Feynman Lectures on Physics:

The Theory of Gravitation

Planetary Motion

“one of the most far-reaching generalizations of the human mind”

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

Kepler’s 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 α a3/2

Development of Dynamics

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

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

Newton’s 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 Kepler’s 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

Newton’s law of Gravitation

“It falls away from the straight line that it would pursue”

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

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

Cavendish Experiment

The experiment is called – “weighing the Earth’”

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

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

Gravity and Relativity

“In spite of all the excitement it caused Newton’s law of Gravitation is not correct”

Einstein modified Newton’s laws of Gravitation

Added in the effects of relativity to make Einstein’s 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