Physics
Etymology of the word “Physics”
Physics comes from the Greek word “Fυσικά,” title of one of Aristotle’s book;
it is an adjective meaning everything related to nature (“Fύσις”.)
Thus, any type of motion as the solar system evolution, all we can investigate with our senses
that is related to nature belongs to the field of Physics.
In antiquity, sciences appeared and were prematurely systematized not strictly separated, as in
modern times, but they all belonged to the global science of “philosophy;” today, of course,
sciences are fully separated and specialized, while philosophy has been reduced to a discipline
of the humanitarian sciences.
•Archeological findings reveal that nature and celestial motions had attained
the interest of different cultures and nations of the ancient times. Assyrians
(the ancient people of Syria nowadays),
Babylonians (the inhabitants of Mesopotamia, Iran-Iraq nowadays) and ancient
Egyptians had advanced knowledge of astronomy, forming very accurate
calendars
Of course, that was not due to scientific research and theoretical interest, but
to necessity for their land cultivation; they had observed specific phenomena
as for example, the rise of the waters of the Nile which occurred periodically
every year and that initiated their knowledge.
The indigenous populations of America, Mayas and Aztecs, also had
important astronomical knowledge as demonstrated by their monuments and
very accurate (even for our time) calendars (of course their culture reached its
peak almost two millenniums after, around 600 AD to 1000 AD!)
The first to present a systematized scientific thought were ancient Greeks, who
passed from the everyday problems to theoretical research and, thus, gave
birth to sciences.
Interestingly, many modern theories converge or even are inspired for future
research by theories presented by Greek philosophers: the “Big Bang” theory
itself is believed to have many common points with Heraclitus theory of the
“Big Fire Sphere” “Sφαῖρος” from which everything initiates and to which
everything eventually returns at the end of Times.
There is a famous quote by Heraclitus: “Πάντα ῥεῖ καὶ οὐδὲν μένει” (English:
Everything flows, nothing stands still); Quantum Physics claim today that even
at absolute zero
(0°K) quantum-mechanical motion disturbances appear,
confirming Heraclitus 2 and a half milleniums after!
One of the most fundamental theories of the ancient Greek thought which is
also our modern society point of view about the construction and evolution of
nature and space is the theory of “atomism;” the philosophical belief (and
experimentally proved nowadays) that everything is composed entirely of
various impenetrable, indivisible elements called the “atoms” (“άτομον,” adjective
meaning that it can no further be divided, cut). Theory expressed by three
philosophers:
(a) Leucippus or Leukippos (Greek Λευκιππος, first half of 5th century BC)
There are no existing writings which we can attribute to Leucippus, since his writings seem to have
been enfolded into the work of his famous student Democritus. The most famous among Leucippus'
lost works were titled Megas Diakosmos (The Great Order of the Universe or The great worldsystem) and Peri Nou (On mind).
A single fragment of Leucippus survives: Nothing happens at random (maten), but everything from
reason (ek logou) and by necessity. Leucippus, Diels-Kranz 67 B1
(b) his student Democritus, 460-370 B.C.,
Democritus argued the eternity of existing nature, of void space, and of motion.
He supposed the atoms are originally similar, and that even the human soul
consists of globular atoms of fire, which impart movement to the body.
and (c) Epicurus, 341-270 B.C.
Epicurus adopted the atomism of Leucippus and Democritus, maintaining that
all objects and events—including human lives—are in reality nothing more than
physical interactions among minute indestructible particles. His difference was
that he believed there is no necessity {Gk. anagkh [anankê]} about any of this,
of course; everything happens purely by chance.
The most important scientific
contribution of the Greeks to modern
times was their knowledge and
extensive research on the celestial
world; Greek mythology found its place
not only on our solar system, with the
name of the sun and the six known
planets of antiquity, but also to all the
constellations of the northern
hemisphere and partially the southern,
giving names which still hold in our
times.
Celestial Sphere
Map of the
Mediterranean Sea
That research was the
outcome of their need for navigation
(even during night),
thus
rendering constellations their
secure compass over night.
Moreover,
their moon-based 12
month calendar (even though the
names of the months
are
completely changed and their
duration changed slightly)
and seven
days weekly calendar
was adopted by
the Romans, the
Christians and was
inherited to
our modern societies.
