Introduction
Class 9 Physics chapter 10 Gravitation notes are the best companion for you while you prepare this chapter. This chapter involves the universal laws of gravity, different types of Gravitational forces, and their impact on us. The Physics notes illustrated all the concepts in easy-to-grasp language and presented real-life examples for the students’ ease. In this blog article, the summary and revision notes on Class 9 Physics chapter 10 have been provided. The acceleration caused by gravity, centripetal force, and relative density are also briefly discussed here.
Gravitation and Gravity
Class 9 Physics chapter 10 Gravitation involves a detailed discussion on the concept of Gravitation and gravity.
- Gravitation- Gravitation denotes the force of attraction between any two objects in this universe.
- Gravity – Gravity is the force that attracts an object towards the Earth.
The concept of Gravitation has a universal approach; on the contrary, Gravity is specifically the effect of the Earth’s gravitational pull.
Newton’s Law of Gravitation
According to Newton’s Law of Gravitation :
- Gravitational force is the same as the product of the masses of two objects (m1 and m2).
- The square of the distance (r) between the centers of the objects is inversely proportional.
- As per the formula F=Gm1m2/r2 (G stands for the Gravitational constant)
- The Gravitational constant means the force between two 1 kg masses is 1 N when they are 1 meter apart.
Characteristics of Gravitational Force
Characteristics of the Gravitational Force have been discussed in the following points:
- Universal attraction
This force acts among all objects having mass.
- Inverse square law
Force between two objects will decrease when the distance between them increases.
- Newton’s third law
The force is opposite as well as equal.
- Dependent on mass
With the increasing mass of an object, the force will increase.
- Line of action
It acts along the line that connects the centres of the masses.
- Long range
This force impacts long distances.
Importance of the Law of Gravitation
The following phenomena indicate the universal law of gravitation:
- The moon moves around the Earth
- The force binds us to the earth
- The Earth moves around the sun
- The attraction between the sun and the moon causes tides.
- A ball moves downwards after being thrown upwards direction
Free fall
When an object is thrown upwards in the air, after reaching a certain height, it will start to fall downwards due to the gravity of the Earth. This is called free fall.
The direction remains unchanged in the free fall, but the velocity changes constantly, which is called acceleration.
Its unit is m/s2.
The equation of motion under gravity
V = u + at
S = ut+1/2 at2
v2 =u2 +2as
Where u refers to the initial velocity, v indicates the final velocity, s stands for displacement, t is time, and a denotes acceleration.
Newton’s law of gravitation and Newton’s third law of motion
According to Newton’s third law of motion, “Every object applies an equal and opposite force on another object but in the opposite direction”.
As per Newton’s law of gravitation, “Every mass in the universe attracts every other mass”.
Example – A stone that is falling freely towards the surface of the Earth is being attracted by the Earth.
Newton’s third law of motion states that the stone attracts the earth towards it by applying the same force as the earth.
F= mxa
As the mass of the stone is less,
The acceleration in its velocity is 9.8 m/s2
The earth’s acceleration towards the stone is 1.65x 10 -24.
Acceleration due to Gravity (g)
A free-falling object experiences uniform acceleration caused by the gravitational force of the Earth, which is called the acceleration due to Gravity.
It is indicated as ‘g’ and it works towards the center of the Earth.
g = GM/R2
G = Universal gravitational constant
M = Mass of the Earth
R = Radius of the Earth
For earth, g = 9.8 m/s2
The relation between “g” and “G”
G= Gravitational constant
g = Acceleration due to gravity
g = GM/R2
The following table illustrates the difference between the Gravitational constant (G) and the Acceleration due to gravity (g)
| Gravitational constant | Acceleration due to gravity |
| It remains constant | |
| Its value is 6.67×10-11 Nm2/kg2 | Its value is 9.8 m/s2 |
| Its unit is Nm2/kg2 | Its unit is m/s2 |
| It is a scalar quantity | It is a vector quantity |
Centripetal force
An object moves in circular motion, it changes its direction every moment, and its velocity also changes. The force that is produced from the center of a circular path and attracts the moving object is called the Centripetal force.
In the absence of centripetal force, no object can move in a circular path; it would always move in a straight line.
F= mv2/r
Mass and weight
1. Mass- The matter present in an object is called the mass of the object.
The SI unit is kg.
It is a scalar quantity.
2. Weight – the weight of the object refers to the Earth’s gravitational force applied to an object.
SI Unit- newton(N)
W = mg(g stands for the acceleration produced by gravitational force, and m denotes the mass of the object)
3. Pressure – Pressure denotes the type of force that acts on the per unit area of an object.
It is denoted by P and it refers to the thrust acting on an object’s per unit area.
P= Thrust / area= force/area = F/A
SI Unit- N/ m² or Pa (Pascal)
Example – 1N force on 1 m² produces 1 Pa.
4. Thrust – Thrust refers to the kind of force that is applied in the perpendicular direction. This is a vector quantity.
5. Fluid – The substances can flow and do not have a fixed shape; they exert pressure in all directions, and their pressure depends on the height of the fluid column on a surface.
6. Buoyancy – It is an upward force applied by a fluid that makes an object seem to be lighter in a submerged condition.
As per Archimedes’ laws, the buoyant force is equal to the weight of the dispersed fluid.
7. Apparent weight
The weight of an object in air minus the buoyant force. Due to the upward force acting on an object engulfed in a fluid partially or wholly, then an apparent loss in weight of the object. The net weight of an object engulfed in a fluid is called the apparent weight.
Apparent weight: the weight of an object in air – E
- Density = mass per volume kg/m3
- Relative density – When the density of a substance is compared to the density of water at 4 °C.
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Conclusion
Class 9 Physics chapter 10 Gravitation notes covers all the important topics such as the laws of gravitation, free fall, acceleration due to gravity, and the concept of mass and weight. The complex topics are presented in simple language. The relevant formulas as well as their derivations are described in precise form, which is easy for quick revision. The concept of Centripetal force, as well as the relation between Newton’s third law of motion and Newton’s law of gravitation, has been included here in this note.
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