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 = 4π2R
Acceleration ∞ Force so substitute F for Ac
2
T
F ∞ 4π2R
T2
Keplers law of planetary motion showed that T2 ∞ R3
So substituting R3 for T2 the formula becomes F ∞ 4π2R
R3
cancel the R and you get F ∞ 4π2
R2
4π2 is a constant so the related values are F ∞ _1_
R2
This is the inverse square law !!!
Force of gravity equals ___1___
(distance)2
F= _1_
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 = 4π2R
T2
The average radius( R ) of the moons orbit is (3.83E8 meters)
The Period ( T ) of the moons orbit is 2.36E 6 sec
Plug the numbers into Ac = 4π2R
T2
How did he do????
and check Newton’s result !!!
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 M2
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
Fg = M1 m2
D2
inertia mass
F= ma
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.