Galileo on
Motion
“My purpose is to set forth a very new
science dealing with a very ancient
subject. There is, in nature, perhaps
nothing older than motion, concerning
which the books written by philosophers
are neither few nor small; nevertheless I
have discovered by EXPERIMENT
some properties of it which are worth
knowing and which have not hitherto
been either observed or demonstrated.”
~ Galileo
Scientific
revolution
Experiment
Supporter of
Copernicanism
Legacy
Telescope
So, who’s
Galileo?
Scientific
method
“Father of
science”
First and
second laws of
motion
“Father of
modern
physics”
“Father of
modern
Astronomy”
Freedom of
thought & the
Catholic
Church
Bridging the Gap:
Aristotle to Galileo
Philoponus
Ockham
Oresme
533 A.D.
1300’s
1330’s
Avicenna
980 A.D.
Buridan
Galileo’s Life
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


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Born in 1564 in Pisa of Vincenzo Galilei and Guilia Ammannati
Florence
University of Pisa; 1581-1585
Chair of Mathematics Univ. of Pisa 1589
Father dies in 1591; Professory of Mathematics at the University of
Padua
 Meets Maria Gamba; Virginia (later Sister Maria Celeste) is born
(1600), Livia (1601), later Sister Arcangela, and Vincenzio (1606)
 1609; the spyglass
 Cardinal Robert Bellarmine
 1616 wrote the Letter to the Grand Duchess
 1630 wrote Dialogue Concerning the Two Chief Systems of the World
 Inquisition bans the Dialogue and Galileo is sentenced to lifetime
house arrest
 1634 his daughter, Virginia, dies
 Disourses are smuggled out
 Died 1642
“Philosophy is written in this grand book, the universe, which
stands continually open to our gaze. But the book cannot be
understood unless one first learns to comprehend the
language and read the characters in which it is written. It is
written in the language of mathematics, and its characters
are triangles, circles, and other geometric figures without
which it is humanly impossible to understand a single word
of it; without these one is wandering in a dark labyrinth.”
Galileo – from The Assayer
Works
 De Motu (1602-1604) never published
 Dialogue Concerning the Two Chief
Systems of the World (1625-1630) 1632
 Discourses and mathematical
demonstrations concerning the two new
sciences (1633-1642)-1638
Galileo’s Qualifiers
 Rectilinear vs. Circular Motion
 Straight line motion or motion parallel with
the surface of the earth?
 Criterion of Simplicity
 Agreement with experiment
Kinematics: Quantitative
description of the motion of
bodies
VS.
Dynamics: Causes of such
motion
Naturally Accelerated
Motion/Constant Acceleration
 Vs. uniform motion – motion in which
equal distances are covered in equal
increments of time
 “A motion is said to be uniformly
accelerated when, starting from rest, its
speed receives equal increments in
equal times”
Inclined Plane Experiments
~ 100 trials
with various
inclinations for
various lengths
of time
http://brunelleschi.imss.fi.it/genscheda.asp?appl=SIM&xsl=multimed&lingua=ENG&chiave=500045
http://brunelleschi.imss.fi.it/genscheda.asp?appl=SIM&xsl=multimed&lingua=ENG&chiave=500065
What do we make of this?
 Finds that the distance of descent
always varies closely to the square of
the time
 Algebra had yet to be applied to such
problems
 The final velocity of the ball depended
on how high the plane was, not directly
to the plane’s incline angle
Projectile Motion
1.
2.
Constant downward acceleration
Maintains constant horizontal velocity
• Treat vertical and horizontal motions
independently
• Combination of two components will agree
with observed trajectory
•Observed trajectory is parabolic
Projectile Motion
 Derived Equations
 Horizontal
x = v ot
 Vertical
y = 1/2gt2
 Parabolic
 Trajectory
y = ( g ) x2
2v2o
Projectile Motion
http://brunelleschi.imss.fi.it/genscheda.asp?appl=SIM&xsl=multimed&lingua=ENG&chiave=500098
The Components of Motion
http://www.makingthemodernworld.org.uk/larning_modules/maths/04.TU.02/?secti
on=7
Inertia
 Momentum – product of the weight of a
body times its velocity
 Inertia - a body in motion tends to stay
in motion (doesn’t specify straight-line)

http://id.mind.net/~zona/mstm/physics/mechanics/forces/g
alileo/galileoInertia.html
http://brunelleschi.imss.fi.it/genscheda.asp?appl=SIM&xsl=multimed&lingua=ENG&chiave=500135
Break from Aristotelian
Philosophy?
 Objects are no longer “purposeful” – break
from teleology
***Disclaimer: Concepts of inertia did not begin
only with Galileo
Galileo on Aristotle
 Pg 84 in book and open discussion
Pisa
 Dropped balls diff.
mass
 Independent of mass
 Thus, disproving
Aristotle
Questions?
Summary:
http://brunelleschi.imss.fi.it/genscheda.asp?appl=SIM&xsl=multimed&lingua=ENG&chiave=
500012
All truths are easy to understand once they
are discovered; the point is to discover
them.
Galileo