Newton's law of universal gravitation and
Kepler's third
law
In our earlier article \Conic sections and Newton's law of gravity," we
brie y mentioned
that Newton's law of gravity properly explains Kepler's
rst law.
In this
article, we will see
how it can explain Kepler's third law.
Newton's law of gravity asserts that there are universal attractive force
between any two
massive bodies.
If the mass of each body is
m
1
and
m
2
, and the distance between them
r
,
the force is given as follows:
F
=
Gm
1
m
2
r
2
(1)
Here
G
is a proportionality constant called \Newton's constant."
Notice also
that this is the
force that the
rst object attracts the second object as much as the
force that the second
object attracts the
rst object in light of Newton's third law.
This force is
responsible for

objects on Earth such as apples falling downward.
On the other hand,
the apples attract the
Earth by the same amount of force, but this attraction is practically
negligible since Earth is

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- Fall '10
- noris
- Mass, General Relativity