Constellation of Orion
Names of the 7 days:
French:
Lundi
Mardi
Mercredi
Jeudi
Vandredi
Samedi
Dimanche
English:
(lune: moon)
(mars)
(mercure)
(jeus)
(venus)
(Christian)
(Christian)
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday
planet/God(ess)
(moon)
(G.Eq.*)
(G.Eq.)
(G.Eq.)
(G.Eq.)
(Saturn)
(Sun)
Moon(Diane)
Mars/Tyr,Tew
Mercury/Woden Wodnes
Jupiter/Thor
Venus/Freya-Frige
Saturn
Sun
Interesting enough, Greeks no longer use these names! (They name days after the Bible)
*G.Eq.: German Equivalent
Except from the great contribution of Aristotle, Plato and the other
Greek philosophers, an important figure was the astronomer
Aristarchus of Samos, (310-230 B.C.) who firstly introduced the idea
of the heliocentric solar system at 231 B.C., i.e. that the center of our
solar system was not the earth as believed but the sun.
To arrive at his conclusion he proved that the bigger of the three
bodies, the sun the earth and the moon, is the sun and by far, and
therefore the sun is at the center of the solar system.
One last, but not least, reference should be made for the so
believed most important and influential of all ancient Greek
astronomers, Hipparchus of Rhodes, (190 – 120 B.C.),
the person who made the most important contribution before that of
Copernicus in the early 17th century A.D.
His approach to science ranks him far above other ancient
astronomers.
It was based on data from accurate observations, and is essentially
modern in that he collected his data and then formed his theories to
fit the observed facts.
Most telling regarding his understanding of the scientific method is
the fact that he proposed a theory of the motion of the sun and the
moon yet he was not prepared to propose such a theory for the
planets.
He realized that his data was not sufficiently good or sufficiently
plentiful to allow him to base a theory on it.
However, he made observations to help his successors to develop
such a theory. Most of his work was incorporated in the work of
Claudius Ptolemaeus, Greek:Κλαύδιος Πτολεμαῖος, English: Ptolemy (90-168)
Greek-speaking geographer, astronomer, and astrologer who lived in the
Hellenistic culture of Roman Egypt.
Ptolemy was the author of several scientific treatises, three of which have been of
continuing importance to later Islamic and European science.
The first is the astronomical treatise that is now known as the Almagest (in Greek
Η μεγάλη Σύνταξις, "The Great Treatise").
The second is the Geography, which is a thorough discussion of the geographic
knowledge of the Greco-Roman world.
The third is an astrological treatise known as the Tetrabiblos ("Four books").
Astronomy
In the Almagest, one of the most influential books of classical antiquity, Ptolemy relied
mainly on the work of Hipparchus of three centuries earlier.
It was preserved, like most of Classical Greek science, in Arabic manuscripts (hence its
name) and only made available in Latin translation in the 12th century.
Ptolemy formulated a geocentric model that was widely accepted until the
heliocentric solar system of Copernicus.
Likewise his computational methods were of sufficient accuracy to satisfy the needs of
astronomers and navigators, until the time of the great explorations.
They were also adopted in the Arab world and in India.
The Almagest also contains a star catalogue, which is probably an updated version of a
catalogue created by Hipparchus.
It’s list of forty-eight constellations is ancestral to the modern system of
constellations, but unlike the modern system they did not cover parts of the southern
hemisphere (only the sky Ptolemy could see).
The Almagest is also known as the Great Syntaxis of Astronomy.
Other works
In his Optics, a work which survives only in a poor Arabic translation, he writes about properties of
light, including reflection, refraction and colour. The work is a significant part of the early history of
optics. His other works include Planetary Hypothesis, Planisphaerium and Analemma.
Ptolemy's theorem
If the quadrilateral inscribed in a circle is given by its four
vertices A, B, C, and D in order, then the theorem states that:
where the overbar denotes the lengths of the line segments
between the named vertices.
"If a quadrilateral is inscribed in a circle then the sum of the
products of its two pairs of opposite sides is the product of its
diagonals".
The converse of Ptolemy's theorem is also true (In a
quadrilateral, if the sum of the products of its two pairs of
opposite sides is the product of its diagonal, then it can be
inscribed in a circle).
