# NCERT Exemplar Class 9 Science Chapter 10 Gravitation Solutions

NCERT Exemplar Solutions for Class 9 Science Chapter 10 Gravitation covers all the important questions and answers as well as advanced level questions. It helps in learning about the Universal Law of Gravitation, freefall, mass, inertia, greater the mass the greater will be the inertia, weight, mass of an object is constant, buoyancy, buoyant law, thrust and pressure.

The NCERT Exemplar solutions for class 9 science is very important in the examination. NCERT Exemplar Solutions for Class 9 Science Chapter 10 Gravitation is provided by our experts. They prepared the best solutions which help the students in understanding the solutions in an easy way. This chapters also covers the other topics like Archimedes principle i.e. weight of the object immersed in water is equal to the weight of the fluid that is displaced by the object, law of basic fluid mechanics and relative density.

 Chapter Name Chapter 10 Gravitation Book Title NCERT Exemplar for Class 9 Science Related Study NCERT Solutions for Class 9 Science Chapter 10 GravitationRevision Notes for Class 9 Science Chapter 10 GravitationMCQ for Class 9 Science Chapter 10 GravitationImportant Questions for Class 9 Science Chapter 10 Gravitation Topics Covered MCQShort Answers QuestionsLong Answers Questions

### Multiple Choice Questions

1. Two objects of different masses falling freely near the surface of Moon would :
(a) Have same velocities at any instant
(b) Have different accelerations
(c) Experience forces of same magnitude
(d) Undergo a change in their inertia

Solution

(a) Have same velocities at any instant

Acceleration of an object depends on acceleration due to gravity irrespective of its mass.

2. The value of acceleration due to gravity :
(a) Is same on equator and poles
(b) Is least on poles
(c) Is least on equator
(d) Increases from pole to equator

Solution

(c) Is least on equator

As the distance between surface of the earth and its centre is more on equator than in poles, the acceleration due to gravity is least at the equator.

3. The gravitational force between two objects is F. If masses of both objects are halved without changing distance between them, then the gravitational force would become :
(a) F/4
(b) F/2
(c) F
(d) 2 F

Solution

(a) F/4

4. A boy is whirling a stone tied with a string in an horizontal circular path. If the string breaks, the stone :
(a) Will continue to move in the circular path
(b) Will move along a straight line towards the centre of the circular path
(c) Will move along a straight line tangential to the circular path
(d) Will move along a straight line perpendicular to the circular path away from the boy

Solution

(c) Will move along a straight line tangential to the circular path

At any instance of time an object in circular motion will tend to be in rectilinear motion. The object keeps on moving due to the centripetal force and it moves along a straight line tangential to the circular path when strings break.

5. An object is put one by one in three liquids having different densities. The object floats with 1/9, 2/11 and 3/7 parts of their volumes outside the liquid surface in liquids of densities d1, d2 and drespectively. Which of the following statement is correct?

(a) d1> d2> d3

(b) d1> d2< d3

(c) d1< d2> d3

(d) d1< d2< d3

Solution

(d) d1< d2< d3

6. In the relation F = G M m/d2, the quantity G :
(a) Depends on the value of g at the place of observation
(b) Is used only when the Earth is one of the two masses
(c) Is greatest at the surface of the Earth
(d) Is universal constant of nature

Solution

(d) Is universal constant of nature

G is called as Newton’s constant. It is the force of gravity on a body. Value of G is 6.66×10-11Nm2kg-2.

7. Law of gravitation gives the gravitational force between :
(a) The Earth and a point mass only
(b) The Earth and Sun only
(c) Any two bodies having some mass
(d) Two charged bodies only

Solution

(c) Any two bodies having some mass

8. The value of quantity G in the law of gravitation :
(a) Depends on mass of Earth only
(b) Depends on radius of Earth only
(c) Depends on both mass and radius of Earth
(d) Is independent of mass and radius of the Earth

Solution

(d) Is independent of mass and radius of the Earth

As G is the universal constant, it is independent of mass and radius of the earth.

