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.
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GRAVITATION
INTEXT QUESTION ANSWER
Page NO. :- 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 the gravitational force. The force acting between two objects is directly proportional to the
product of their masses and inversely proportional to the square of the
distance between their centers.
Masses of two objects of = m₁ and m₂
Distance between the two objects = d
Force of attraction acting between them = F
It will be given by 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 :- Mass of the Earth = Me
Mass of an object on its surface = m.
Radius of the Earth = R
According to the universal law of gravitation, the gravitational force
(F) acting between the Earth and the object is given by the relation:
F = G
(m₁ × m₂) / r²
Thus, the formula will be: -
F = G (Mₑ · m) / r²
Page No. :- 104
Ques 1:- What do
you mean by free fall ?
Ans :- Gravity of earth attracts every
object towards its center. When an object is dropped from a certain height , it
begins to fall towards Earth's surface under the influence of gravitational for
Such a motion of 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 earth from a certain height, then its velocity changes. This change in velocity produces
acceleration in the object which is known as acceleration due to gravity
denoted by letter g. The value of acceleration due to gravity is : -
g = 9.8 m/s².
Page NO. :- 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 the body.
Weight is the force of
gravity acting on the body
It is the measure of
inertia of the body.
It is the measure of
gravity.
Mass is a constant
quantity.
Weight is not a
constant quantity . It is different at different places.
It only has magnitude.
It has magnitude as well as direction.
It's SI Unit is the
kilogram(Kg).
It's SI Unit is the
Newton.
(kg m/s²)
Ques 2 :- Why is
the weight of an object on the moon ¼ th its weight on the earth?
Ans :- The moon's gravitation force is
determined by the mass and the size of the moon. The mass of moon is 1/100 times
and its radius ¼ times that of earth. As a result, the gravitational attraction on the moon is about one sixth
when compared to earth. Hence, the weight of an object on the moon is 1/6 th its weight on the earth.
Page NO. :- 109
Ques 1 :- Why is
it difficult to hold a school bag having a strap made of a thin and strong
string ?
Ans :- Pressure ∝ 1 / (surface area)
As the pressure is inversely proportional to the surface area on which
force acts. The smaller is the surface area; the larger will be the pressure on
the surface.
In the case of a thin strap, the contact surface area is very small.
Hence, the pressure exerted on the shoulder is very large.
Ques 2 :- What do
you mean by buoyancy ?
Ans :- The property of fluids (gases and
liquids) by virtue of which they exert an upward thrust on a body immersed into
it is called the 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 because of two reasons.
(i) If its density is greater than that of water, an object sinks in
water.
(ii) If its density is less than that of water, an object floats in
water.
Page NO. :- 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, the weight acts downwards while the upthrust due to air acts upwards.
As, true weight = (apparent weight + up thrust)
So our apparent weight becomes less than the true weight. This apparent weight is measured by the weighing machine and hence, the mass indicated
is less than the actual mass. So our actual mass will be 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 an
iron bar as it experiences a larger air thrust than the iron bar.
As, true weight = (apparent weight + up thrust)
The cotton bag's density is less than that of the iron bar, so, the
volume of the cotton bag is more compared to the iron bar. Hence, the cotton bag experience more upthrust due to the presence of
air. So, in the presence of air, the cotton bag's true weight is more
compared to the true weight of the iron bar.
Page NO. :- 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 the gravitational force. The force acting between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.
Masses of two objects of = m₁ and m₂
Distance between the two objects = d
Force of attraction acting between them = F
It will be given by 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 :- Mass of the Earth = Me
Mass of an object on its surface = m.
Radius of the Earth = R
According to the universal law of gravitation, the gravitational force (F) acting between the Earth and the object is given by the relation:
F = G (m₁ × m₂) / r²
Thus, the formula will be: -
F = G (Mₑ · m) / r²
Page No. :- 104
Ques 1:- What do you mean by free fall ?
Ans :- Gravity of earth attracts every
object towards its center. When an object is dropped from a certain height , it
begins to fall towards Earth's surface under the influence of gravitational for
Such a motion of 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 earth from a certain height, then its velocity changes. This change in velocity produces acceleration in the object which is known as acceleration due to gravity denoted by letter g. The value of acceleration due to gravity is : -
g = 9.8 m/s².
Page NO. :- 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 the body. |
Weight is the force of
gravity acting on the body |
|
It is the measure of
inertia of the body. |
It is the measure of
gravity. |
|
Mass is a constant
quantity. |
Weight is not a constant quantity . It is different at different places. |
|
It only has magnitude. |
It has magnitude as well as direction. |
|
It's SI Unit is the
kilogram(Kg). |
It's SI Unit is the Newton. (kg m/s²) |
Ques 2 :- Why is the weight of an object on the moon ¼ th its weight on the earth?
Ans :- The moon's gravitation force is determined by the mass and the size of the moon. The mass of moon is 1/100 times and its radius ¼ times that of earth. As a result, the gravitational attraction on the moon is about one sixth when compared to earth. Hence, the weight of an object on the moon is 1/6 th its weight on the earth.
Page NO. :- 109
Ques 1 :- Why is it difficult to hold a school bag having a strap made of a thin and strong string ?
Ans :- Pressure ∝ 1 / (surface area)
As the pressure is inversely proportional to the surface area on which
force acts. The smaller is the surface area; the larger will be the pressure on
the surface.
In the case of a thin strap, the contact surface area is very small. Hence, the pressure exerted on the shoulder is very large.
Ques 2 :- What do you mean by buoyancy ?
Ans :- The property of fluids (gases and liquids) by virtue of which they exert an upward thrust on a body immersed into it is called the 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 because of two reasons.
(i) If its density is greater than that of water, an object sinks in water.
(ii) If its density is less than that of water, an object floats in
water.
Page NO. :- 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, the weight acts downwards while the upthrust due to air acts upwards.
As, true weight = (apparent weight + up thrust)
So our apparent weight becomes less than the true weight. This apparent weight is measured by the weighing machine and hence, the mass indicated is less than the actual mass. So our actual mass will be 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 an iron bar as it experiences a larger air thrust than the iron bar.
As, true weight = (apparent weight + up thrust)
The cotton bag's density is less than that of the iron bar, so, the volume of the cotton bag is more compared to the iron bar. Hence, the cotton bag experience more upthrust due to the presence of air. So, in the presence of air, the cotton bag's true weight is more compared to the true weight of the iron bar.
----------------------------------------------------------------------------------------------------------------------
In this chapter, we learned important concepts of gravitation
such as universal law of gravitation, gravitational constant, free fall,
acceleration due to gravity, mass, weight, and related numerical problems. The
NCERT intext questions help students understand how gravitational force works
in real life and how to apply formulas correctly. These solutions will make
your exam preparation easier and strengthen your understanding of Class 9
Physics concepts.
In this chapter, we learned important concepts of gravitation
such as universal law of gravitation, gravitational constant, free fall,
acceleration due to gravity, mass, weight, and related numerical problems. The
NCERT intext questions help students understand how gravitational force works
in real life and how to apply formulas correctly. These solutions will make
your exam preparation easier and strengthen your understanding of Class 9
Physics concepts.
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