# Gravitation class 9 physics Notes Free.

## S.Chand book explaination.

Hey! Students today we are gonna to understand the one of important chapter of Physics class 9
Gravitation.

#### Gravitation

Gravitation is a force between any two bodies in the universe.

#### Gravity

The force with which earth attracts any object towards its surface of earth is called force of Gravity.

#### Effects of gravity

1)when we throw a ball it fall towards earth because of gravity.

2)Rainfall occurs due to gravity.

3)The gravitational force of earth causes water to flow in the rivers.

4) Planets revolves in uniform motion because of gravitational force.

#### *UNIVERSAL LAW OF GRAVITATION

Universal law of gravitation states that every body (object) in the universe attract every object/body with a force which is directly proportional to product of their masses and inversely proportional to square of distance between them.

1)F∝m1*m2

2)F∝1/r^2

So the
Formula of Gravitation

F=m1*m2/r^2
Where
m=mass of objects
r=radius / distance between theme(cente to centre)

Gravitation class 9 physics Notes Free.

#### VALUE OF GRAVITATIONAL CONSTANT (G)

The value of "G" is 6.67*10^-11Nm^2/kg^2

GRAVITATION FORCE BETWEEN OBJECTS OF SMALL SIZE AND BIG SIZE.

When the masses of both objects are so small then their GRAVITATIONAL force between them will si weak
(Negligible) which can't be observed.
Ex- GRAVITATIONAL force between two cricket balls.

But when the masses of the one or both objects is/are large then the gravitational force between them is also large which can be easily observed.

Ex-Gravitational force between a ball and the earth is clearly observed.
Earth attracts ball towards its centre of earth due to which it comes downward the surface of earth.

Gravitation class 9 physics Notes Free

### KEPLER'S LAW OF PLANETRY MOTION

There are three laws of KEPLER'S planetary motion.

1)All the planets moves in elliptical orbits around the sun ,with the sun at one of its two focii(plural of focus).

2) Every plantes revolves around the sun in such way that the line joining the planets to the sun sweeps over equal areas in equal interval of time.

Conclusion:(A planet moves faster when it is nearer to the sun and moves slower when it is farther from the sun.

3)The cube of mean distance of the  planet from the sun is directly proportional to the square of time it takes to move around the sun.

=r^3∝T^2

Note: Gravity follows inverse square law.

Gravitation class 9 physics Notes Free

#### FREE FALL

When a body falls towards earth with Gravitational force acting on it and no other force acting on it then the body is said to be in free fall.

Ex Falling of apple toward the surface of earth.

ACCELERATION DUE TO GRAVITATY

When a body is realeased from a height an uniform acceleration is produced in it by the gravitational pull of the earth which is reffered as ACCELERATION DUE TO GRAVITATY.

Acceleration of a body doesn't depends on the mass.

It is represented by "g"
And its value is 9.8 m/s^2

VARIATION OF ACCELERATION DUE GRAVITY

Point to be noted that the value of acceleration due to gravity is not constant.
It's variable.
For example the radius at the pole is minimum so the acceleration due to gravity at poles are maximum.
And at equator the radius is maximum so the acceleration due to gravity is minimum.

*As we go down inside the earth the value of g will decrease.
*The value of g is maximum at the surface of earth.

g=G*M/R^2

#### 1) v= u+gt

Where,
u=initial velocity
v=final velocity
g=acceleration due to gravity
h= height

#### 2)h=ut+1/2gt^2

Where,
u=initial velocity
v=final velocity
g=acceleration due to gravity
h= height

#### 3)v^2=u^2+2gh

Where,
u=initial velocity
v=final velocity
g=acceleration due to gravity
h= height

#### MASS.

The matter contained in a body is said to be it's mass.
Of
Mass is the measure of inertia.
Higher the mass higher the inertia.

*It's SI Unit is Kilograms (Kg)
*It's a scalar quantity.
*It can never be zero.
* It doesn't changes place to place.

#### The weight is nothing but the gravitational force by which a body is being attracted towards the centre  of the  earth.

*It's SI Unit is Newton (N).
*It is a vector quantity.
* It can be zero.
*It changes place to place.

#### THRUST AND PRESSURE

Pressure is the force acting on per unit area .
It's si unit is Pascal (P).
Pressure=Force/Area

THRUST is the pressure exerting perpendicularly.
Its SI Unit is also Pascal (P).

#### BUOYANCY

BUOYANCY is the upward force exerted by the fluids(generally water).