Sir Isaac Newton (1642 - 1727)
The Giant of the 18th Century

"Newton is clearly the most influential scientist who ever
 lived. His accomplishments in mathematics, optics, and
 physics laid the foundations for modern science and
 revolutionized the world."
 

By "standing on the shoulders" of Galileo, Kepler
and others, Newton was able to understand (not
only describe) the laws of planetary motion. His
three laws of motion  describe
all types of motion - not  only the motion of the
planets. The movement of the planets in their orbits
is governed by the gravitational force between the
Sun and the Planets. If the Sun were suddenly to
vanish the planets would escape from their orbit
and would travel through space in a straight line.
 

Newton's Three Laws of Motion:

1) A body continues at rest or in uniform motion in a
      straight line unless acted upon by some external
      force.

2) A body's change of motion (velocity) is proportional
     to the force acting on it and is in the direction of the
     force

    F  = ma

F = Force aplied to the object
m= mass of the object
a = resulting acceleration

3) When a body exerts a force on a second body, the
     second body must exert an equal an opposite force
     back on the first body.


   Newton's seminal contribution to planetary motion is
    his Law of Gravitation.

a) The force is prorportional to the PRODUCT of the
     masses. That is, the force with which, say the Sun acts
     on a Planet, is proportional to the mass M1 of the Planet
     and to the mass M2 of the Sun.

b) The force is inversely proportional to the SQUARE of
     the Sun-Planet distance r. If the distance between the
     Sun and the Planet is increased by a factor of two, the
     force becomes 4 times smaller (weaker).

c) The force is always ATTRACTIVE: the Sun wants to
     "engulf" the Planet - it never pushes it away.

d) G is  a UNIVERSAL constant: G=6.672x10-11 m3/kg/s2
     It is the same for any two objects in the Universe!!!