__Question 8.13__:^{8}km.

__Solution__:*r*= 1.5 × 10

^{11}m

Time taken by the Earth to complete one revolution around the Sun,

*T*= 1 year = 365.25 days

= 365.25 × 24 × 60 × 60 s

Universal gravitational constant, G = 6.67 × 10

^{–11}Nm

^{2}kg

^{–2}

Thus, mass of the Sun can be calculated using the relation,

M = 4π

^{2}r

^{3}/ GT

^{2}

= 4 X 3.14

^{2}X (1.5 X 10

^{11})

^{3}/ [ 6.67 X 10

^{-11}X (365.25 X 24 X 60 X 60)

^{2}]

= 2 X 10

^{30}kg

Hence, the mass of the Sun is 2 × 10

^{30}kg.

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