4. Movement, or a tendency to
move, towards a centre of
gravity, as in the falling of
bodies to the earth is known as
5. The force of attraction between
any two particle in the universe
is called Gravitation or
Gravitational force. For
example: moving of planet
around the sun in universe or
falling of apple from tree toward
7. Johannes kepler gave the following three
law to explain the motion of the planets:
1. Law of orbit: Each planet moves
around the sun in an elliptical orbit
with the sun at one of the foci*of the
* Foci is the plural of the term focus.
8. 2.Law of areas : The line joining the sun and
planet sweeps out equal area in equal interval
let a planet moves from position A
to B in time t and then it moves from
position X to position Y in the same time t .
Then according to kepler’s second law, area
SAB= area SXY
9. 3. Law of period: The square of time taken by
a planet A to complete a revolution around the
sun is directly proportional to the cube of
semi-major axis of the elliptical orbit
i.e., T ∝ r or T = (constant) 𝑟3
𝑟3 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
10. According to this law, the force of attraction
between two Particle or bodies is (i)directly
proportional to the product of their masses
and (ii) inversely proportional to the square
of the distance between these particle or
11. Consider two bodies A and B having mass
m1 and m2 respectively. Let distance
between them be r.
According to the law of gravitation, the
force of attraction (F) or force of
gravitation between these bodies is directly
proportional to :
12. (i) product of masses of bodies
i.e., F∝ m1 m2
And, (ii) inversely proportional to square of
distance between bodies.
Combining eqns. (1)&(2),we get
G is constant and is known as Universal
14. The acceleration with
which a body falls towards
the earth due to earth
gravitation pull is known
as acceleration due to
gravity. It is denoted as
15. Acceleration due to gravity (g) is
i. Directly proportional to the mass of
the earth. And
ii. Inversely proportional to the square
of the radius of the earth.
16. The falling body of which only which
only force of gravitation of the earth
acts is known as freely falling body and
such fall of a body is known as free fall.
A freely falling body has acceleration
equal to acceleration due to gravity.
18. Mass :
ass of a body is defined as the
quantity of matter contained in the
body. Since mass of body is a measure
of inertia of the body, so mass is also
know as Inertia mass.
The unit of mass is kilogram (KG)
19. Weight :
T h e f o r c e w i t h w h i c h t h e
b o d y i s a t t r a c t e d b y t h e
e a r t h i s k n o w n a s t h e
w e i g h t o f t h e b o d y .
20. MASS Weight
Mass of a body remains constant. Weight of a body changes from
place to place.
Mass of body is never zero. Weight of a body at the centre of
the earth is zero.
Mass is a scalar quantity. Weight is a vector quantity.
Mass is measured in Kg. Weight is measured in Kg wt. or N.
Mass is measured by a beam
Weight is measured by a weighing
machine or a spring balance.
21. Radius of earth = 6.4x106 m
Mass of earth = 5.98x1024 Kg
G = gravitational constant; 6.673x10-11
Nm2 / sec2
g = acceleration due to
acceleration (g) on the moon=>