UPSC Prelims 2023 · GS Paper I
Question 65: Geography
Consider the following statements :
- 1.In a seismograph, P waves are recorded earlier than S waves.
- 2.In P waves, the individual particles vibrate to and fro in the direction of wave propagation whereas in S waves, the particles vibrate up and down at right angles to the direction of wave propagation.
Which of the statements given above is/are correct?
AnswerUPSC’s answer is (c).
Both statements are correct. P waves are the fastest seismic waves, so a seismograph records them before S waves. P waves are longitudinal: particles vibrate to and fro along the direction of travel, producing compression and rarefaction. S waves are transverse: particles move at right angles to the direction of travel, such as up and down. S waves cannot pass through liquids, which led to the discovery of the liquid outer core.
- 1. P waves travel faster and arrive first.
- 2. P waves are longitudinal and S waves transverse.
How you could have got this
- If you were sure statement 1 is correct, (b) and (d) go: each of them leaves 1 out.
What you needed to know: P waves travel faster and arrive first.
- Then, if you were sure statement 2 is correct, (a) goes: it leaves 2 out.
What you needed to know: P waves are longitudinal and S waves transverse.
Only (c) is left. This one needs every statement.
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) Statement 2 is also correct, so '1 only' undercounts.
- (b) Statement 1 is also correct, so '2 only' undercounts.
- (c) Correct: both statements hold.
- (d) Both statements are true, so not neither.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2023, General Studies Paper I, Series A. The paper on upsc.gov.in.