UPSC Prelims 2015 · GS Paper I
Question 70: Geography
Tides occur in the oceans and seas due to which among the following?
- 1.Gravitational force of the Sun
- 2.Gravitational force of the Moon
- 3.Centrifugal force of the Earth
Select the correct answer using the code given below.
AnswerUPSC’s answer is (d).
Tides arise from the combined gravitational pull of the Moon and the Sun on the oceans, together with the centrifugal force due to the Earth's motion around the common centre of mass of the Earth-Moon system. The Moon, being nearer, has about twice the effect of the Sun. Spring tides occur when the Sun, Moon and Earth are in line, and neap tides when they are at right angles. All three factors are accepted.
- 1. The Sun's gravity contributes to tides, as in spring and neap tides.
- 2. The Moon's gravity is the main tide-generating force.
- 3. Centrifugal force from the Earth's motion balances the gravitational pull on the far side.
How you could have got this
- If you were sure statement 1 is correct, (b) goes: it leaves 1 out.
What you needed to know: The Sun's gravity contributes to tides, as in spring and neap tides.
- Then, if you were sure statement 2 is correct, (a) and (c) go: each of them leaves 2 out.
What you needed to know: The Moon's gravity is the main tide-generating force.
Only (d) is left. You didn’t need statement 3.
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) Incomplete: the Moon's pull and centrifugal force also matter.
- (b) Incomplete: the Sun's pull also contributes.
- (c) Incomplete: the Moon's pull is the main cause.
- (d) Correct: the Sun's and Moon's gravity plus centrifugal force produce tides.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2015, General Studies Paper I, Series A. The paper on upsc.gov.in.