Canon of Kings
The Canon of Kings was a dated list of kings used by ancient astronomers as a
convenient means to date astronomical phenomena, such as eclipses. The
Canon was preserved by the astronomer Claudius Ptolemy. It is one of the most
important bases for our knowledge of ancient chronology. The Canon derives
originally from Babylonian sources.
Canon Contents
(1) Babylonian Kings, 747-539 BC, (2) Persian Kings, 538-332 BC , (3) Macedonian
Kings, 331-305 BC (4) Ptolemies of Egypt, 304-30 BC , (5) Roman Emperors, 29 BCAD 160
Ptolemy was the last of the circle of ancient scientists.
After his work science faced retrogression and decline; the Roman Empire was not particularly interested in
scientific work (as in the Hellenistic period), the famous Library of Alexandria was burnt, destroying more
than 90% of the written knowledge of Antiquity, and the religion of Christianity appeared as a revolution and
resistance to the Roman empire, religion and everything related to the ancient world, including all scientific
advancements and forbidding any research or doubt over the “absolute will of the Lord.”
During the Middle Age, the “Dark Ages” in Western Europe, the only light of wisdom were Byzantium -The
Eastern Roman Empire- that preserved the rest of the books, continuing the work and philosophy of the
Academy of Plato up to the 7th century A.D., and the Arabs; The liberating form of the religion of Islam at that
time gave space for scientific research and evolution at these lands once more. An important contribution of the
Arabs are the so called “Arabic numbers,” the ones we use nowadays.The numbers were developed in India
by the Hindus around 400 BCE, and the Arabs adopted and relayed this system to the West.
For almost 1000 years Aristotle and Ptolemy were the absolute and undoubted truth of the Western
populations, with their work known mainly through Arabic translations.
The destruction of Byzantium in the middle of the 15th century A.D., forced Greeks from Constantinople and
elsewhere in Greece to immigrate towards Italy (Florence, Venice, etc.), bringing books of the ancient Greek
philosophers and rendering the ancient philosophers work known to the Western countries, thus initiating (in
combination with the invention of press) the beginning of the period known as the Renaissance.
People like Galileo Galilei and others started doubting the “absolute truth” of the religion and the Church, and
gave birth to what we call nowadays “scientific research and enterprise.”
In the following a list of the different chronological steps in scientific thought and the main figures who initiated
them, and who led humanity from the “Dark Ages” to our modern way of thinking, is presented.
Celestial Dynamics, Terrestrial Mechanics:
Copernicus (“De revolutionibus orbioum coelestium”: “On the revolutions of the heavenly spheres”) (1543)
Galileo Galilei (the “Dialogue”) (1564-1642)
Kepler Johannes (1571-1630) (“Mysterium Cosmographicum”: “Cosmographic Mystery”)
Bacon
The Mechanical Philosophy:
Van Helmont (influenced by Paracelsus)
Descartes
The Mechanical Science:
Pascal
Huygens (essay “on light”)
The Mechanical Chemistry:
Paracelsus
Nicolas Lemery
Robert Boyle
Organization of the Scientific Enterprise:
Robert Boyle (definition of the experimental method)
Descarte
The Science of Mechanics:
Descarte
Newtonian Dynamics:
Galileo Galilei
Newton
Nicolaus Copernicus (February 19, 1473 – May 24, 1543)
Astronomer who provided the first modern formulation of a heliocentric
(sun-centered) theory of the solar system in his epochal book, De
revolutionibus orbium coelestium (On the Revolutions of the Celestial
Spheres).
Copernicus was one of the great polymaths of the Renaissance.
Mathematician, astronomer, jurist, physician, classical scholar, governor,
administrator, diplomat, economist, and soldier. His formulation of how
the sun rather than the earth is at the center of the universe is considered
one of the most important scientific hypotheses in history. It mark the
starting point of modern astronomy and of modern science,
encouraging young astronomers, scientists and scholars to take a more
skeptical attitude toward established dogma.