9. Two particles are placed at some distance. If the of each of the two particles is doubled, keeping the distance between them unchanged, the value of gravitational force between them will be :
(a) 1/4 times
(b) 4 times
(c) 1/2 times
(d) Unchanged

Solution

(b) 4 times

10. The atmosphere is held to the Earth by :
(a) Gravity
(b) Wind
(c) Clouds
(d) Earth’s magnetic field

Solution

(a) Gravity

11. The force of attraction between two unit point masses separated by a unit distance is called :
(a) Gravitational potential
(b) Acceleration due to gravity
(c) Gravitational field
(d) Universal gravitational constant

Solution

(d) Universal gravitational constant

Here, point masses are separated by unit distance.

That is m1, m2 and r = 1

Therefore, F = G which is a universal constant.

12. The weight of an object at the centre of the Earth of radius R is :
(a) Zero
(b) Infinite
(c) R times the weight at the surface of the Earth
(d) 1/R2 times the weight at surface of the Earth

Solution

(a) Zero

The acceleration due to gravity is zero at the centre of the earth. As the weight is the product of mass and gravity. Weight of the object at the centre of the earth will be zero.

13. An object weighs 10 N in air. When immersed fully in water, it weighs only 8 N. The weight of the liquid displaced by the object will be :
(a) 2 N
(b) 8 N
(c) 10 N
(d) 12 N

Solution

(a) 2 N

Object of weight displace d by liquid = weight in air-weight in liquid

=10N - 8N

= 2N

14. A girl stands on a box having 60 cm length, 40 cm breadth and 20 cm width in three ways. In which of the following cases, pressure exerted by the brick will be :
(a) Maximum when length and breadth form the base
(b) Maximum when breadth and width form the base
(c) Maximum when width and length form the base
(d) The same in all the above three cases

Solution

(b) Maximum when breadth and width form the base

Pressure is inversely proportional to the surface area. So, the pressure will be maximum when the surface area is minimum.

15. An apple falls from a tree because of gravitational attraction between the Earth and apple. If F1 is the magnitude of force exerted by the Earth on the apple and F2 is the magnitude of force exerted by apple on Earth, then :
(a) F1 is very much greater than F2
(b) F2 is very much greater than F1
(c) F1 is only a little greater than F2
(d) F1 and F2 are equal

Solution

(d) F1 and F2 are equal

Newton’s third law of motion states that for every action there is an equal and opposite reaction.

16. What is the source of centripetal force that a planet requires to revolve around the Sun? On what factors does that force depend?

Solution

Gravitational force provides the necessarycentripetal force which makes the planetsrevolve around the sun.

1. Mass of the planet and the Sun, i.e., depends on the product of the masses of the planet and the Sun.
2. Distance between the planet and the Sun, i.e. depends on the square of distance between the planet and the Sun.

17. On the Earth, a stone is thrown from a height in a direction parallel to the Earth’s surface while another stone is simultaneously dropped from the same height. Which stone would reach the ground first and why?

Solution

Both the stones will reach the ground at the same time because the two stones fall from the same height.
As both the stones will have initial velocity (u) = 0,
a = g (acceleration due to gravity), distance (s) = s; t = ?
Using equation of motion,

18. Suppose gravity of Earth suddenly becomes zero, then in which direction will the Moon begin to move if no other celestial body affects it?

Solution

If gravity of Earth suddenly becomes zero then the moon will start moving in a straight line tangent to its circular path. This change happens because the circular motion of the moon is due to the centripetal force provided by the gravitational force of the Earth.

19. Identical packets are dropped from two aeroplanes, one above the equator and the other above the north pole, both at height h. Assuming, all conditions are identical, will those packets take same time to reach the surface of Earth. Justify your answer.

Solution

The value of acceleration due to gravity is constant. But it depends upon the surface of the earth. The surface of the earth varies from place to place as it is not completely spherical. The earth is flattened at the poles, hence the value of ‘g’ is maximum at the poles. There is bulge at the equator which has the minimum value of ‘g’. The value of ‘g’ value increases as we move towards the poles. Therefore, the packets fall gradually at the equator in comparison to the poles. This is the reason why the packets stay in air for longer when dropped at the equator.

20. The weight of any person on the Moon is about 1/6 times that on the Earth. He can lift a mass of 15 kg on the Earth. What will be the maximum mass, which can be lifted by the same force applied by the person on the Moon ?

