Class Ix (Science) : SESSION: 2021-2022 Handout On Gravitation (Physics) CHAPTER-11 (PART-1)

CLASS IX (SCIENCE)
SESSION: 2021-2022
HANDOUT ON GRAVITATION (PHYSICS)
CHAPTER-11 (PART-1)
Introduction
→ Gravitational Force of Earth: If we release a small stone without pushing it from
height, it accelerates towards earth.
→ The stone is when accelerated towards earth, means some force is acting on it. The
force which pulls the objects towards the centre of the earth is known as gravitational
force of the earth.
→ Here, stone also attracts earth. It means every object in universe attracts other
object.
Newton’s Universal Law of Gravitation

→ Sir Isaac Newton in 1687 proposed a law about the force of attraction between the two objects
the universe which is known as Newton’s law of gravitation.
According to Universal law of Gravitation

→ Every mass in this universe attracts every other mass with a force which is directly proportional
the product of two masses and inversely proportional to the square of the distance between them.
Let masses (M) and (m) of two objects are distance (d) apart, then force of attraction (F) between
them
F ∝ M× m
F ∝ 1/d2
F ∝ Mm/d2
F = (G M m) /d2
where,
G is a constant and is known as Gravitational constant.
→ If unit of F is in Newton, m is in kg, d is in metre, then unit of G can be calculated as :
G = (F×d2)/Mm, therefor unit will be Nm2/kg2
Value of G = 6.673 × 10-11 Nm2 kg-2
Importance of universal law of gravitation

(i) The force that binds us to the earth.
(ii) The motion of moon around the earth.
(iii) The motion of earth around the sun.
(iv) The tides due to moon and the sun.
Free Fall
Acceleration due to gravity – Whenever an object falls towards the Earth there is an acceleration
associated with the movement of the object. This acceleration is called acceleration due to gravity.
 Denoted by: g
 SI Unit: m s-2
 We know that, F= ma
 Therefore, F = mg
 The following figure demonstrates the mathematical derivation of ‘g'
The force (F) of gravitational attraction on a body of mass m due to earth of mass M and radius R is given
by
We know from Newton’s second law of motion that the force is the product of mass and acceleration.
∴ F = ma
But the acceleration due to gravity is represented by the symbol g. Therefore, we can write
F = mg ….. (2)
From the equation (1) and (2), we get
When body is at a distance ‘r’ from the centre of the earth then
Value of ‘g’ on the surface of earth
Value of ‘g' may vary at different parts of the earth –

 From the equation g = GM/ r 2 it is clear that the value of ‘g' depends upon the distance of the object from
the earth's centre.
 This is because the shape of the earth is not a perfect sphere. It is rather flattened at poles and bulged out
at the equator.
 Hence, the value of ‘g' is greater at the poles and lesser at the equator. However, for our convenience, we
take a constant value of ‘g' throughout.
Difference between G (Gravitational constant) and g (Acceleration due to gravity)

Equation of motion when an object is falling freely towards earth or thrown vertically upward.
When an object is falling towards earth with initial velocity (u)
Velocity (v) after t seconds, v = u + gt
Height covered in t seconds, h = u t + ½gt2
Relation between v and u when t is not given: v2 = u2 + 2gh
Mass
→ The mass of a body is the quantity of matter contained in it. Mass is a scalar quantity which
only magnitude but no direction.
→ SI unit of mass is kilogram which is written in short form as kg.
• Mass of a body is constant and does not change from place to place.
• Mass of a body is usually denoted by the small ‘m’.
• Mass of a body cannot be zero.
Weight
→ The force with which an object is attracted towards the centre of the earth, is called the we
the object.
Force = m × a
In case of earth, a = g
∴ F = m× g
But the force of attraction of earth on an object is called its weight (W).
∴W=m×g
→ Weight is the force and its SI unit is Newton (N). It depends on ‘g’ and is a vector quantity.
Relation between 1 kg wt and express it into Newton
We know that W = m × g
If mass (m) = 1 kg, g = 9.8 m/s2, then
W = 1 kg × 9.8 m/s2
⇒ 1 kg wt = 9.8 N
→ The gravitational force of earth that acts on an object of mass 1 kg is called as 1 kg wt.
Weight of an object on the Moon

Just like the Earth, the Moon also exerts a force upon objects. Hence, objects on moon also have some
weight. The weight will not be same as than on the earth. So, weight on the Moon can be calculated as -

Class Ix (Science) : SESSION: 2021-2022 Handout On Gravitation (Physics) CHAPTER-11 (PART-1)

Uploaded by

Copyright:

Available Formats

Class Ix (Science) : SESSION: 2021-2022 Handout On Gravitation (Physics) CHAPTER-11 (PART-1)

Uploaded by

Document Information

Original Title

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Class Ix (Science) : SESSION: 2021-2022 Handout On Gravitation (Physics) CHAPTER-11 (PART-1)

Uploaded by

Copyright:

Available Formats

CLASS IX (SCIENCE)

Newton’s Universal Law of Gravitation

According to Universal law of Gravitation

Importance of universal law of gravitation

Value of ‘g’ on the surface of earth

Value of ‘g' may vary at different parts of the earth –

Difference between G (Gravitational constant) and g (Acceleration due to gravity)

Relation between 1 kg wt and express it into Newton

Weight of an object on the Moon

You might also like