The Copernican heliocentric system
Copernicus cited Aristarchus and Philolaus in an early manuscript of his book which survives,
stating: "Philolaus believed in the mobility of the earth, and some even say that Aristarchus of
Samos was of that opinion." Inspiration came to Copernicus not from observation of the planets,
but from reading two authors: Hicetas and Plutarch provided an account of the Pythagoreans
Heraclides Ponticus, Philolaus, and Ecphantes. These philosophers had proposed a moving
earth, which did not, however, revolve around a central sun.
It has been argued that in developing the mathematics of heliocentrism Copernicus drew on, the Greek
and the Islamic tradition of mathematics and astronomy.
Copernicus' major theory was published in the book, De revolutionibus orbium coelestium (On the
Revolutions of the Heavenly Spheres) in the year of his death, 1543.
He held that the Earth is another planet revolving around the fixed sun once a year, and turning on its
axis once a day. He arrived at the correct order of the known planets and explained the precession of
the equinoxes correctly by a slow change in the position of the Earth's rotational axis. He also gave a
clear account of the cause of the seasons: that the Earth's axis is not perpendicular to the plane of its
orbit. But while Copernicus put the Sun at the center of the celestial spheres, he did not put it at the exact
center of the universe, but near it.
His model had a large influence on later scientists such as Galileo and Johannes Kepler, who adopted,
championed and (especially in Kepler's case) sought to improve it. The Copernican system can be
summarized in seven propositions, as Copernicus himself
The seven parts of Copernicus' theory are:
1. There is no one center in the universe
2. The Earth's center is not the center of the universe
3. The center of the universe is near the sun
4. The distance from the Earth to the sun is imperceptible compared with the
distance to the stars
5. The rotation of the Earth accounts for the apparent daily rotation of the
stars
6. The apparent annual cycle of movements of the sun is caused by the Earth
revolving around the sun
7. The apparent retrograde motion of the planets is caused by the motion of the
Earth, from which one
observes
Johannes Kepler (December 27, 1571 – November 15,
1630)
German mathematician, astronomer, astrologer, and
an early writer of science fiction stories, a key
figure in the scientific revolution.
He is best known for his laws of planetary motion,
based on his works Astronomia nova, Harmonice
Mundi and the textbook Epitome of Copernican
Astronomy.
Kepler's laws
Kepler inherited from Tycho Brahe, court
mathematician to Emperor Rudolf II, a wealth
of the most accurate raw data ever collected
on the positions of the planets.
The difficulty was to make sense of it. The
orbital motions of the other planets are viewed
from the vantage point of the Earth, which is
itself orbiting the sun. As shown to the graph,
this can cause the other planets to appear to
move in strange loops.
Kepler concentrated on trying to understand the orbit of Mars, but he had
to know the orbit of the Earth accurately first. So, he used Mars and the Sun
as his baseline, since without knowing the actual orbit of Mars, he knew that
it would be in the same place in its orbit at times separated by its orbital
period. Thus the orbital positions of the Earth could be computed, and from
them the orbit of Mars.
He finally arrived at his three laws of planetary motion:
the sun in
one focus.
planet to the
amounts of time.
Kepler's elliptical orbit law: The planets orbit
elliptical orbits with the sun at
Kepler's equal-area law: The line connecting a
sun sweeps out equal areas in equal
Kepler's law of periods: The time required for a
planet to orbit
the sun, called its period, is
proportional to the long axis of the
ellipse raised
to the 3/2 power. The constant of proportionality is
the
same for all the planets.
He was the first astronomer to successfully predict a transit of Venus (for
the year 1631). Kepler's laws were the first clear evidence in favor of the
heliocentric model of the solar system. Isaac Newton eventually showed
that the laws were a consequence of his laws of motion and law of
universal gravitation.
constellation
since in the Milky Way.
In 1604, Kepler observed a supernova in the
Ophiuchus, the first and only ever
Kepler also made fundamental investigations
into
combinatorics, geometrical
optimization, and natural
phenomena such as
snowflakes, always with an emphasis
on form
and design. He was also one of the founders of
modern optics, defining for example antiprisms and the
Kepler telescope.
Remnant of Kepler's Supernova SN 1604
In
addition, since he was the first to recognize
the nonconvex regular solids (such as the
stellated
dodecahedra),
they are named Kepler solids in his honor.
