10. Gravition Class 9 NCERT Exemplar Solutions Chapter 10 (CBSE Science)
CBSE Class 9 Science Chapter 10: Gravition – NCERT Exemplar Questions with Answers
Chapter 10: Gravition
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
Answer: (a) have same velocities at any instant
[ Near the Moon’s surface, all objects fall with the same acceleration (ignoring air resistance, which is negligible on the Moon). So, their velocities at any instant during free fall are the same, regardless of their masses.]
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
Answer: (c) is least on equator.
[ The value of acceleration due to gravity (g) is not the same everywhere on Earth. It is least at the equator and maximum at the poles.]
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) 
(b) 
(c) F (d) 2F
Answer: (a)
[ The gravitational force (F) between two masses m₁ and m₂ separated by distance r is 
Since both masses are halved , then 
So, the gravitational force becomes one-fourth of the original. ]
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
Answer: (c) will move along a straight line tangential to the circular path
[ When the string breaks, the centripetal force disappears. According to Newton’s first law, the stone continues in a straight line in the direction of its velocity at that instant, which is tangential to the circle.]
5. An object is put one by one in three liquids having different densities. The object floats with 
parts of their volumes outside the liquid surface in liquids of densities d1 , d2 and d3 respectively. Which of the following statement is correct?
(a) 
(b) 
(c) 
(d) 
Answer: (a)
[ The fraction of the volume outside the liquid is inversely related to the liquid’s density. The higher the liquid’s density, the less of the object floats above.
Given, 
outside → most dense ; 
outside → medium dense and 
outside → least dense
So, the correct order of densities is: ]
6. In the relation 
, 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
Answer: (d) is universal constant of nature.
[ The gravitational constant G is the same everywhere in the universe and does not depend on location, the value of g or whether Earth is involved.]
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
Answer: (c) any two bodies having some mass
[ Newton’s law of gravitation states that every two masses in the universe attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. ]
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
Answer: (d) is independent of mass and radius of the Earth
[ The universal gravitational constant G is a fundamental constant of nature and does not depend on the Earth’s mass or radius.]
9. Two particles are placed at some distance. If the mass of each of the two particles is doubled, keeping the distance between them unchanged, the value of gravitational force between them will be
(a) 
times (b) 4 times (c) 
times (d) unchanged
Answer: (b) 4 times
[ The gravitational force (F) between two masses m₁ and m₂ separated by distance r is
Since both masses are doubled
So, the gravitational force becomes 4 times the original value. ]
10. The atmosphere is held to the earth by
(a) gravity (b) wind (c) clouds (d) earth’s magnetic field
Answer: (a) gravity
[The Earth's atmosphere is a layer of gases surrounding the planet. These gases have mass, so they are pulled downward towards the centre of the Earth by the force of 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
Answer: (d) universal gravitational constant.
[ The gravitational force (F) between two masses m₁ and m₂ separated by distance r is
Here, 
Now,
So, the force of attraction between two unit point masses separated by unit distance is exactly equal to G, the universal gravitational 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) 
times the weight at surface of the earth
Answer: (a) zero
[ Since weight W = mg and g = 0 at the centre, the weight of the object is 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
Answer: (a) 2 N
[ When an object is immersed fully in water, it experiences an upward buoyant force equal to the weight of the water displaced .
Given, weight in air = 10 N , Apparent weight in water = 8 N
Loss in weight = weight in air – apparent weight in water = 10 N – 8 N = 2 N ]
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
Answer: (b) maximum when breadth and width form the base.
[ We know that, Pressure = Force /Area .
Since, pressure is maximum when the area of the base is minimum.
Areas of bases:
Length × Breadth = 60 × 40 = 2400 cm²
Breadth × Width = 40 × 20 = 800 cm²
Width × Length = 20 × 60 = 1200 cm²
The smallest area is 40 × 20 = 800 cm².
So, pressure is maximum when breadth and width form the base.]
