Every object in the universe attracts every
other object with a force which is called the force of gravitation
Newton’s Law of
Gravitation
Gravitational
force is a attractive force between two masses m1
and m2 separated by a distance
r.
The
gravitational force acting between two point objects is proportional to the
product of their masses and inversely proportional to the square of the
distance between them.
Gravitational
force. 
where
G is universal gravitational constant.
Dimensional formula of Gis [M-1L3T-2].
The value of G
is 6.67 X 10-11 Nm2 kg-2 and is same
throughout the universe.
The
value of G is independent of the nature and size of the bodies well as the
nature of the medium between them.
(i)
Gravitational force is a central as well
as conservative force.
(ii)
It is the weakest force in nature.
(iii)
It
is 1036 times
smaller than elect
(iv)
The law of gravitational is applicable
for all bodies, irrespective of their size, shape and
position.
(v)
Gravitational force acting between sun
and planet provide it centripetal force for orbital motion.
(vi)
Gravitational pull of the earth is
called gravity.
(vii)
Newton’s third law of motionmeansthe
holds gravitation forces between two bodies are action-reaction pairs.
Following three
points are important regarding the gravitational force
(i)
Unlike the electrostatic force, it is
independent of the medium between the particles.
(ii)
It is conservative in nature.
(i)
It expresses the force between two point
masses (of negligible volume). However, for external points of spherical bodies
the whole mass can be assumed to be concentrated at its centre of mass.
Acceleration
Due to Gravity
The
uniform acceleration produced in a freely falling object due to the
gravitational pull of the earth is known as acceleration due to gravity.
It
is denoted by g and its unit is m/s2.
It is a vector quantity and its direction is towards the centre of the earth.
The
value of g is independent of the mass of the object which is falling freely
under gravity.
The
value of g changes slightly from place to place. The value of g is taken to be
9.8 m/s2 for all practical
purposes.
The
value of acceleration due to gravity on the moon is about. one sixth of that On
the earth and on the sun is about 27 times of that on the earth.
Among
the planets, the acceleration due to gravity is minimum on the mercury.
Relation between g and a is given by
g = Gm / R2
where
M = mass of the earth = 6.0 * 1024
kg and R = radius of the earth = 6.38 * 106
m. Acceleration due to gravity at a height h above the surface of the earth is
given by
gh = Gm / (R+h)2 = g (1 –2h / R)
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