Understanding Gravity

1
Understanding Gravity

• Rebecca Drozd, Megan Vallely, David Reber Jr.,
  Zachary Gianola, Patricia Arble

2
What is Gravity?

• Gravity is the force of attraction that acts
  between all objects
• For examplegravity brings this child back into
  their parents arms

3
Before Discovery

• Before gravity was discovered people plainly
  thought that objects fell because they were
  supposed to
• In the 300s B.C. Aristotle, a Greek philosopher
  and scientist documented the first thoughts on
  the law of gravity

4
Aristotles Thoughts on Gravity

• Aristotle thought that a stone falls because it
  has a nature within it and that nature brings it
  to its natural place, the center of Earth
• At this time people believed there were 4
  natures in the world earth, air, wind and fire
• Aristotle believed that heavier objects fell
  faster than light ones
• These views were accepted and unchallenged for
  almost the next 2000 years

5
Galileos Thoughts on Gravity

• Galileo discovered that dropped objects have a
  constant acceleration
• Contradicting Aristotle, he believed that if 2
  falling objects of different weights were
  attached and dropped then the lighter one would
  slow the larger one. This did not occur, thus
  Galileo believed all objects fall at the same
  speed unless air resistance or some other force
  acts upon them

6
Newtons Expansion on Gravity

• Sir Isaac Newton decided to study gravitational
  forces outside of the Earth.
• He came up with the idea that gravitational
  forces would reach the moon, thus the moon
  orbiting the earth is a result of gravitation to
  the earth.

7
Newtons Law of Universal Gravitation

• He decided to try and prove his theory of
  gravitation and came up with a concept through
  the thought of the following experiment.
• Suppose that you fired a cannon horizontally from
  a high mountain, the ball would eventually fall
  to the earth due to gravity. Now as you
  increased the velocity the ball would travel
  farther and farther around the earth until
  eventually the ball would be continuously falling
  toward the earth but never reaching the earth.
  This is because the earth is curving away at the
  same rate that the ball is falling toward the
  earth.
• He called this the Law of Universal Gravitation

8
After the Discovery Scientists who made
gravitational discoveries following Isaac Newton
include

• Lorentz
• Poincaré
• Einstein

• Ricci
• Rita-Civita
• Hilbert

9
Their Discoveries

• Lorentz and Poincaré thought that gravitation
  could be related to actions which increase with
  the velocity of light and did not believe
  Newtons law of gravitation was valid
• Einstein proposed the special theory of
  relativity (1905) and the equivalence principle
  (1907) ... we shall therefore assume the
  complete physical equivalence of a gravitational
  field and the corresponding acceleration of the
  reference frame. This assumption extends the
  principle of relativity to the case of uniformly
  accelerated motion of the reference frame.

10
Discoveries Continued

• Einstein Continued realized that the bending of
  light in a gravitational field could be checked
  with astronomical observations (1911), noted
  Lorentzs theories will not apply in a general
  setting (1912), realized that gravitational field
  equations were bound to be non-linear and the
  equivalence principle only appeared locally
  (1912)
• Ricci and Rita-Civita developed tensor calculus
  (1913) pointed out errors on Einstein's work on
  tensors and exchanged ideas with Einstein (1915)
  

11
Lastly

• Einstein and Hilbert recognized flaws in
  Einsteins previous discoveries on the theory of
  general relativity. They
• discovered the same form of gravitational field
  equations within days of each other (1915)
• collaborated their discoveries to complete the
  theory
• The Theory of Relativity was finally explained
  in simple terms by Einstein (1916) and plays a
  role in areas including cosmology and the big
  bang theory. It has also been experimented to a
  high level of accuracy

12
References

• Paintings of Newton and Galileo
• http//www-groups.dcs.st-and.ac.uk/history/PictDi
  splay/Galileo.html
• http//www-groups.dcs.st-and.ac.uk/history/PictDi
  splay/Newton.html
• Information
• http//www-history.mcs.st-andrews.ac.uk/HistTopics
  /Gravitation.html
• http//www-history.mcs.st-andrews.ac.uk/HistTopics
  /General_relativity.html