Class 9 chapter 10 Gravitation

Gravitation is the force of
attraction between two
objects in the universe.
i) 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 earth’s gravity.

If a 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.

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 circular
path.

The 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’.

The universal law of gravitation successfully
explained several phenomena which were
believed to be unconnected:
(i) the force that binds us to the earth;
(ii) the motion of the moon around the earth;
(iii) the motion of planets around the Sun;
and
(iv) the tides due to the moon and the Sun

 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.
 This acceleration is called acceleration due to
gravity or gravitational acceleration.
 The acceleration due to gravity is denoted by g.
 The unit of g is same as the unit of acceleration
ms -2

 As the radius of the earth increases from the
poles to the equator, the value of g becomes
greater at the poles than at the equator.
 As we go at large heights, value
of g decreases.

Value of universal gravitational constant, G =
6.7 × 10–11 N m2/ kg2,
Mass of the earth, M = 6 × 1024 kg, and
Radius of the earth, R = 6.4 × 106 m
Putting all these values in equation (iii), we get:

S.no. Gravitation Constant
(G)
Gravitational
acceleration (g)
1. Its value is 6.67×10-11
Nm2/kg2.
Its value is 9.8 m/s2 .
2. It is a scalar quantity. It is a vector quantity.
3. Its value remains
constant always and
everywhere.
Its value varies at
various places.
4. Its unit is Nm2/kg2. Its unit is m/s2

Let an object is falling towards earth with initial
velocity u. Let its velocity, under the effect of
gravitational acceleration g, changes to v after
covering the height h in time t.
Then the three equations of motion can be
represented as:
Velocity (v) after t seconds, v = u + ght
Height covered in t seconds, h = ut + ½gt2
Relation
between v and u excluding t, v2 = u2 + 2gh

The value of g is taken as positive in case of
the object is moving towards earth and
taken as negative in case of the object is
thrown in opposite direction of the earth.

 The amount of matter in a body is call 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.
 Mass of a body can never be zero .

 The 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,
W = m x g
Since g at a place is constant , W α m
 The weight of a body changes from place to
place.
 The SI unit of weight is the same as force –
Newton (N).

Mass Weight
It is a scalar Quantity. It is vector Quantity.
The value of the mass
remains constant.
The value of weight varies
according to the position of
an object.
It is represented by “m”. It is represented by “W”.
Its SI unit is kg. Its SI Unit is Newton.
It is measured instruments
like beam balance.
It is measured instruments
like spring balance.
Its formula is: m = w/g Its formula is : W = mg

 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.
 Mass of an object is same on earth as well as
on moon. But weight is different.

Weight of an object is given as:

 Hence, weight of the object on the moon =
(1/6) × its weight on the earth.
 We can calculate the ratio of weight of an
object on the surface of moon and on the
surface of earth by putting the values from
the following table in the steps below-

Class 9 chapter 10 Gravitation

Class 9 chapter 10 Gravitation

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Class 9 chapter 10 Gravitation