Kepler also was in contact with Wilhelm Schickard, inventor of the first
automatic calculator, whose letters to Kepler show how to use the
machine for calculating astronomical tables.
Galileo Galilei (February 15, 1564 – January 8, 1642)
Italian physicist, astronomer, astrologer, and philosopher
who is closely associated with the scientific revolution. His
achievements include improvements to the telescope, a
variety of astronomical observations, the first and second
laws of motion, and effective support for Copernicanism.
He has been referred to as the "father of modern
astronomy," as the "father of modern physics," and as the
"father of science." Galileo's career coincided with that of
Johannes Kepler.
The work of Galileo is considered to be a significant break
from that of Aristotle.
Scientific methods
Famous quote:
Epur si muove
And yet it does move
In the pantheon of the scientific revolution, Galileo Galilei
takes a high position because of his pioneering use of
quantitative experiments with mathematically analyzed
results. There was no tradition of such methods in
European thought at that time. Galileo also contributed to
the separation of science from philosophy or religion.
These are the primary justifications for his description as
the "father of science".
Galileo showed a remarkably modern appreciation for the
proper relationship between mathematics, theoretical
physics, and experimental physics.
For example:
- He understood the mathematical parabola, both in terms of conic sections and
in terms of the square-law.
- He asserted that the parabola was the theoretically-ideal trajectory, in the
absence of friction and other disturbances.
- He recognized that his experimental data would never agree exactly with any
theoretical or mathematical form, because of the imprecision of measurement,
and because of irreducible friction, et cetera.
Astronomy
Galileo first noted an observation of the moons of Jupiter. This observation
upset the notion of that time that all celestial bodies must revolve around the
Earth. He firstly discovered Jupiter's four largest satellites (moons): Io,
Europa, Callisto, and Ganymede. He observed that Venus exhibited a full set of
phases similar to that of the Moon. These observations of the phases of Venus
proved that it orbited the Sun and lent support to (but did not prove) the
heliocentric model.
He was one of the first Europeans to observe sunspots, which formerly had
been attributed (impossibly) to a transit of Mercury, and he was also the first
to report lunar mountains and craters, whose existence he deduced from the
patterns of light and shadow on the Moon's surface.
He also observed the Milky Way,
to be nebulous, and
previously believed
found it to be a multitude of stars
densely that they appeared to be clouds
Earth. He also located many other stars too
distant to be visible with the naked eye.
packed so
Finally, Galileo observed the
planet Neptune in 1612, but
did not realize that it was a
planet and took no particular
notice of it.
from
Galileo
• At age 19, Galileo discovered the pendulum,
the basis for later clocks
• Galileo questioned Aristotelian physics and
performed many of his own experiments,
especially with gravity; his work on physics
summarized in De Moto
• Most optical telescopes used today derive
from the
two types of telescopes developed in the 17th
century by Galileo and Newton, which
amateur and professional astronomers use
today
Physics
Galileo's theoretical and experimental work on the motions of bodies,
along with the largely independent work of Kepler and René Descartes,
was a precursor of the Classical mechanics developed by Sir Isaac Newton.
He was a pioneer, at least in the European tradition, in performing rigorous
experiments and insisting on a mathematical description of the laws of
nature.
He performed experiments involving rolling balls down inclined planes,
which proved that: falling or rolling objects (rolling is a slower version of
falling, as long as the distribution of mass in the objects is the same) are
accelerated independently of their mass.
He determined the correct mathematical law for acceleration: the total
distance covered, starting from rest, is proportional to the square of the
time. He also concluded that objects retain their velocity unless a force —
often friction — acts upon them, refuting the generally accepted
Aristotelian hypothesis that objects "naturally" slow down and stop
unless a force acts upon them. Galileo's Principle of Inertia stated: "A body
moving on a level surface will continue in the same direction at constant
speed unless disturbed." This principle was incorporated into Newton's
laws of motion (first law).
independently of
He also noted that a pendulum's
take the same amount of time,
the amplitude. c
first to
He is lesser known for being one of the
understand sound frequency.
swings always
He also put forward the basic principle of relativity, that the laws of
physics are the same in any system that is moving at a constant speed in a
straight line, regardless of its particular speed or direction. This
principle provided the basic framework for Newton's laws of motion and is
the infinite speed of light approximation to Einstein's special theory of
relativity.
