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Gravitation is an important chapter in Class 9 Science that explains why objects fall to the ground, how planets stay in orbit, and much more. Chapter 9 introduces students to important concepts like gravitational force, free fall, Archimedes’ Principle, thrust, pressure, and buoyancy.
NCERT Solutions for Class 9 Science Chapter 9 Gravitation are provided here to help students prepare well for school exams and build a strong foundation for competitive exams like NSEJS and Olympiads. In these NCERT solutions, students will find accurate and step by step answers to all textbook questions . These solutions for class 9 also include Important Topics, important Formula, and extra practice problems to enhance your problem-solving. These NCERT solutions for class 9 science are designed as per the latest CBSE syllabus by our expert faculty.
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Below are the solved intext questions from NCERT Class 9 Science Chapter 9 Gravitation. These step by step solutions will help you understand the concepts easily and build your problem solving skills
Q1. State the universal law of gravitation.
Answer:
The universal law of gravitation states that everybody in the universe attracts every other body by the virtue of its mass. This force is directly proportional to the product of the masses of the two bodies and inversely proportional to the square of the distance between them.
Let there be two bodies of masses
where G is the universal gravitational constant and is equal to
Answer:
Let
Mass of Earth,
d would be approximately equal to the radius of the earth.
Radius of Earth
Q1. What do you mean by free fall?
Answer:
We say an object is freely falling when it is dropped from some height and is attracted by the gravitational force of earth only and is under the influence of no other considerable force.
Q2. What do you mean by the acceleration due to gravity?
Answer:
Each object on the Earth is under the influence of the gravitational force of the earth. The acceleration due to the earth's gravitational force is known as acceleration due to gravity.
Q1. What are the differences between the mass of an object and its weight?
Answer:
The following are the differences between the mass of a body and its weight
Mass | Weight |
(i) Mass is the amount of matter contained in the body. | (i) Weight is the gravitational force experienced by the body. |
(ii) Mass of a body is always constant. | (ii) Weight of a body depends on the place where it is at. |
(iii) Mass is the measure of the inertia of the body. | (iii) Weight is the measure of the gravitational force acting on the body. |
(iv) Mass only has magnitude. | (iv) Since weight is a force it naturally has a direction associated with it as well |
(v) It's SI unit is the kilogram (kg). | (v) It's SI unit is Newton (kg m |
Q2. Why is the weight of an object on the moon
Answer:
The weight of an object on the moon would be given by
where M M is the mass of the moon, m is mass of the body, r M is the radius of the moon and G is the gravitational constant.
The weight of an object on the Earth would be given by
where M E and r E are the mass and radius of the earth respectively.
The above ratio is approximately equal to 1/6 and this is why the weight of an object on the moon (1/6) th of its weight on the earth .
Q1. Why is it difficult to hold a school bag having a strap made of a thin and strong string?
Answer:
It is difficult to hold a school bag having a strap made of a thin and strong string because as the surface area of the string in contact with the shoulders is very less and due to this, its weight applies a large pressure on the shoulders.
Q2. What do you mean by buoyancy?
Answer:
When an object is placed in a fluid it displaces a volume of liquid equal to its own volume. Due to this, the liquid exerts an upward force on the body called the Buoyant force. This tendency of a liquid to exert the upward buoyant force is called buoyancy.
Q3. Why does an object float or sink when placed on the surface of the water?
Answer:
When an object is placed on the surface of the water it displaces a certain volume of water. If the density of the object is less than that of water the buoyant force due to this displacement of water is equal to the weight of the object and it floats on the surface of the water. If the density of the object is more than the density of water, the volume of water displaced would be equal to the volume of the object itself and the buoyant force acting upwards due to this displacement of water would be less than the weight of the object and the object would sink
Q1. You find your mass to be
Answer:
The weighing scale shows the reading according to the weight applied to it. The weight on the weighing scale would be slightly less than our weight because there is a small upward force acting on us due to the buoyancy of the atmosphere and the reading would be slightly lower than our actual weight, therefore, our mass must be more than 42 kg.
Q2. You have a bag of cotton and an iron bar, each indicating a mass of
Answer:
The volume of the bag of cotton would be much more than the iron bar and the upward buoyant force on the bag of cotton would be more than that acting on the iron bar and therefore the value is shown in case of the bag of cotton must be lesser than the actual value by a larger amount and therefore the bag of cotton is heavier than the iron bar.
