# NCERT Class 11 Physics Solutions: Chapter 8 – Gravitation-Part 2

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Question 8.3:

Suppose there existed a planet that went around the sun twice as fast as the earth. What would be its orbital size as compared to that of the earth?

Lesser by a factor of times

Explanation:

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

Orbital radius of the Earth in its orbit,

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

From Kepler’s third law of planetary motion, we can write:

Hence, the orbital radius of the planet will be smaller than that of the Earth.

Question 8.4:

, one of the satellites of Jupiter, has an orbital period of days and the radius of the orbit is . Show that the mass of Jupiter is about one-thousandth that of the sun.

Orbital period of

Satellite is revolving around the Jupiter

Mass of the latter is given by the relation:

Where,

Mass of Jupiter

Universal gravitational constant

Orbital period of the earth,

Mass of Sun is given as:

Hence, it can be inferred that the mass of Jupiter is about one-thousandth that of the Sun.

Question 8.5:

Let us assume that our galaxy consists of stars each of one solar mass. How long will a star at a distance of from the galactic centre take to complete one revolution? Take the diameter of the Milky Way to be