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Home CBSE Notes Class 9 Science Chapter 9

Gravitation
Gravitation describes the fundamental force that causes objects to attract each other. Sir Isaac
Newton introduced the Law of Universal Gravitation to explain this phenomenon. Gravitation is a key
force that pulls two bodies toward one another, influencing the movement of celestial objects and
objects on Earth.

1.0
Gravitation and Gravity
Gravitation-It is the force of attraction that exists between any two objects in the universe.
Gravity specifically refers to the force of attraction between Earth and other material objects. While
gravitation is a universal concept, gravity is the localized effect of Earth's gravitational pull.

2.0
Newton’s Universal Law of Gravitation
Newton’s Universal Law of Gravitation states that:

Gravitational force(F) is directly equitable to the product of the masses of two objects (m1 and
m2).

It is inversely equitable to the square of the distance r between their centers.

The formula for this law is F = Gmr12m2 where G is the Universal Gravitational Constant,
​ ​

approximately 6.673 × 10−11 N m2 Kg −2

This constant represents the force between two 1 kg masses separated by 1 meter.

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3.0

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Characteristics of Gravitational Force

1. Universal Attraction: It acts between all objects with mass.

2. Line of Action: It acts along the line joining the centers of the masses.

3. Newton’s Third Law: The force is equal and opposite on both objects.

4. Inverse Square Law: The force decreases as the distance between objects increases.

5. Dependence on Mass: The force increases with greater mass.

6. Long Range: It has an effect over long distances.

4.0
Importance of The Universal Law of Gravitation
Anchors to Earth: It keeps us grounded on Earth.

Planetary Orbits: Explains why planets orbit the Sun.

Tides: Accounts for tides caused by the Moon and Sun.

5.0
Free Fall(Motion Under Gravity)
Free fall alludes to the motion of an object subjected only to the force of gravity. This occurs when
an object drops from rest, or is thrown downward or upward. The equations of motion under gravity
are:

v = u + at

s = ut + 12 at2 ​

v 2 = u2 + 2as

Where u is the initial velocity, v is the final velocity, aaa is acceleration (equal to g for free fall), s is
displacement, and t is time.

6.0
Acceleration Due To Gravity(g)
It is the rate at which an object accelerates when in free fall due to Earth's gravitational pull. On
Earth’s surface, this acceleration g is approximately 9.8 sm2 ​

GM
g= R2
​

Where G is the Universal Gravitational Constant, M is the mass of Earth, R is the radius of Earth.

For Earth g=9.8m/s2

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7.0
Acceleration Due To Gravity On Earth’s Surface

Fg =​
GM m
r2
..............(1)
​

Fg = ma.................(2)
​

From equation (1) and (2)

GM m
ma = r2
​

a= GM
r2
​ (a=g)
g= GM
r2
​

(6.67×10−11 )(6×1024 )
g= (6.4✕106 )2
​
= 9.8m/s2

8.0
Mass And Weight
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Mass-Mass: The amount of matter in an object. It is constant regardless of location and is measured
in kilograms (kg) in the SI system. Mass is a scalar quantity and always positive.
Weight: The gravitational force exerted by Earth on an object. It varies with the local gravitational
field and is measured in newtons (N). Weight is calculated using:

W=mg (m-mass of an object, g=acceleration due to gravity)

On the Moon, an object weighs about 16 of its weight on Earth due to the Moon’s weaker gravity.
​

Thrust: Thrust is the force acting perpendicular to a surface.

Pressure: Pressure is the thrust per unit area.
F orce N
P ressure = Area
​ = m2
​ = Pascal(Pa)

For example, a 1 N force on 1 m² yields 1 Pa.

Fluids: Substances that flow and lack a fixed shape—exert pressure in all directions and this pressure
depends on the height of the fluid column above.
Buoyancy: It is the upward force employed by a fluid, which makes objects appear lighter when
submerged. According to Archimedes' Principle, the buoyant force equals the weight of the
displaced fluid.

Buoyant Force = Weight of fluid displaced = ml g ​ (ml =mass of liquid displaced, vl

​ ​

=volume of liquid displaced)

ml = ρ l × v l
​ ​ ​

F B = ρ l × vl × g
​ ​ ​

Apparent Weight: It is the weight of an object in air minus the buoyant force.
Apparent weight-Because an upward force acting on a body is engulfed in a fluid, either wholly or
partially, there occurs an apparent loss in weight of the body. The net weight of an object engulfed in
a fluid is called apparent weight.
Apparent Weight W’ = Weight of body in air − Buoyant Force

Density is mass per volume Kg

m3
​

Relative Density compares a substance’s density to water’s density at 4°C.

CONTENTS

Frequently Asked Questions

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What do you mean by free fall?

Why don’t heavier objects fall faster than lighter ones?

If the Moon attracts the Earth, why doesn’t the Earth move towards the Moon?

How does Earth's shape affect acceleration due to gravity?

What is acceleration due to gravity, and how does it differ from general acceleration?

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