Galileo's writings
- Two New Sciences 1638 Lowys Elzevir (Louis Elsevier) Leiden (in Italian, Discorsi e Dimostrazioni
Matematiche, intorno a due nuoue scienze Leida, Appresso gli Elsevirii 1638)
- Letters on Sunspots 1613
- The Assayer (In Italian, Il Saggiatore) 1623
- Dialogue Concerning the Two Chief World Systems 1632 (in Italian, Dialogo dei due massimi sistemi del
mondo)
- The Starry Messenger 1610 Venice (in Latin, Sidereus Nuncius)
- Letter to Grand Duchess Christina 1615
THE SCIENTIFIC REVOLUTION
NEW DIRECTIONS IN ASTRONOMY & PHYSICS
 GALILEO GALILEI (15641642)
 Constructed first
telescope
 Described motion of
bodies on earth
Isaac Newton
• Kepler's Laws were a revolution in
regards to understanding
planetary motion, but there was no
explanation why they worked
• That explanation would have to
wait until Isaac Newton formulated
his laws of motion and the concept
of gravity
• Newton's discoveries were
important because they applied to
actions on Earth and in space
• Besides motion and gravity,
Newton also developed calculus
Newton (1642-1727)
Some terms
• Force: the push or pull on an object that in some way
affects its motion
• Weight: the force which pulls you toward the center of the
Earth (or any other body)
• Inertia: the tendency of an object to keep moving at the
same speed and in the same direction
• Mass: basically, the amount of matter an object has
• The difference between speed and velocity
– These two words have become identical in common language, but in
physics, they mean two different things
– Speed is just magnitude of something moving (25 km/hr)
– Velocity is both the magnitude and direction of motion (35 km/hr to
the NE)
Newton's First Law
• Newton's first law states: An object at rest will remain at
rest, an object in uniform motion will stay in motion UNLESS acted upon by an outside force
Outside Force
• This is why you should always wear a seat belt!
Newton's Second Law
• Acceleration is created whenever there is a change in
velocity
– Remember, this can mean a change in magnitude AND/OR
direction
• Newton's Second Law states: When a force acts on a
body, the resulting acceleration is equal to the force
divided by the object's mass
F
a
m
or
F  ma
• Notice how this equation works:
– The bigger the force, the larger the acceleration
– The smaller the mass, the larger the acceleration
Newton's Third Law
• Newton's Third Law states:
For every action, there is an
equal and opposite reaction
• Simply put, if body A exerts
a force on body B, body B
will react with a force that is
equal in magnitude but
opposite direction
• This will be important in
astronomy in terms of
gravity
– The Sun pulls on the Earth
and the Earth pulls on the Sun
Isaac Newton
(1642-1727)
• Possibly the greatest scientist who
ever lived - born on the day Galileo
died
• math/physics/astronomy
• author of Principia Mathematica in
1687
– bringing together Galileo’s discoveries
about motion on Earth and Kepler’s
discoveries in the heavens
– to do so he had to develop calculus
• explained heavenly motion that was
tied to observed motion on Earth.
Isaac Newton
• Provided a
synthesis
superior to
Aristotle
• notion of
inertia - only
have to
explain
change
• Three Laws of
Motion
1 Bodies move in
straight lines
unless impeded
(inertia)
2 Every action has
an equal and
opposite action
3 every body
attracts every
other body with a
force proportional
to the distance
between
Nature and nature’s laws lay hid in night
God said, “Let Newton be.” and all was light
- Pope.
Isaac Newton (1643 - 1727)
-Wrote “Principia Methematica” which contained mathematical
Descriptions of how the world works (up to the speed of light, as
Einstein later proves…)
Law 1:
Every object continues in its state of rest or of uniform motion in a straight
line, unless it is
compelled to change that state by forces impressed upon it.
Law 2:
The acceleration of an object is directly proportional to the net force acting
on the object, is in the
direction of the net force, and is inversely proportional to the mass of the
object.
Law 3:
Whenever one object exerts a force on a second object, the second object
exerts an equal and
opposite force on the first.
