2007 General Studies, all questions

UPSC Prelims 2007 · General Studies

Question 110: Science and Technology

Assertion (A): A jet aircraft moving at Mach number equal to 1 travels faster at an altitude of 15 km than while moving at Mach number equal to 1 near sea level. Reason (R): The velocity of sound depends on the temperature of the surrounding medium.

aBoth A and R are true and R is the correct explanation of A
bBoth A and R are true but R is NOT the correct explanation of A
cA is true but R is false
dA is false but R is true

AnswerThe answer is (d).

Mach 1 means exactly the speed of sound. The speed of sound falls as the air gets colder. At 15 km the air is far colder (about minus 55 degrees C) than at sea level, so sound travels at roughly 295 m/s there against about 340 m/s near the ground. An aircraft at Mach 1 at 15 km therefore moves slower, so the Assertion is false, while the Reason is true.

  • A. A jet at Mach 1 moves slower at 15 km, where the air is colder.
  • R. The speed of sound depends on temperature.

How you could have got this

If you were sure statement 1 is not correct, only (d) is left: every other option says it is correct.

This follows an unofficial answer: UPSC has not published its key for this year, and if that answer is wrong, so is this path.

Which options go is worked out from UPSC’s key and the options, not by a model.

Why each option is right or wrong
  • (a) A is false, so the Reason cannot explain it.
  • (b) A is false, so both cannot be true.
  • (c) R is true, so this fails.
  • (d) Correct: A is false and R is true.

Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2007, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.