15. An apple falls from a tree because of gravitational attraction between the earth and apple. If 
is the magnitude of force exerted by the earth on the apple and 
is the magnitude of force exerted by apple on earth, then
(a) is very much greater than (b) is very much greater than
(c) is only a little greater than (d) and are equal
Answer : (d) and F₂ are equal
[ According to Newton’s third law of motion, for every action there is an equal and opposite reaction. So, the Earth pulls the apple with a certain force, and the apple also pulls the Earth with the same magnitude of force.]
Short Answer Questions
16. What is the source of centripetal force that a planet requires to revolve around the Sun? On what factors does that force depend?
Answer: The source of centripetal force required by a planet to revolve around the Sun is the gravitational force exerted by the Sun on the planet.
This force depends on:
(i) Mass of the Sun (MM)
(ii) Mass of the planet (mm)
(iii) Distance between them (rr)
According to the universal law of gravitation, the force is directly proportional to the product of the masses and inversely proportional to the square of the distance between them.
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?
Answer: Both stones will reach the ground at the same time. The horizontal motion of the first stone does not affect its vertical motion. Since both stones fall through the same height under the same gravitational acceleration they take equal time to reach the ground.
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?
Answer: If the gravity of the Earth suddenly becomes zero the Moon will no longer have the centripetal force required for circular motion. Therefore it will move in a straight line along the tangent to its orbit at that point. This happens because without Earth’s gravitational pull there is no force to change its direction and according to Newton’s first law of motion it continues to move in a straight path.
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.
Answer: No , the packets will not take the same time to reach the Earth. The packet dropped at the north pole will reach first because the value of acceleration due to gravity is greater at the poles than at the equator. Hence the packet at the pole falls faster.
20. The weight of any person on the moon is about 
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: Here, 
and 
We have, Weight = mass × g
So, on the Moon he can lift 90 kg using the same force.
21. Calculate the average density of the earth in terms of g, G and R.
Answer: We know that acceleration due to gravity on the surface of Earth is 
The mass of the Earth, 
Therefore, the average density of the earth, ρ 
22. The earth is acted upon by gravitation of Sun, even though it does not fall into the Sun. Why?
Answer: The Earth does not fall into the Sun because it has a high tangential velocity. The gravitational force of the Sun provides the necessary centripetal force that keeps the Earth moving in its orbit around the Sun continuously.
Long Answer Questions
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?
Answer: We know that, weight of an object is 
Where m is mass of object, M is mass of Earth, R is radius of Earth, G is gravitational constant.
Given, 
and 
New acceleration due to gravity,
Therefore, the new weight becomes 16 times the original.
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.
Answer: The gravitational force between two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
i.e., 
No, I do not agree with the student’s hypothesis. All bodies fall towards the Earth with the same acceleration due to gravity irrespective of their masses if air resistance is neglected.
25. Two objects of masses 
having the same size are dropped simultaneously from heights 
and 
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.
Answer: When an object is dropped from a height h, the time taken to reach the ground is 
Since , initial velocity is 0 (u = 0) .
The times taken by the two objects are 
and 
The ratio of time is, 
Thus, the ratio depends only on the heights, not on the masses.
(i) If one object is hollow and the other is solid, the ratio will remain the same because acceleration due to gravity is the same for all objects (ignoring air resistance).
(ii) If both are hollow and of the same size, the ratio will still remain the same for the same reason.
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 the cube is reduced to 4 cm and then immersed in water, what will be the effect on the 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/m³ is completely immersed in water of density 10³ kg/m³. Find the force of buoyancy on it. (Given g = 10 m/s² .)
Solution: (a) The cube will experience greater buoyant force in saturated salt solution because its density is greater than water. When the side of the cube is reduced from 5 cm to 4 cm its volume decreases. Since buoyant force depends on the volume of displaced liquid the buoyant force also decreases in water.
(b) Given, Mass of ball = 4 kg , Density of ball = 4000 kg/m³ , Density of water = 10³ kg/m³ , g = 10 m/s²
Volume of ball 
Buoyant force 
Therefore, the force of buoyancy acting on the ball is 10 N.
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