Isaac Newton
Newton solved the premier scientific problem of his time --- to
explain the motion of the planets.
To explain the motion of the planets, Newton developed three
ideas:
1.
2.
3.
The laws of motion
The theory of universal gravitation
Calculus, a new branch of mathematics
F
a
m
Gm1m2
F
2
r
“If I have been able to see farther than others it is because I stood
on the shoulders of giants.”
--- Newton’s letter to Robert Hooke,
perhaps referring to Galileo and Kepler
7/14/06
ISP 209 - 3A
37
Isaac Newton
• Mathematical theories explained the
observed motions of the planets.
• Considered the greatest scientific
genius until Einstein.
• 1684-wrote his major work,
Mathematical Principles of Natural
Philosophy
• Theory of gravitation.
– Planets bound to the sun by gravitation
– Basic force of gravitation and
– proved that it explained the motions of
the planets.
Newton’s Beliefs
• 1665: Newton developed calculus
• 1664-1666: Discovered that light is made
up of different colors by passing sunlight
through a prism
• 1666: Began to develop the theory of
gravity
• 1687: Wrote a book on gravity and the
laws that make objects move---Principia
Newton and the Apple - Gravity
• After formulating his three
laws of motion, Newton
realized that there must be
some force governing the
motion of the planets around
the Sun
• Amazingly, Newton was able
to connect the motion of the
planets to motions here on
Earth through gravity
• Gravity is the attractive force
two objects place upon one
another
The Gravitational Force
Gm1m2
Fg 
r2
• G is the gravitational constant
– G = 6.67 x 10-11 N m2/kg2
• m1 and m2 are the masses of the two
bodies in question
• r is the distance between the two bodies
Gravity - Examples
• Weight is the force you feel due to the gravitational force
between your body and the Earth
– We can calculate this force since we know all the variables
Gm1m2
Fg 

2
r
(6.67 10
11
N m
24
)(
72
kg
)(
5
.
97

10
kg)
2
kg
6
2
(6.378 10 m)
2
Fg  705 N
1 Newton is approximately 0.22 pounds
0.22lbs
Fg  705 N 
 155lbs
1N
Gravity - Examples
• What if we do the same calculation for a person standing on
the Moon?
– All we have to do is replace the Earth's mass and radius with the
Moon's
Gm1m2
Fg 

2
r
(6.67 10
11
N m
22
)(
72
kg
)(
7
.
35

10
kg)
2
kg
6
2
(1.738 10 m)
2
Fg  117 N
1 Newton is approximately 0.22 pounds
0.22lbs
Fg  117 N 
 26lbs
1N
Gravity - Examples
• If gravity works on any two bodies in the universe, why don't
we all cling to each other?
– Replace the from previous examples with two people and the distance
with 5 meters
Gm1m2
Fg 

2
r
(6.67 10
11
N m
)(72kg)(65kg)
2
kg
2
(5m)
2
8
Fg  0.0000000125N  1.25 10 N
1 Newton is approximately 0.22 pounds
0.22lbs
Fg  1.25 10 N 
 2.75 10 9 lbs
1N
8
Physics: Isaac Newton
• English scientist who used the scientific method in science and
mathematics
• He was a below average student at the University of Cambridge
that was helped by a tutor who recognized his talent
• Newton studied the works of Copernicus and Galileo
• In 1665, the plague forced Newton back to rural family farm
• There he continued to study and created his theory of gravity
• In 1687, Newton published his theories of gravity, etc. in his book
Mathematical Principles of Natural Philosophy aka Principia in
which he expanded the theories of Copernicus, Galileo, and
Kepler
-explained Newton’s theory of universal gravitation
-Newton developed calculus in order to prove his theory of
gravity
The legend is that
Newton saw an apple
fall in his garden,
thought of it in terms
of an attractive
gravitational force
towards the earth,
and realized the same
force might extend as
far as the moon.
Newton’s Theory of Universal Gravitation
Newton and the Apple
Newton asked good questions  the key to his success.
Observing Earth’s gravity
acting on an apple, and seeing
the moon, Newton asked
whether the Earth’s gravity
extends as far as the moon.
(The apple never fell on
his head, but sometimes a
stupid person will say
that, trying to be funny.)
