NCERT Exemplar Solutions for Class 9 Science Chapter 10 Gravitation (MCQ, SAQ and LAQ)
Chapter Name | NCERT Exemplar Solutions for Class 9 Science Ch 10 Gravitation |
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Objective Type Questions for Gravitation
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
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
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
5. 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
6. 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
7. 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
8. 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
9. The atmosphere is held to the Earth by :
(a) Gravity
(b) Wind
(c) Clouds
(d) Earth’s magnetic field
Solution
(a) Gravity
10. 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
11. 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
12. 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
13. 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
14. 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
Short Answer Questions for Gravitation
15. 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 is the source of centripetal force that a planet requires to revolve around the Sun. It depends upon the following factors :
- Mass of the planet and the Sun, i.e., depends on the product of the masses of the planet and the Sun.
- Distance between the planet and the Sun, i.e. depends on the square of distance between the planet and the Sun.
16. 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 take the same time to reach the ground 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, 
Therefore, time taken for both the stones to reach the ground will be the same.
17. 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 Moon will begin to move a straight line in the direction in which it was moving at that instant. This straight line will be a tangent to the 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.
18. 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
At a given place, the value of acceleration due to gravity is constant but it varies from one place to another place on the Earth surface. It is due to this fact that Earth is not a perfect sphere. It is flattened at the poles and bulges out at the equator (ellipsoidal shaped). Thus, the value of `g’ is minimum at the equator and maximum at the poles. It means `g’ increases as we go from equator to pole. Therefore, the packet falls slowly at equator compared to the poles. Thus, the packet will remain in air for a longer time, when it is dropped at the equator.
19. 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
20. 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
21. 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 gravitational force is responsible for providing the necessary centripetal force which allows the Earth to move around the Sun in a defined orbit. Gravitational attraction of the Sun deflects the Earth from a straight path.
Long Answer Questions for Gravitation
22. 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
From Newton’s law of gravitation, we get Weight of an object is directly proportional to the mass of the Earth and inversely proportional to the square of the radius of the Earth.
Weight of a body = mass of the body × gravity of the Earth
W = mg …..(1)
Mass of the Earth is same everywhere
Weight of a body = GMm/R2 ….(2)
Where,
M = mass of Earth;
m = mass of the body;
R = radius of the Earth
G = universal gravitational constant
g = 9.8 m/s2 on Earth
Thus,
Weight of a body ∝ M
Weight of a body ∝ 1/R2
Original weight Wo = mg
The weight of the same object will become 16 times its original weight.
23. 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/a2 …(2)
The hypothesis of the student is wrong because acceleration due to gravity is independent of the mass of the falling body. The two objects of different masses (mass of two bricks tied together and the other having the mass of a single brick) will hit the ground at the same time because, when an object is in free fall, it is “weightless”. This is because there is no force pushing up on it. Since, both of these objects are experiencing the same force, namely, gravitational force, then they both will fall at the same velocity and they will both hit the ground at the exact same time. Therefore, the two bricks tied together will fall with the same speed and will reach the ground at the same time (free fall) equal to the speed and time of a single brick. If no air resistance is present, the rate of descent depends only on how far the object has fallen, no matter how heavy the object is. This means that two objects will reach the ground at the same time if they are dropped simultaneously from the same height.
24. 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. 
(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.
25. (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) As the density of salt solution is greater than that of water so 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 displayed
= density of water × volume of water displayed × gravity
= 1000 × 4/4000 × 10 = 10 N