Class 9 Science Chapter 10 Gravitation Notes

9 Class Science Chapter 10 Gravitation Notes

Textbook	NCERT
Class	Class 9
Subject	Science
Chapter	Chapter 10
Chapter Name	Gravitation
Category	Class 9 Science Notes
Medium	English

Class 9 Science Chapter 10 Gravitation Notes. here we will be learn about Earth’s gravity, Newton’s law of gravity, free fall, acceleration due to gravity, mass, weight, thrust, pressure, buoyancy, Archimedes’ principle etc.

Class 9 Science Chapter 10 Gravitation Notes

Chapter - 10

Gravitation

Gravitational Force of Earth :-

If we release a small stone without pushing it from a 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 every 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 in the universe which is known as Newton’s law of gravitation.

According to this law :-

Every mass in this universe attracts every other mass with a force which is directly proportional to the product of two masses and inversely proportional to the square of the distance between them.

Let masses (m₁) and (m₂) of two objects are distance (d) apart, then force of attraction (F) between them

F∝m₁×m₂
F∝1/d²
F∝m₁m₂/d²
F = Gm₁m₂/d²

where G is a constant and is known as Gravitational constant.

Value of G = 6.67 × 10⁻¹¹ Nm²/kg²
G is called universal 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/m₁×m₂ so unit be Nm²/kg² or Nm²/kg²

Relation between Newton’s third law of motion and Newton’s law of gravitation :-

According to Newton’s third law of motion, “Every object exerts equal and opposite force on other object but in opposite direction.”

According to Newton’s law of gravitation, “Every mass in the universe attracts the every other mass.”

In case of freely falling stone and earth, stone is attracted towards earth means earth attracts the stone but according to Newton’s third law of motion, the stone should also attract the earth and really it is true that stone also attracts the earth with the same force F = m × a but due to very less mass of the stone, the acceleration

(a) in its velocity is 9.8m/s² and acceleration
(a) of earth towards stone is 1.65 x 10⁻²⁴ m/s² which is negligible and we cannot feel it.

Importance of universal law of gravitation :-

The force that binds us to the earth.
The motion of moon around the earth.
The motion of earth around the sun.
The tides due to moon in the sea.

Free fall of an object and acceleration during free fall :-

When an object is thrown upward, it reaches certain height, then it starts falling down towards earth. It is because the earth’s gravitational force exerts on it.

This fall under the influence of earth is called ‘free fall of an object’.

During this free fall direction do not change but velocity continuously changes which is called acceleration due to gravity.

It is denoted by ‘g’.
Its unit is same as acceleration m/s².

Gravitational Acceleration and its value at the surface of earth :-

The uniform acceleration produced in a freely falling object due to the gravitational force of earth, is called acceleration due to gravity.

It is represented by ‘g’ and it always acts towards the centre of the earth.

Value of ‘g’ on the surface of earth

The force acting on an object is F = GMem/R²
Where
Me = Mass of earth
m = Mass of an object
R = Radius of earth

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

Gravitation Constant (G)	Gravitational acceleration (g)
Its value is 6.6734 × 10⁻¹¹ Nm²/kg².	Its value is 9.8 m/s².
Its value remains constant always and everywhere.	Its value varies at various places.
Its unit is Nm²/kg².	Its unit is m/s².
It is a scalar quantity.	It is a vector quantity.

Mass :-

The mass of a body is the quantity of matter contained in it. Mass is a scalar quantity which has 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 weight of the object.

Force = m × a
In case of earth, a = g
So, F = m×g

But the force of attraction of earth on an object is called its weight (W). So, W = m x g

So, weight is the force and its SI unit is Newton (N). It depends on ‘g’ and is a vector quantity.

Different between Mass and Weight :-

Mass	Weight
We can measure mass of an object by its inertia.	Weight = Weight mass × acceleration or m×g.
The total quantity of matter contained in an object is called mass of an object.	The gravitational force by which earth attracts an object is called weight of the object.
Mass of the object remains constant at all the places.	Weight of the object is different at different places.
Measurement of mass is done by using a pan or beam balance.	Measurement of weight is done by using a spring balance.
Mass does not change even value of g is zero at any place.	Weight of the object becomes zero if g is zero.

Factors affecting value of g :-

Earth is not a perfect sphere. The radius of earth increases when we go from pole to equator. Therefore, in most of the calculation, we can take g as constant at the surface of earth or closer to it. But, as we move away from earth, we can use equation g = GM/D² for solving problems.

Thrust and Pressure :-

Thrust :- The force acting on an object prependicular to the surface is called thrust.

Pressure :- The effect of thrust per unit area is called pressure.

Pressure (P) = Force (F)/Area (A) = Newton/meter = N/m²
SI unit is N/m² or Nm⁻².
SI unit of pressure is Pascal (Pa).

Factors on which pressure depends :-

Pressure depends on two factors :-

Force applied
Area of surface over which force acts

Buoyancy :-

The upward force experienced by an object when it is immersed into a fluid is called force of buoyancy. It acts in upward direction and it depends on the density of the fluid.

Force of gravitational attraction of the earth on the surface of the object < buoyant force exerted by fluid on the surface of the object.

Result :- The object floats.

Force of gravitational attraction of the earth on the surface of the object > buoyant force exerted by fluid on the surface of the object.

Result :- The object sinks.

That is why, allpin sinks and boat/ship floats on the surface of water. (Archimedes’ principle)

Density :-

The mass per unit volume is called density of an object. If M is the mass and V is the volume, then density (d) is

Density (d) = Mass (M)/Volume (V)
SI unit = kg/m³

Archimedes’ Principle :-

It states, when a body is immersed fully or partially in a fluid, it experiences a upward force that is equal to the weight of the fluid displaced by it.

Applications of Archimedes’ Principle :-