Great Idea #2: Newton’s
universal laws of motion and
gravity predict the behavior of
objects on Earth and in space.
How Do Objects Move?
Key Idea: One set of laws describes motions
on Earth and in space
1. Universal laws of motion
2. The universal law of gravity
Key Terms:
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Force
Uniform Motion
Acceleration
Weight vs. Mass
Newgrange, Ireland:
5,000-year old “passage tomb”
Stonehenge
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On the Salisbury Plain of southern England
4500 years old
Exact history is uncertain
Stonehenge Decoded by Richard Hawking
John Snow (1813-1858)
He used the scientific method
to discover that contaminated
water at the Broad Street Pump
was the source of a deadly
cholera outbreak in London.
His research saved
countless thousands
of lives.
The Idealized Scientific Method
Measurements and Observations
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How would you measure a star’s position?
1. Compass – direction to an object (degrees from North)
2. Protractor w/ straw (degrees from horizon)
3. Plot changes from night to night
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Planets (Mars) display
retrograde motion
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Patterns emerge
Ptolomy’s Model of the Solar
System
• Model of the universe
with Earth at the center
• Epicycle (wheels within wheels)
• Why is it important?
• Navigation
• Astrology
• Lasted 1500 years
Dead White Males
• Nicolas Copernicus (1473-1543)
• Sun-centered solar system
• Tycho Brahe (1546-1601)
• Meticulous measurements of planet positions
• Johannes Kepler (1571-1630)
• Mathematical analysis of planetary orbits
• Galileo Galilei (1564-1642)
• Founder of experimental physics
Ptolomeic vs. Copernican models
• Earth- vs. Sun-centered
• Perfect circles (still need epicycles)
Ptolomy
Copernicus
Tycho Brahe (1546-1601)
•Observed a new star
•Showed heavens can change
over time
•Designed and used new
astronomical instruments
•Collected data on planetary
movements
The Quadrant
Tycho Brahe (1546-1601)
Johannes Kepler (1571-1630)
• Mathematician
• Assistant to Tycho Brahe
• Inherited his notebooks
• Traditional ideas of the solar
system are wrong (Kepler’s Laws)
Kepler’s Laws of
Planetary Motions
First Law: Planets have elliptical orbits
Elliptical orbits
Kepler’s Laws
Second Law: Orbits sweep out equal
areas in equal times
Kepler’s Laws
Third Law: Distant orbits take longer.
(Average orbital radius)3 = k (orbital period)2
R3/P2 = constant
Galileo Galilei (1564-1642)
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Mathematician
Improved the telescope
Observational astronomy
Founder of experimental science
Musical training
Arrogant and contemptuous of others
Published in Italian (vernacular) promoting
Copernican view
• Heretic (threatened with torture) so he
recanted
Galileo – The Founder of
Experimental Science
Galileo studied the relationship among
distance, time, velocity and acceleration
He observed that objects accelerate while
falling:
Galileo’s Discoveries
• Constant acceleration
–Balls on a plane: v = at
• Freefall
–Constant acceleration at g
–g = 9.8 m/s2 = 32 feet/s2
–Distance traveled (d) = ½at2
Speed, Velocity, and
Acceleration
• Speed = distance traveled over time
• Velocity = speed with direction
• Equation for speed:
d
v
t
• Acceleration = rate of change of velocity
• Equation for velocity:

v
a
f
 vi 
t
Galileo’s Laws of Motion
1. Trajectories are parabolas
2. All objects fall at the same velocity
3. Distance of fall = k(Time)2
Remember Kepler (1571-1630)
1. Orbits are ellipses
2. Equal areas in equal times
3. (Radius)3 = k(Time)2
Now on to Sir Issac Newton (1642-1727)
Sir Issac Newton (1642-1727)
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Born prematurely
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Abandoned by his mother
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Forced to run family farm
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Sent to Cambridge
University
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Made fundamental
advances in several
scientific fields
Issac Newton’s – Plague Years
(1665-1666)
1. Calculus
2. Laws of optics
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Broke white light into colored light
3. Universal laws of motion
4. Universal law of gravitation
Two Types of Motions
Uniform motion: No change in velocity or
direction
Acceleration: Change in velocity and/or
direction
Issac Newton’s Laws of Motion
1st Law: Nothing happens without a force
“A moving object will continue moving in
a straight line at a constant speed, and a
stationary object will remain at rest,
unless acted on by an unbalanced force.”
This law defines “force”
Issac Newton’s Laws of Motion
1st Law: Nothing happens without a force
2nd Law: Force = mass X acceleration (F=ma)
“The acceleration produced on a body by
a force is proportional to the magnitude
of the force and inversely proportional to
the mass of the object.”
This law defines “mass”
Issac Newton’s Laws of Motion
1st Law: Nothing happens without a force
2nd Law: Force = mass X acceleration (F=ma)
3rd Law: Forces always act in pairs
“For every action
there is an equal
and opposite
reaction.”
Gravity (a universal force)
The Universal Force of Gravity
Between any two masses there
exists a force that is proportional
to the masses and inversely
proportional to the square of the
distance between them.
F = Gm1m2/d2
The Force of Gravity
F = Gm1m2/d2
Why mass x mass?
Think of “rays of mass” radiating in all directions
gravitational force = # of rays intersecting
m1
m2
The Force of Gravity
F = Gm1m2/d2
Why inverse square of distance?
Think about a flashlight:
light intensity is greater
at shorter distances
Now double the distance:
(4X the area = 1/4 the light)
The Force of Gravity
F = Gm1m2/d2
What is G?
• G = universal constant
of direct proportionality
• Henry Cavendish
G = 6.67 x 10-11m3/s2-kg or
6.67 x 10-11N-m2/kg2
Weight and Gravity
• Weight = Gravity acting on an
object’s mass
• Weight depends on gravity
Different on Earth vs. Moon
• Mass is constant
Natural Laws Raise Deep
Philosophical Questions
• What about free will?
• Natural laws in other domains
Economies (Adam Smith)
Law (cause and effect)
Politics (George Mason)