CDS (II) 2023 General Knowledge · Question 5 of 120

Correct — A, greater than the speed of sound. A body moving through air pushes pressure disturbances ahead of itself, and those disturbances travel outwards at the speed of sound. So long as the body is slower than sound the disturbances outrun it and spread away smoothly. Once the body goes faster than sound it overtakes its own pressure waves, which pile up and merge into a single sharp front — a shock wave, trailing behind the body as a cone. When that cone sweeps past a listener the pressure jumps up and then back down, and the ear hears it as the crack or double crack called a sonic boom. The condition is therefore purely one of speed relative to sound in the same medium, which is what a Mach number greater than one means. At sea level and 20 °C sound travels at roughly 1,192 km per hour, so that is the threshold an aircraft has to cross.

• (b)greater than the speed of light — Nothing carrying energy or information moves faster than light in a vacuum, and in any case a sonic boom is an air-pressure phenomenon governed by the speed of sound, which is about a million times smaller.
• (c)lesser than the speed of sound — Below Mach 1 the pressure waves stay ahead of the source and disperse gently. That is ordinary subsonic flight — audible as a rising and falling engine note, not as a bang.
• (d)equal to the speed of sound — At exactly Mach 1 the waves bunch up into a flat front travelling with the aircraft rather than a cone trailing behind it. This is the transonic drag rise pilots call the sound barrier; the boom that reaches the ground belongs to speeds past it.

Sound in air is a longitudinal pressure wave whose speed depends on the medium, not on the source — about 343 metres per second in air at 20 °C, roughly 1,500 in water and about 5,000 in steel. The ratio of a body's speed to the speed of sound in the same medium is its Mach number: below one is subsonic, one is transonic, above one supersonic and above five hypersonic. A supersonic body cannot get rid of its pressure disturbances forwards, so they coalesce into a conical shock front whose half-angle satisfies sin α = v_sound / v_object — the faster the object, the narrower the cone.

The item is one line long and hinges on a single word, so the only real risk is misreading 'greater than' as 'equal to'. Keep the physical picture rather than the phrase: the boom exists because the source has outrun its own sound. Two clarifications save marks elsewhere. First, the boom is not a one-off event at the moment of crossing Mach 1 — the shock cone trails a supersonic aircraft continuously, and anyone the cone passes over hears a boom, which is why supersonic airliners were barred from overland routes. Second, a listener hears a double bang because the pressure profile has two steps, one as it rises and one as it returns to normal. Sonic booms have been in the Indian news through the Gaganyaan and hypersonic-missile programmes and, in 2023, through attempts to revive quiet supersonic passenger flight, but the physics in the stem has not moved since Ernst Mach's photographs of the 1880s.

• A sonic boom is produced when a source moves through the air faster than sound travels in that air, that is at a Mach number greater than one.
• The speed of sound in air is about 343 metres per second at 20 °C, roughly 1,192 km per hour.
• The pressure waves pile up into a conical shock front whose half-angle obeys sin α = v_sound / v_object, so the cone narrows as the object goes faster.
• Ground observers usually hear a double boom, because the pressure first jumps up and then drops back through normal.
• The speed of sound depends on the medium and its temperature — it rises with temperature in air, and it is far higher in water and higher still in steel.

• Thinking the boom happens only once, at the instant the aircraft crosses Mach 1; the shock cone follows it for as long as it stays supersonic.
• Bringing the speed of light into a question that is entirely about pressure waves in air.
• Assuming the speed of sound is a fixed number — it changes with the medium and rises with temperature.

As a definition item on the condition for a boom, as a Mach-number classification, or as a comparison of the speed of sound in different media.

• practice — not a real PYQ

  An aircraft is said to be flying at Mach 2. This means that its speed is

  • (a)twice the speed of light
  • (b)twice the speed of sound in air
  • (c)half the speed of sound in air
  • (d)2 kilometres per second

  Answer(b) twice the speed of sound in air — the Mach number is the ratio of the body's speed to the speed of sound in the same medium, so Mach 2 is supersonic.

• practice — not a real PYQ

  In which one of the following media does sound travel fastest?

  • (a)Air at 0 °C
  • (b)Air at 30 °C
  • (c)Water
  • (d)Steel

  Answer(d) Steel — sound travels fastest in solids, where the particles are most tightly coupled; steel carries it at roughly 5,000 metres per second against about 1,500 in water and 343 in air at 20 °C.