9th class gravitation CBSE NOTES
1) Gravitation : �Gravitation
is the force of attraction between two objects in the universe.
Gravitation may be the attraction of objects by the earth.� �
�Eg :- If a body is dropped from a certain height, it falls downwards due to
body is thrown upwards, it reaches a certain height and then falls downwards
due to the earth�s gravity.
ii) Gravitation may
be the attraction between objects in outer space.��
Eg :- �Attraction
between the earth and moon.
��������� Attraction between the sun and planets.
Centripetal force :-
When a body moves in a circular path,
it changes its direction at every point. The force which keeps the body in
the circular path acts towards the centre of the circle. This force is
called centripetal force.
If there is no
centripetal force, the body will move in a straight line tangent to the
Universal law of gravitation :-
universal law of gravitation states that, �Every object in the universe
attracts every other object with a force which is directly proportional to
product of the masses and inversely proportional to the square of the
distance between them.�
objects A and B of masses M and m lie at a distance d from each other. Let F
be the force of attraction between them.
According to the
universal law of gravitation the force between the objects is directly
proportional to the product of their masses and inversely proportional to
the square of the distance between them
M x m��
F α �1/
Combining the two equations� F
Mxm/ d2�� ��������������
����Or� F = G� �Mxm/ d2�� ���
is a constant of proportionality called �universal gravitation constant��
Cross multiplying we
F x d2�
= G M x m��� or�� G =� F x d2/ M x
The SI unit of G is N
m2 kg -2� and its value is� 6.673 x 10-11 N
m2 kg -2
3) �Free fall :-
The earth attracts
objects towards it due to gravitational force. When an object falls towards
the earth due to the earth�s gravitational force it is called free fall.
�When an object falls
towards the earth there is a change in its acceleration due to the
gravitational force of the earth. So this acceleration is called
acceleration due to gravity.
The acceleration due
to gravity is denoted by g.
The unit of g� is
same as the unit of acceleration� ms -2
�From the second law
of motion, force is the product of mass and acceleration.����
� F = ma
�For free fall, force
is the product of mass and acceleration due to
� F = mg��
� mg = G �M x m/d2
� g = G m/d2�
where M is the mass
of the earth and d is the distance between the object and the earth.
For objects near or
on the surface of the earth d is equal to the radius of the earth R�
�� mg = G M x m/ R2����
or� g =� G M/ R2
The value of� g is�
9.8 ms -2
4 a) Mass :-
The mass of a body is
the measure of its inertia. If the mass of a body is more its inertia is
more.��� The mass of a body is constant and does not change from place to
place.��� The SI unit of mass is kg.
weight of a body is the force with which the earth attracts the body.
The force with which a body is attracted by the earth depends on its mass
m and acceleration due to gravity g.
��� F = m x g
��� Since weight of a
body is the force with which the earth attracts the body,�
m x g
g at a place is constant , W
The weight of a body changes from place to place.
The SI unit of weight
is the same as force � Newton (N).
c) Weight of an object on
the moon :
The weight of an
object on the earth is the force with which the earth attracts the object
and the weight of an object on the moon is the force with which the moon
attracts the object.
The mass of the moon is less than the mass of the earth. So the moon exerts
lesser force on the objects than the earth.
�The weight of an
object on the moon is one sixth (1/6th) of its weight on the
5) Thrust and pressure
Thrust is the force
acting on an object perpendicular to the surface.�� Eg :- When you stand on
loose sand the force (weight) of your body is acting on an area equal to the
area of your feet. When you lie down, the same force acts on an area equal
to the contact area of the whole body. In both cases the force acting on the
sand (thrust) is the same.
Pressure is the force
acting on unit area of a surface.��
�������������������������� ������Pressure =� Thrust/ Area
�� Eg :- The effect
of thrust on loose sand is larger while standing than while lying down.
� The SI unit of
thrust is N/m2� or� N m-2 . It is called Pascal (Pa).
6 a) Pressure in fluids
(Liquids and gases)
Fluids exert pressure
on the base and walls of the container. Fluids exert pressure in all
directions. Pressure exerted on fluids is transmitted equally in all
b) Buoyancy (Upthrust)
�When an object is immersed in a fluid it experiences an upward force called
buoyant force. This property is called
buoyancy or upthrust. ����The force of gravity pulls the object
downward and the buoyant force pushes it upwards.� The magnitude of the
buoyant force depends upon the density of the fluid.
Why objects float or sink in water ?
�If the density of an
object is less than the density of a liquid, it will float on the liquid and
if the density of an object is more than the density of a liquid, it will
sink in the liquid.
Take some water in a
beaker. Take a piece of cork and an iron nail of the same mass. Place them
on the water. The cork floats and the nail sinks.
The cork floats
because the density of cork is less than the density of water and the
upthrust of water is more than the weight of the cork.
The nail sinks
because the density of the iron nail is more than the density of water and
the upthrust of water is less than the weight of the nail.
Archimedes� principle :-
states that, � When a body is partially or fully immersed in a fluid it
experiences an upward force that is equal to the weight of the fluid
displaced by it.�
has many uses. It is used in designing ships and submarines,
Hydrometers used to determine the
density of liquids, lactometers used to determine�
purity of milk etc.
Density and relative density :-
The density of a substance is the mass of a unit volume of the
Density = Mass/� Volume
The unit of density
is kilogram per metre cube (kg m -3).
Relative density :-�
The relative density of a substance is the ratio of the density of a
substance to the density of water.
� Relative density =�
Density of a substance/� Density of water
Since relative density is a ratio of similar quantities, it has no unit.