Q70 · UPSC Prelims 2015 · Set A · Geography

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Tides occur in the oceans and seas due to which among the following?1.Gravitational force of the Sun2.Gravitational force of the Moon3.Centrifugal force of the EarthSelect the correct answer using the code given below

A 1 only
B 2 and 3 only
C 1 and 3 only
D 1, 2 and 3

Correct answer: (d) 1, 2 and 3

Explanation

  1. A

    1 only

    The Sun’s gravity does contribute to tides (especially spring and neap inequality), but the Moon and Earth’s rotation/centrifugal effect also act. Statement 1 alone is incomplete. This option is not the key.

  2. B

    2 and 3 only

    Moon and centrifugal effect matter, but omitting the Sun drops a real tide-raising force. This option is therefore wrong.

  3. C

    1 and 3 only

    Sun plus centrifugal force without the Moon omits the principal tide-raising body. This option is not the key.

  4. D

    1, 2 and 3

    Tides arise from the gravitational pull of the Moon and the Sun and from the centrifugal force due to the Earth–Moon system’s rotation about the barycentre. All three statements are therefore correct. This option is the official key.

Summary. Official key is (d) 1, 2 and 3. Lunar gravity is the main tide-raising force; solar gravity modulates spring and neap tides. A centrifugal component in the Earth–Moon system completes the equilibrium-tide picture used in this question. No statement can be dropped. Options 1 only or any pair are incomplete.

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