Feynman Lectures on Physics:

1
Feynman Lectures on Physics

• The Theory of Gravitation

2
Planetary Motion

• Story begins with the discovery that the planets
  go around the sun
• Copernicus re-discovered this
• Tycho Brahe decided to measure the planets
  positions (15th century)
• Kepler studied Brahe work and came up with a set
  of laws

• one of the most far-reaching generalizations of
  the human mind

3
Keplers laws

• Found each planet goes round sun in an ellipse
  with sun at focus
• Planets move faster when nearer the sun
• The relationship between the orbit size and the
  period of any planet can be given by T a a3/2

4
Development of Dynamics

• People believed that Planets kept going round as
  angels were pushing them.
• Galileo was studying why the planets orbit
• A body will keep moving until a force is acted
  upon it (inertia)
• The force needed to keep a planet in orbit acts
  towards the sun not tangentially

• An eclipse is not just an oval, but is a very
  specific and precise curve that can be obtained
  by using two tacks

5
Newtons law of Gravitation

• It falls away from the straight line that it
  would pursue

• Newton noticed the law of areas is because the
  force is acting towards the sun
• Newton showed from Keplers third law and the
  fact that the force is toward the sun to show
  that there is a force which acts between to
  objects and it is inversely proportional to the
  distance
• Newton generalised the force of gravity

6
Newtons law of Gravitation

• Newton compared ball falling on Earth to moon
  falling in space to show the validity of inverse
  square law
• Objects fall 16 feet in one second whether
  horizontal or vertical release
• If you shoot a bullet quick enough the height in
  one second will be the same as when it left as
  the earth is curved
• The law of Gravitation explains the tides

• It falls away from the straight line that it
  would pursue

7
Universal Gravitation

• We can deduce that the planets, sun and moon
  should be nearly spheres
• Explained the moons of Jupiter (and the light)
• Explained the orbit of Uranus and found planet
  (Neptune)
• Explained the motion of double stars around one
  another
• Works out to distances as far as Galaxies

8
Cavendish Experiment

• Cavendish designed an experiment to weight the
  Earth
• Relates the attraction between the masses to the
  torsion in the wire and the oscillatory movement
• Found that G 6.67 (10-11)
• Used G to calculate the weight of the Earth

• The experiment is called weighing the Earth

9
What is Gravity?

• There are many theories of what gravity is not
  just how it works
• One such theory was an ether
• No relation of gravity to the other forces like
  electromagnetic
• Behaviour of forces is very similar (unified
  theory)
• Ratio between forces is massive

10
Gravity and Relativity

• Einstein modified Newtons laws of Gravitation
• Added in the effects of relativity to make
  Einsteins law of gravitation
• Since light has energy and energy is mass, then
  light can be deflected by an object
• Challenge to quantify gravity to use quantum
  mechanics

• In spite of all the excitement it caused
  Newtons law of Gravitation is not correct