7/14/06
ISP 209 - 3A
49
Newton
Cont.
“To explain all nature is too
difficult a task for any one man
or even for any one age. `Tis
much better to do a little with
certainty, and leave the rest for
others that come after you, than
to explain all things.” - Newton
Replica of Newton’s
Refracting telescope
“...from the same principles, I now demonstrate the
frame of the System of the World”
Principia Mathematica. Hypotheses non fingo.
“I feign no hypotheses”
Principia Mathematica.
Original copy of Newton’s Principia
If I have seen further it
is by standing on the
shoulders of giants.
Isaac Newton, Letter to
Robert Hooke,
February 5, 1675
Newton’s Findings
Newton developed calculus, new kind of math
• Used calculus to predict effects of gravity
• German philosopher Gottfried von Leibniz also developed
calculus at same time
• Each accused the other of plagiarism
• Historians believe it was simple case of independent
discovery
Isaac Newton and Universal
Physics
• The implications of Newton’s law were
enormous for he demonstrated with one
universal law, mathematically proved, that all
motion in the universe, from the movements of
the planets to an apple falling off a tree, could be
explained
• This Newtonian synthesis created a new
cosmology in which the world was largely seen
in mechanistic terms
• Would remain the dominant cosmology until
Einstein’s concept of relativity
Revisions to Kepler's 1st Law
• Newton's law of gravity required
some slight modifications to
Kepler's laws
• Instead of a planet rotating around
the center of the Sun, it actually
rotates around the center of mass
of the two bodies
• Each body makes a small elliptical
orbit, but the Sun's orbit is much
much smaller than the Earth's
because it is so much more
massive
Revisions to Kepler's 3rd Law
• Gravity also requires a slight
modification to Kepler's 3rd
Law
3
a
P 
M1  M 2
2
• The sum of the masses of
the two bodies is now
included in the equation
• For this equation to work,
the masses must be in units
of solar mass (usually
written as M )

• Why did this equation work
before?
Remember - for this
equation to work:
P must be in years!
a must be in A.U.
M1 and M2 must be in
solar masses
Michael Faraday: 1791 –
1867AD
• English
• Attended the lectures
at the Royal
Institution
• 1825: discovered
benzene when
looking at the
ingredients of oil from
a dead whale
•
•
•
•
•
Did experiments with electricity
In 1831, Faraday discovered
electromagnetic induction, the
principle behind the electric
transformer and generator.
discovery was crucial in
allowing electricity to be
transformed from a curiosity
into a powerful new technology.
During the remainder of the
decade he worked on
developing his ideas about
electricity.
He was partly responsible for
coining many familiar words
including 'electrode', 'cathode'
and 'ion'
Michael Faraday
Henry Cavendish
• Experimented with
electricity
• Tested the electric
current by giving
himself shocks and
noting the pain he felt
• In 1747, Henry
Cavendish started
measuring the
conductivity (the
ability to carry an
electrical current) of
different materials
and published his
results.
Henry Cavendish
• Cavendish had the ability to make a seemingly limited
study give far-reaching results.
• An example is his study of the origin of the ability of
some fish to give an electric shock. He made up
imitation fish of leather and wood soaked in salt water,
with pewter (tin) attachments representing the organs of
the fish that produced the effect.
• By using Leyden jars (glass jars insulated with tinfoil) to
charge the imitation organs, he was able to show that
the results were entirely consistent with the fish's ability
to produce electricity.
• This investigation was among the earliest in which the
conductivity of aqueous (in water) solutions was
studied.
Henry Cavendish: 1731 – 1810
AD• He discovered a whole
• British
• Was worth over one
hundred million
pounds
• Son of a Lord and the
grandson of the Duke
of Devonshire
• Spent most of his life
doing science
experiments at home
alone
range of new gasses
• In 1776 he added acid
to marble and the gas
that was given off was
called fixed air.
• He then dripped acid
onto iron, and another
gas was given off. It
seemed that this gas
was lighter than air and
burnt so easily, so he
called it “fire air.”--What we know now as
hydrogen.
Cavendish
• Cavendish was so
shy that he never
told anyone about
his discoveries.
• It was not until 100
years later when
James Clerk
Maxwell came
across his
notebooks.