Find here the detailed and correct solutions for all the exercise questions from Chapter 9 Gravitation of Class 9 Science. These answers will help you prepare better for exams.
Answer:
Let there be two bodies of masses m 1 and m 2 and let the distance between them be r. The gravitational force F between them would be given by
Let the distance between them be halved by two. The gravitational force between them would now be given by
The force of gravitation between two objects would increase by 4 times if the distance between them is halved.
Answer:
The gravitational force acting is definitely more in the case of a heavier object than a light object but the acceleration depends on the ratio of the force acting on the body to the mass of the body and since gravitational force acting on a body is proportional to its mass the acceleration due to gravity is the same for all bodies irrespective of their masses.
Q.3. What is the magnitude of the gravitational force between the earth and a
Answer:
Mass of Earth
Mass of the body m is 1 kg
The radius of the earth
Universal gravitational constant, G =
The magnitude of the gravitational force between the earth and the body would be
Q4. The earth and the moon are attracted to each other by gravitational force. Does the earth attract the moon with a force that is greater or smaller or the same as the force with which the moon attracts the earth? Why?
Answer:
The earth attracts the moon with the same force as the moon attracts the earth.
We know from the third law of motion that each force has an equal and opposite force and the universal gravitational law also states the same i.e. the gravitational force of attraction between two bodies is the same.
Q5. If the moon attracts the earth, why does the earth not move towards the moon?
Answer:
The moon and the earth attract each other with the same gravitational force. It is because of the much larger mass of the earth than the mass of the moon the earth does not move towards the moon.
Q6. (i) What happens to the force between two objects, if
Answer:
As the force between the objects is directly proportional to the product of masses of the objects if the mass of one object is doubled the force between them will also double.
Q6.(ii) What happens to the force between two objects, if
The distance between the objects is doubled and tripled?
Answer:
The force between two objects is inversely proportional to the square of the distance between them. Therefore if the distance between the objects is doubled and tripled the force between them would become one fourth and one-ninth of the initial value respectively.
Q.6.(iii) What happens to the force between two objects, if
Answer:
As the force between the objects is directly proportional to the product of masses of the objects if the masses of both objects are doubled the force will become four times the initial value.
Q7. What is the importance of the universal law of gravitation?
Answer:
The importance of universal law of gravitation lies in the fact that it proves that every object in the universe is attracted by every other object in the universe by the virtue of their masses.
Q8. What is the acceleration of free fall?
Answer:
The acceleration of free fall on earth is 9.8 m
Q9. What do we call the gravitational force between the earth and an object?
Answer:
The gravitational force between the earth and an object is called the weight of the object.
Answer:
As the value of g is more at the poles than that at the equator the weight of the same amount of gold would be more at the poles than that at the equator and therefore the friend will not agree with the weight of gold bought.
Q11. Why will a sheet of paper fall slower than one that is crumpled into a ball?
Answer:
A sheet of paper has much more area than the same paper crumbled and due to this the sheet experiences more air resistance and thus falls at a speed slower than when it is crumbled.
Answer:
The weight of an object on earth is given by w = mg where m is the mass of the object and g is the gravitational acceleration.
g = 9.8 m
Weight of a 10 kg object on earth = 10 X 9.8 = 98 N
As the gravitational force on the surface of the moon is only 1/6th of that on earth the gravitational accelerations on the moon would be equal to g/6.
Weight of an object of mass 10 kg on the moon is therefore given as follows
Q13.(i) A ball is thrown vertically upwards with a velocity of
(i) The maximum height to which it rises,
Answer:
The initial velocity of the ball u = 49 m
Final velocity at the highest point would be v = 0
The magnitude of the acceleration is equal to the acceleration due to gravity g
Acceleration, a = -g = -9.8 m
Let the maximum height to which it rises be s
Using the third equation of motion we have
The ball would rise to a maximum value of 112.5 m.
Q13.(ii) A ball is thrown vertically upwards with a velocity of
(ii) The total time it takes to return to the surface of the earth.
Answer:
Let the time taken by the ball to reach the highest point be t
At the same time, t would be taken to come back to the ground from the highest point.
Therefore the total time it takes to return to the surface of the earth = 2t = 10 s.
Answer:
Initial velocity u = 0
Acceleration, a = g = 9.8 m
Distance travelled, s = 19.6 m
Let the final velocity be v
According to the third equation of motion
Its final velocity just before touching the ground will be 19.6 m
Answer:
Initial velocity
Acceleration
Final velocity at the highest point would be
Let the maximum height reached be s
As per the third equation of motion
The net displacement would be zero as the stone will return to the point from where it was thrown.
