeds.
• Thinking sound travels fastest in air because air is the lightest of the three states; it is in fact slowest in gases.
• Forgetting that ultrasonic waves, for all their uses, are still sound and equally blocked by a vacuum.

NDA returns to this fact almost every session, usually as a 'which statement is not correct' item with 'sound can travel through vacuum' planted among true statements.

• practice — not a real PYQ

  An astronaut outside a spacecraft can see a fellow astronaut clearly but cannot hear him speak without a radio link. The reason is that

  • (a)sound travels too slowly in space
  • (b)sound requires a material medium while light does not
  • (c)the spacesuit absorbs all sound
  • (d)sound is reflected back by the vacuum

  Answer(b) sound requires a material medium while light does not — space is very nearly a vacuum, so there is nothing for the sound wave to travel through.

• practice — not a real PYQ

  Sound travels fastest through

  • (a)air
  • (b)water
  • (c)steel
  • (d)vacuum

  Answer(c) steel — a solid has far greater elasticity than a liquid or a gas, and sound cannot travel through a vacuum at all.