PHY1033C/HIS3931/IDH 3931 : Discovering Physics:
The Universe and Humanity’s Place in It
Fall 2015
Prof. Peter Hirschfeld, Physics
Announcements
• HW 1 due Friday
• Lab 1 on Tuesday during class – need
notebook
• HITT clicker points start Tuesday-practice
today. Have you registered your clicker?
• Reading: Almagest, simulations, Physical
Cosmos (all online from syllabus links)
Last time
Aristotle 384-322 BCE
• founded Lyceum most famous ancient school
• foundations of Greek philosophy, science influenced
educated thought for 2000 years
• approach to physical science distinctly not modern
although he used some practical arguments, mostly
dialectical in nature
• objects had natural place, sought to return there
• elements: earth, air fire and water
• objects fell with speed  mass, inversely  to resistance
• natural vs. violent motion. Stone’s natural motion is to
fall, to return to earth, its origin; can be endowed with
violent motion by throwing it.
• planets natural motion (celestial realm) was to go in circles
(5th elements: quintessence or “ether”
Clicker quickies
Q1: Aristotle taught that a falling body dropped from rest
A) Fell upward if it was made mostly of fire or air,
downward if it was made mostly of water or earth
B) Fell at a rate proportional to its weight
C) Did not ever really fall, but rose to the heavens to join
the gods
D) Both A) and B)
E) Both B) and C)
Q2: Which of the following did the ancient
Greeks not believe?
A. The Earth is round
B. The stars were fixed on a sphere that rotated
around earth every 24 hours
C. We live on the inside of a hollow spherical
shell
D. All celestial movements are at uniform speed
E. The sun rotated around the Earth
Falling objects: what Aristotle couldn’t know
(but might have figured out with a bit of observation)
Recall Aristotle reasoned that heavier objects fell faster, so v  W
Brian Cox visits world’s biggest vacuum chamber
https://www.youtube.com/watch?v=E43-CfukEgs
Q: How might Aristotle have figured out he was wrong?
Solar system terms (modern)
Plane of ecliptic: plane containing sun, earth’s orbit around it,
and most planetary orbits
Failure of Aristotelian models
Observations over 100s of years – deviations accumulate
Simplified observational table of position of Jupiter (numbers invented!)
Time of observation
Angle in plane of
ecliptic relative to
Polaris
Angle relative to
horizon
Predicted
Observed
Predicted
Midnight, January 1,
300 BCE
30
30
Midnight, January 1,
295 BCE
44
45
Midnight, January 1,
200 BCE
137
141
Midnight, January 1,
195 BCE
151
155

Observed
Two innovations introduced between
Aristotle and Ptolemy to improve fit
P = planet
B
A
C=Center
E=Earth
D
F
The eccentric circle
Attributed to Apollonius of Perga ~20 -190 BCE)
Accounts for 1) variation in brightness of planets
2) changes in angular speed
Two innovations introduced between
Aristotle and Ptolemy to improve fit
The epicycle on deferent
Accounts for changes in acceleration, retrograde motion
Simulation: http://astro.unl.edu/naap/ssm/animations/ptolemaic.swf
Equant point-the epicycle has
constant angular speed moving
around this point
Center point of deferent
q
Orbit of planet
(Ptolemy)
+
Earth
Deferent-the large circle
carrying the epicycle
Sun
Planet
Simulation:
http://astro.unl.edu/naap/ssm/animations/ptolemaic.swf
Epicycle-the small circle
carrying the planet
Questions for discussion
• Why do you think moving the Earth away from
the center of the orbit allowed for a better fit to
the data, e.g. for the Sun’s position?
• Why do we have seasons? Is it because Earth is
sometimes closer, sometimes farther from the
sun?
• What problem would people from another solar
system where theirs was the only planet orbiting
around a star have deducing which was in the
center?
Position of an object
Define the position of an object as where it is at some
time t relative to some coordinate system.
the position of a car on University Avenue
We sometimes use the word displacement to mean the same thing as position.
Speed & velocity
Def: Speed–rate of change of object’s position. Defined always as positive number.
Def: Velocity – rate of change of position in a particular direction. Can be +/-.
Def: average velocity
(f means final and i means initial)
Exercise: you drive a pickup truck down a straight
road for 5.2 mi at 43 mi/hr, at which point you run
out of fuel. You walk 1.2 mi farther, to the nearest
gas station, in 27 min (=0.450 h). What is your
average velocity?
Exercise: you drive a pickup truck down a straight
road for 5.2 mi at 43 mi/hr, at which point you run
out of fuel. You walk 1.2 mi farther, to the nearest
gas station, in 27 min (=0.450 h). What is your
average velocity?
Displacement start to finish Dx = 5.2 mi + 1.2 mi = 6.4 mi
Total time Dt = Dtdrive + Dtwalk=[5.2mi/(43 mi/h)]+0.450h
= 0.121h + 0.450h = 0.57h
Avg. velocity = Dx/ Dt = 6.4mi/0.57h = +11mi/h
If velocity is constant, average v = v(t)
Quickie
Here's a quick (trick) question to see if you're
getting some of the physicists' definitions before we go on:
Q: If you travel from Gainesville to Miami at 20 mph
and return to Gainesville at 40 mph, what is your average
velocity?
A: Zero
B: 30 mph
C: Impossible to say since distances aren’t given.
D: 30 mph south
A: Zero, because
and the total displacement Dx =0 (you came back to
where you started!).