The total distance covered by the stone
Answer:
Mass of the earth, M E
Mass of the Sun, M S
Distance between the earth and the sun,
Universal gravitational constant,
The force of gravitation between the earth and the Sun would be given as
Answer:
Let the distance travelled by the stone which is dropped from the top upto the instant when the two stones meet be x
Initial velocity u = 0
Acceleration a = g = 9.8 m
Using the second equation of motion
The distance travelled by the stone which is projected vertically upwards from the ground up to the instant when the two stones meet would be equal to 100 - x
Initial velocity = 25 m
Acceleration a = -g = -9.8 m
Using the second equation of motion
Equating x from (i) and (ii) we get
The stones meet after a time of 4 seconds at a height of 21.6 meters from the ground.
Q18. A ball thrown up vertically returns to the thrower after 6 s. Find
(a) the velocity with which it was thrown up,
(b) the maximum height it reaches, and
(c) its position after 4 s.
Answer:
(a) Let the ball be thrown with initial velocity u
Time taken to get back to the thrower = 6 s
Time taken to reach the highest point is t = 6/2 = 3 s
Final velocity at the highest point is v = 0
Acceleration a = -g = -9.8 m
Using the first equation of motion
(b) Let the maximum height it reaches be s
Using the second equation of motion
(c) Out of the 4 seconds, 3 have been spent in reaching the highest point
The distance travelled by the ball in the next 1 second is s' given by
Distance from the ground after 4 seconds = s - s' = 44.1 - 4.9 = 39.2 m
The position of the ball after 4 seconds is 39.2 m from the ground.
Q19. In what direction does the buoyant force on an object immersed in a liquid act?
Answer:
The buoyant force acts on an object in the vertically upward direction opposite to that of gravitational force.
Q20. Why does a block of plastic released under water come up to the surface of water?
Answer:
The density of plastic is less than that of water and due to which the upwards acting buoyant force is more than the downwards acting gravitational force. Due to this, a block of plastic released under water comes up to the surface of the water.
Answer:
Mass of the given amount of substance = 50 g
Volume of the given amount of substance = 20
Density of the given substance is
As the given substance has a higher density than that of water it will be sinking in water.
Answer:
Mass of the packet = 500 g
Volume of the packet = 350
Density of the packet is given by
As the density of the packet is less than that of water it will sink in water.
Volume of the water displaced by the packet = volume of the packet = 350 cm3
Mass of the water displaced by the packet = Volume of the water displaced by the packet X Density of water
= 350 X 1
= 350 g
Mass of water displaced is less than the mass of packet, so the packet will sink.
Below are the main topics from Class 9 Science Chapter 9 that will help you understand the chapter better and prepare for your final exam as well as for Olympiads.
10.1 Gravitation
10.1.1 Universal law of gravitation
10.1.2 Importance of the universal law of gravitation
10.2 Free fall
10.2.1 To calculate the value of g
10.2.2 Motion of objects under the influence of gravitational force of the Earth
10.3 Mass
10.4 Weight
10.4.1 Weight of an object on the Moon
10.5 Thrust and pressure
10.5.1 Pressure in fluids
10.5.2 Buoyancy
10.5.3 Why objects float or sink when placed on the surface of water
10.6 Archimedes’ principle
Below are the important formulas from Class 9 Science Chapter 9 Gravitation. These formulas will help you solve numerical problems and understand important concepts better. Make sure to revise them regularly for better exam preparation.
1. Read the question carefully and identify what is given (mass, height, area, force, etc.).
2. Note the given values with proper units.
3. Choose the correct formula, like:
4. Substitute values into the formula step by step.
5. Use correct units (e.g.,
6. Draw a small diagram(FBD) if needed especially for buoyancy or floating objects.
7. Solve neatly, and check your final answer.
Gravity holds us and all else down to the Earth. It's also the reason the Moon orbits and oceans have tides.
Yes, space is not gravity-free. It works between any two masses, whether there is air or not.
Because in a vacuum there is no air resistance, and all objects travel at the same speed because of gravity.
They are not weightless because they are in free fall with their spacecraft, even though gravity still acts upon them.
Gravitation provides the centripetal force required to keep planets orbiting the Sun.
The Class 9 Science chapter on Gravitation, with its question-and-answer format, establishes a fundamental understanding of gravity that proves beneficial for higher-level studies in physics and related disciplines.
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