Solution

Weight of person on moon = 1/6 thof weight on earth
Therefore, ‘g’ on moon = 1/6th ‘g’ on earth
The force that is applied by the man to lift mass ‘m’ is
F = mg = 15g (on earth)
If he can lift a certain mass ‘m’ by applying the same force on moon, then
m = F ÷ g
= 15× 6
=90kg
Therefore on moon a person can lift a mass 6 times heavier than that on earth.

21. Calculate the average density of the Earth in terms of g, G and R. ‘g’ is related to Earth’s mass “M” and radius “R”.

Solution

22. The Earth is acted upon by gravitation of Sun, even thought it does not fall into the Sum. Why ?

Solution

According to Newton’s first law of motion, an object in motion tries to move in straight-line at a constant speed unless external pressure is not applied. When the Earth comes close to the Sun that has a large gravitational force, the path of the Earth is altered due to the unbalanced force of gravity on it. The Sun exerts an attractive force on the Earth, accelerating the Earth directly towards the Sun. It moves toward the Sun as described by Newton’s second law. To avoid falling into the Sun, the Earth also counteracts the force that is pulling it towards the Sun. Thus, a force, which tends to make a body move in a curved path, is called a centripetal force is applied by the Earth. Therefore, we can say that the Sun provides the necessary centripetal force to keep the earth in its orbit and the earth provides centrifugal force due to its motion. These two forces counter balance each other and this prevents the earth from falling into the sun.

23. How does the weight of an object vary with respect to mass and radius of the Earth? In a hypothetical case, if the diameter of the Earth becomes half of its present value and its mass becomes four times of its present value, then how would the weight of any object on the surface of the Earth be affected ?

Solution

Let R and M be the radius and mass of the earth.

Then,

Hypothetically m and R becomes 4m and R/2.

24. How does the force of attraction between the two bodies depend upon their masses and distance between them? A student thought that two bricks tied together would fall faster than a single one under the action of gravity. Do you agree with his hypothesis or not? Comment.

Solution

From Newton’s law of gravitation, we get Force of attraction between two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.
F ∝ m1m2 …(1)
F ∝ 1/d2 …(2)
The two bricks, like a single body, fall with the same speed to reach the ground at the same time in case of free-fall. This is because acceleration due to gravity is independent of the mass of the falling body.

25. Two objects of masses m1 and m2 having the same sizes are dropped simultaneously from heights h1 and h2 respectively. Find out the ratio of time they would take in reaching the ground. Will this ratio remain the same if

(i) one of the objects is hollow and the other one is solid and

(ii) both of them are hollow, size remaining the same in each case? Give reason.

Solution

Two objects of masses m1 and m2 having the same sizes are dropped, therefore, initial velocity (u) = 0 and a = g (Acceleration due to gravity–falling object) and it is independent of mass.
By using Newton’s equation of motion namely.

For object 1 = h1

For object 2 = h2

Time taken by object 1 = t1

Time taken by object 2 = t2

(i) This ratio of time will be the same even if one of the objects is hollow and the other one is solid. This is because acceleration due to gravity is independent of the mass of the falling body.

(ii) This ratio of time will be the same even if both of them are hollow, size remaining the same in each case. This is because acceleration due to gravity is independent of the mass of the falling body.

26. (a) A cube of side 5 cm is immersed in water and then in saturated salt solution. In which case will it experience a greater buoyant force. If each side of cube is reduced to 4 cm and then immersed in water. What will be the effect on buoyant force experienced by the cube as compared to the first case for water? Give reason for each case.
(b) A ball weighing 4 kg of density 4000 kg m–3 is completely immersed in water of density 103 Kg m–3. Find the force of buoyancy on it. (Given g = 10 ms–2)

Solution

(a) (i) Buoyant force, F = Vρg

ρ= Density of water,

V = Volume of water displaced by the body

Volume and density of an object decides its Buoyancy.

As the density of the salt solution is greater than that of water the cube will experience a greater buoyant force in the saturated salt solution.

(ii) The smaller cube will experience lesser buoyant force as its volume is lesser than the initial cube.

(b) Buoyant force = weight of the liquid displaced

= density of water × volume of water displaced ×g

= 1000 × 4/4000 × 10

= 10 N