UPSC Prelims 2000 · General Studies

Question 103: Science and Technology

Assertion (A): A man standing on a completely frictionless surface can propel himself by whistling. Reason (R): If no external force acts on a system, its momentum cannot change.

aBoth A and R are true, and R is the correct explanation of A

bBoth A and R are true, but R is not a correct explanation of A

cA is true, but R is false

dA is false, but R is true

AnswerThe answer is (a).

On a frictionless surface no outside horizontal force can act, so the man's total momentum stays zero. When he whistles, he pushes a stream of air in one direction, and that air gains momentum. To keep the total at zero, the man gains an equal momentum in the opposite direction. So he can propel himself, and R explains how.

A. A man on a frictionless surface can move by whistling, as the air expelled gives him a recoil.
R. With no external force the momentum cannot change; this explains the recoil.

How you could have got this

If you were sure statement 1 is correct, (d) goes. Then judge the second statement; if it is correct too, decide whether it explains the first.

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) Right: both are true, and conservation of momentum explains the recoil.
(b) R directly explains A, so it is the correct explanation.
(c) R is true, so it cannot be called false.
(d) A is true, so it cannot be called false.

Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2000, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.