History of Gravity

A Quick History of Gravity 300 BC Aristotle

Things go to a natural spot, heavy things fall, light things float, heavy objects fall faster than light objects.

Experimentation was discouraged and considered to be “beneath the dignity” of philosophers. Middle ages roughly 500AD to 1400AD

Church adopted ideas of Aristotle. Since the Church gave this idea the seal of approval it was unquestioned by Universities and Professors of the time. (don’t argue with the church, you will lose…..your head)

Experimentation is not needed; god will lead you to the truth Rennaissance 1400-1700

Economic growth, people have time and money

Increased interest and activity in artistic, social, scientific, and political thought Shakespeare, Michelangelo, Galileo, Leonardo Da Vinci, Christopher Columbus

(Aristotle already answered the question “Why do they fall?”) Galileo asked, “How do objects fall?” Rolled ball down a ramp found object accelerate when they fall, also found that without friction all objects accelerate at the same rate.

Aristotle was wrong!!

New era in science Experimentation was used to answer questions; conclusions were based upon careful measurements and mathematics Isaac Newton 1642-1727

Newton used Galileo’s information experimental results and then asked, “Why doesn’t the moon fall?”

He thought about a cannonball shot from a tower. The faster it is moving the farther out it will land. If the ball was moving fast enough it would never hit the earth because the earth is round!!!!

He knew the moon orbited the earth He knew that for something to travel in a circular path a force is needed (remember his first law?)

According to the story he watched an apple fall from a tree and he concluded the same force that pulls an apple from a tree also pulls on the moon.

This is a big leap in thought. Newtonian synthesis: What holds true on earth also holds true for the rest of the universe. He Connected the Terrestrial with the Celestial. There are not separate rules for earth and space!!!!!!! He knew that for something to travel in a circular path it is accelerating.

Ac = 4π 2R Acceleration ∞ Force so substitute F for Ac T2 F ∞ 4π 2R T2 Keplers law of planetary motion showed that T2 ∞ R3

So substituting R3 for T2 the formula becomes F ∞ 4π 2R R3 cancel the R and you get F ∞ 4π 2 R2 4π2 is a constant so the related values are F ∞ _1_ R2 This is the inverse square law !!!

Force of gravity equals ___1___ F= _1_ (distance)2 d2

Now all Newton had to do was test his calculations with the moon and find out if he was correct!!!

The moon is 60 times farther from the center of the earth as the apple so…. The force on the moon should be 1/d2 or 1/602 the force on earth

Calculate the acceleration of gravity acting on the moon

1/d2 or 1/602 = 1/3600 he knew gravity on earth was 9.8m/sec2

9.8m/sec2 x 1/3600 = .00272 m/sec2

All he had to do next was to calculate acceleration of the moons orbit around earth

Ac = 4π 2R The average radius( R ) of the moons orbit is (3.83E8 meters) T2 The Period ( T ) of the moons orbit is 2.36E 6 sec

Plug the numbers into Ac = 4π 2R and check Newton’s result !!! T2

How did he do???? He found he could explain the motion of the entire solar system, moons, comets etc. with one law. The universal law of gravity! All objects attract each other and the strength of that attraction is related to the mass of the object and the square of the distance

Fg = M1 m2 D2

Phillip Von Jolly 1809 Found the value of the constant G. This allows us to calculate the force of gravity in Newton’s for any object.

The value of G is 6.67 E -11 N M 2 Kg2

Find the force of gravitational attraction between two 60kg people standing 1 meter apart

Fg = G M1 m2 D2

John Adams 1845 and Urbain Leverrier 1846

Astronomers noticed the orbits of Jupiter and Saturn were perturbed when they past each other. They noticed that the orbit of Uranus was also perturbed. Working independently and using the Universal Law of Gravitation they discovered the planet Neptune. Pluto was discovered using the same technique in 1930. 1900 Roland Eotvos Why do motion and gravity both involve mass? Shouldn’t gravity and motion be different things?

Gravity mass inertia mass

Fg = M1 m2 F= ma D2

Roland Eotvos there are not different types of mass.

Inertia mass or acceleration is equal to gravitational mass

Einstein early 1900’s

Imagine sitting in a room behind a desk with paper pencil etc. one choice is true a. you are in a room on earth b. you are in a room in space being accelerated by a rocket ship

You cannot tell the difference!!! Gravity and acceleration are the same thing!!!

From your frame of reference you can cancel out gravity and acceleration if you are moving in the right way.

If you are in free fall around the earth from your frame of reference you are moving at a constant speed in a straight line( inerta) and no force(gravity) is needed

Mass( matter) affects space and space affects mass, time and distance are dependent on matter.

Clocks on satellites or airplanes run slower than clocks on earth. Starlight bends as in passes the sun. These are real changes

General relativity changes the way we look at time, and space.