NCERT Physics Class 11 Exemplar Ch 8 Gravitation Part 3

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MCQ Type Questions

Q.14 Supposing Newton’s law of gravitation for gravitation forces between two masses at positions read

where is a constant of dimension of mass 2 and n is a number. In such a case,

(a) the acceleration due to gravity on earth will be different for different objects.

(b) none of the three laws of Kepler will be valid.

(c) only the third law will become invalid.

(d) for n negative, an object lighter than water will sink in water.

Answer: (a), (c), (d)

Q.15 Which of the following are true?

(a) A polar satellite goes around the earth’s pole in north south direction.

(b) A geostationary satellite goes around the earth in east-west direction.

(c) A geostationary satellite goes around the earth in west-east direction.

(d) A polar satellite goes around the earth in east-west direction.

Answer: (a), (c)

Q.16 The Centre of mass of an extended body on the surface of the earth and its Centre of gravity

(a) are always at the same point for any size of the body.

(b) are always at the same point only for spherical bodies.

(c) can never be at the same point.

(d) is close to each other for objects, say of sizes less than 100 m.

(e) both can change if the object is taken deep inside the earth.

Answer: (d)

Very Short Answer

Q. 17 Molecules in air in the atmosphere are attracted by gravitational force of the earth. Explain why all of them do not fall into the earth just like an apple falling from a tree.

Answer: Molecules experience the vertically downward force due to gravity just like an apple falling from a tree. Due to thermal motion, which is random, their velocity is not in the vertical direction. The downward force of gravity causes the density of air in the atmosphere close to earth higher than the density as we go up.

Q.18 Give one example each of central force and non-central force.

Answer: Central force; gravitational force of a point mass, electrostatic force due to a point charge. Non-central force: spin-dependent nuclear forces, magnetic force between two current carrying loops.

Q.19 Draw areal velocity versus time graph for mars.

Answer:

Areal velocity vs time

Areal Velocity vs Time

Areal velocity vs time

Q. 20 What is the direction of areal velocity of the earth around the sun?

Answer: It is normal to the plane containing the earth and the sun as areal

Velocity

Q.21 How is the gravitational force between two-point masses affected when they are dipped in water keeping the separation between them the same?

Answer: It remains same as the gravitational force is independent of the medium separating the masses.

Q.22 Is it possible for a body to have inertia but no weight?

Answer: Yes, a body will always have mass but the gravitational force on it can be zero; for example, when it is kept at the centre of the earth.

Q.23 We can shield a charge from electric fields by putting it inside a hollow conductor. Can we shield a body from the gravitational influence of nearby matter by putting it inside a hollow sphere or by some other means?

Answer: No

Q.24 An astronaut inside a small spaceship orbiting around the earth cannot detect gravity. If the space station orbiting around the earth has a large size, can he hope to detect gravity?

Answer: Yes, if the size of the spaceship is large enough for him to detect the variation in g.

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