In this post, we have provided Class 9 Science Chapter 9 –Gravitation Intext Questions and Answers as per the latest NCERT textbook. These solutions will help students understand important concepts such as gravitational force, acceleration due to gravity, free fall, mass, weight, and related numericals . All answers are written in simple and clear language to help you in exams and quick revision.
Chapter 9 – GRAVITATION
INTEXT QUESTION ANSWER
Page 102
Ques 1:- State the universal law of gravitation.
Ans: - The universal law of gravitation states that every object in the universe attracts every other object with a force called gravitational force.
This force is:
• Directly proportional to the product of the masses of the two objects,
• Inversely proportional to the square of the distance between their centres.
If,
Masses of two objects = m₁ and m₂
Distance between their centres = d
Gravitational force = F
Then, according to the universal law of gravitation:
F = G m1 × m2 d2
where, G is the universal constant.
G = 6.67 × 10⁻¹¹ N m² kg⁻²
Ques 2 :- Write the formula to find the magnitude of the gravitational force between the earth and an object on the surface of the earth.
Ans: - According to the universal law of gravitation, the force between the Earth and an object on its surface is:
F = G Me × m R²
where Mₑ is the mass of the Earth, m is the mass of the object and R is the radius of the Earth.
Page 104
Ques 1:- What do you mean by free fall ?
Ans: - Gravity of the Earth attracts every object towards its centre. When an object is dropped from a certain height and falls towards the Earth’s surface only under the influence of gravitational force, such a motion of the object is called free fall.
Ques 2:- What do you mean by acceleration due to gravity?
Ans :-
When an object falls freely towards the surface of the Earth from a certain height, its velocity changes. This change in velocity produces acceleration in the object due to the gravitational force of the Earth, which is known as acceleration due to gravity.
It is denoted by the letter g.
The value of acceleration due to gravity is g = 9.8 m/s².
Page 106
Ques 1:- What are the differences between the mass of an object and its weight?
Ans :-
| Mass | Weight |
|---|---|
| Mass is the quantity of matter contained in a body. | Weight is the force of gravity acting on a body. |
| It is a measure of the inertia of a body. | It is a measure of the gravitational force acting on the body. |
| Mass is a constant quantity. | Weight is not a constant quantity. |
| It remains the same at all places. | It is different at different places. |
| It has only magnitude. | It has magnitude as well as direction. |
| Its SI unit is kilogram (kg). | Its SI unit is newton (N = kg m s⁻²). |
Ques 2 :- Why is the weight of an object on the Moon 1/6th its weight on the Earth?
Ans :- The gravitational acceleration on the Moon depends on its mass and radius. The mass of the Moon is about 1/100 times that of the Earth and its radius is about 1/4 times that of the Earth. Due to this, the gravitational acceleration on the Moon is approximately one-sixth of that on the Earth. Hence, the weight of an object on the Moon is 1/6th of its weight on the Earth.
Page 109
Ques 1 :- Why is it difficult to hold a school bag having a strap made of a thin and strong string ?
Ans :- Pressure is defined as force per unit area, i.e.,
Pressure = Force / Area.
Thus, pressure is inversely proportional to the surface area on which the force acts. The smaller the surface area, the greater is the pressure exerted. In the case of a school bag with a thin strap, the area of contact with the shoulder is very small. Hence, a large pressure is exerted on the shoulder, making it difficult and painful to hold the bag.
Ques 2 :- What do you mean by buoyancy ?
Ans :- The property of fluids (liquids and gases) by virtue of which they exert an upward thrust on a body immersed in them is called buoyancy.
Ques 3 :- Why does an object float or sink when placed on the surface of water?
Ans :- An object floats or sinks when placed on the surface of water depending on its density.
(i) If the density of the object is greater than the density of water, the object sinks in water.
(ii) If the density of the object is less than the density of water, the object floats on water.
Page 110
Ques 1 :- You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg ?
Ans :- When we stand on a weighing machine, our true weight acts vertically downward, while the buoyant force (upthrust) due to air acts upward.
Hence,
Apparent weight = True weight − Upthrust.
Due to this upward buoyant force, the apparent weight becomes slightly less than the true weight. The weighing machine measures this apparent weight, so the mass indicated by it is less than the actual mass.
Therefore, our actual mass is more than 42 kg.
Ques 2 :- You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than other. Can you say which one is heavier and why?
Ans :- The cotton bag is heavier than the iron bar. This is because it experiences a larger buoyant force (upthrust) due to air.
As,
True weight = Apparent weight + Upthrust.
The density of cotton is less than that of iron; therefore, the volume of the cotton bag is greater than that of the iron bar. Due to its larger volume, the cotton bag experiences more upthrust from air.
Hence, although both show the same mass on the weighing machine, the true weight of the cotton bag is greater than that of the iron bar.
Conclusion:
Here we have provided NCERT intext questions and answers for Class 9 Science Chapter 9 – Gravitation with easy explanations. These solutions are helpful for concept clarity, revision, and effective CBSE exam preparation.
Class 9 Science Chapter 9: Gravitation – Intext Questions FAQ
1. What is gravitation?
Gravitation is the force of attraction between any two objects having mass. This force acts along the line joining the centers of the two bodies.
2. State the universal law of gravitation.
Every object in the universe attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.
3. What is the formula for the universal law of gravitation?
F = G (m1 × m2) / r2
where G is the universal gravitational constant.
4. Why do objects fall towards the Earth?
Objects fall towards the Earth due to Earth’s gravitational force, which attracts all objects towards its center.
5. What is free fall?
When an object falls under the influence of gravity alone and no other force acts on it, the motion is called free fall.
6. What is the value of acceleration due to gravity on Earth?
The acceleration due to gravity on Earth is approximately 9.8 m/s2.
7. Why is the weight of an object different on the Moon?
The Moon has much less mass than the Earth, so its gravitational pull is weaker. As a result, the weight of an object on the Moon is 1/6th of its weight on Earth.
8. What is the difference between mass and weight?
Mass is the amount of matter in an object and remains constant everywhere.
Weight is the gravitational force acting on an object and changes from place to place.
9. What is thrust and pressure?
Thrust is the force acting perpendicular to a surface.
Pressure is the thrust acting per unit area.
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