Download - From Aristotle to Newton
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From Aristotle to Newton
The history of the Solar System (and the universe to some extent) from ancient Greek times through to the beginnings of modern physics.
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Kepler (1571-1630)
Used Tycho Brahe's precise data on apparent planet motions and relative distances.
Deduced three laws of planetary motion.
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Kepler's First Law
The orbits of the planets are elliptical (not circular) with the Sun at one focus of the ellipse.
Ellipses
eccentricity =
(flatness of ellipse)
distance between foci major axis length
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Kepler's Second Law
A line connecting the Sun and a planet sweeps out equal areas in equal times.
Translation: planets move fasterwhen closer to the Sun.
slower faster
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Kepler's Third Law
The square of a planet's orbital period is proportional to the cube of its semi-major axis.
P2 is proportional to a3
or
P2 a3
(for circular orbits, a=b=radius).
Translation: the larger a planet's orbit,the longer the period.
ab
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Solar System Orbits
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Kepler's Third Law
The square of a planet's orbital period is proportional to the cube of its semi-major axis. If P measured in Earth years, anda in AU,
P2 a3
(for circular orbits, a=radius).
Translation: the larger a planet's orbit,the longer the period.
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At this time, actual distances of planets from Sun were unknown, but were later measured. One technique is “parallax”.
“Earth-baseline parallax” uses telescopes on either side of Earth to measure planet distances.
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Orbits of some planets (or dwarf planets):
Planet a (AU) P (Earth years)
Venus 0.723 0.615Earth 1.0 1.0Pluto 39.53 248.6
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Clicker Question:
A flaw in Copernicus’s model for the solar system was:A: It didn’t explain retrograde motion.
B: He used circular orbits.
C: The Earth was still at the center.
D: He used the same mass for all the planets.
E: All of the above
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Copernican model was a triumph of the Scientific Method
Scientific Method:
a) Make high quality observations of some natural phenomenonb) Come up with a theory that explains the observationsc) Use the theory to predict future behaviord) Make further observations to test the theorye) Refine the theory, or if it no longer works, make a new one
- Occam’s Razor: Simpler Theories are better-You can prove a theory WRONG but not RIGHT
Observation
TheoryPrediction
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Characteristics of Scientific Theories
Scientific Theories:
a) Must be testableb) Along with their consequences, must be continually testedc) Should be simple (Occam’s Razor) and no more complex than
necessaryd) Should be elegant - simple and able to explain what were thought
to be different phenomenon
- An unproven idea or theory is a hypothesis-You can prove a theory WRONG but not RIGHT
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Newton (1642-1727)
Kepler's laws were basically playing with mathematical shapes and equations and seeing what worked.
Newton's work based on experiments of how objects interact.
His three laws of motion and law of gravity described how all objects interact with each other.
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Newton's Zeroeth Law of Motion
Objects are dumb. They do not know the past and they are not good predictors of the future. They only know what forces act on them right now.
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Newton's Zeroeth Law of Motion
DEMO - Pushing the cart on track
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Newton's First Law of Motion
Every object continues in a state of rest or a state of motion with a constant speed in a straight line unless acted on by a force.
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Newton's First Law of Motion
DEMO - Air Puck motion
DEMO - Smash the HAND
DEMO - Tablecloth
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Newton's Second Law of Motion
When a force, F, acts on an object with a mass, m, it produces an acceleration, a, equal to the force divided by the mass.
a = Fnet
m
acceleration is a change in speed or a change in direction of speed.
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Newton's Second Law of Motion
Demo - Force and Acceleration with fan carts
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Newton's Third Law of Motion
To every action there is an equal and opposite reaction.
Or, when one object exerts a force on a second object, the second exerts an equal and opposite force on first.
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Newton's Third Law of Motion
DEMO: CART
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Clicker Question:
Why didn’t my hand get crushed by the hammer?
A: My bones are actually stronger than steel.
B: The plate has a lot of inertia
C: The plate is very strong
D: The force of gravity kept the plate from moving
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Gravitational Force on a Planet
For an object of mass m at or near the surface of a planet the force of their gravitational attraction is given by:
F = mg
F is the gravitational force.
g is the planetary "gravitational constant".
Your "weight" is just the gravitational force between the Earth and you.
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Newton's Law of Gravity
For two objects of mass m1 and m2, separated by a distance R, the force of their gravitational attraction is given by:
F =G m1 m2
R2
F is the gravitational force.
G is the universal "gravitational constant".
An example of an "inverse-square law".
Your "weight" is just the gravitational force between the Earth and you.
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Clicker Question:
Suppose Matt weighs 120 lbs on his bathroom scale on Earth, how much will his scale read if he standing on a platform 6400 km high (1 Earth radius above sea-level)?A: 12 lbs
B: 30 lbs
C: 60 lbs
D: 120 lbs
E: 240 lbs
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Newton's Correction to Kepler's First Law
The orbit of a planet around the Sun has the common center of mass (instead of the Sun) at one focus.
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Escape Velocity
Velocity needed to completely escape the gravity of a planet.The stronger the gravity, the higher the escape velocity.Examples:
Earth 11.2 km/s Jupiter 60 km/s Deimos (moon of Mars) 7 m/s = 15 miles/hour
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Timelines of the Big Names
Copernicus
Galileo
Brahe
Kepler
Newton1473-1543 1546-16011473-1543
1564-1642
1571-1630
1642-1727
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Electromagnetic Radiation
(How we get most of our information about the cosmos)
Examples of electromagnetic radiation:
LightInfraredUltravioletMicrowavesAM radioFM radioTV signalsCell phone signalsX-rays
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Radiation travels as waves.Waves carry information and energy. Properties of a wave
wavelength ()
crest
amplitude (A)
velocity (v)trough
is a distance, so its units are m, cm, or mm, etc.
Period (T): time between crest (or trough) passages
Frequency (): rate of passage of crests (or troughs),
Also, v =
h
1T
(units: Hertz or cycles/sec)
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Demo: making waves - wave table
Demo: slinky waves
Waves
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Radiation travels as Electromagnetic waves.That is, waves of electric and magnetic fields traveling together.
Examples of objects with magnetic fields:
a magnetthe EarthClusters of galaxies
Examples of objects with electric fields:
Protons (+)Electrons (-)
} "charged" particles that make up atoms.
Power lines, electric motors, …
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Scottish physicist James Clerk Maxwell showed in 1865 that waves of electric and magnetic fields travel together => traveling “electromagnetic” waves.
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The speed of all electromagnetic waves is the speed of light.
c = 3 x 10 8 m / sor c = 3 x 10 10 cm / sor c = 3 x 10 5 km / s
Sun
Earth
light takes 8 minutes
c =
or, bigger means smaller
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c =
1 nm = 10 -9 m , 1 Angstrom = 10 -10 m
The Electromagnetic Spectrum
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Demo: white light and a prism
A Spectrum
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All waves bend when they pass through materials of different densities. When you bend light, bending angle depends on wavelength, or color.
Refraction of light
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Clicker Question:
Compared to ultraviolet radiation, infrared radiation has greater:A: energy
B: amplitude
C: frequency
D: wavelength
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Clicker Question:
The energy of a photon is proportional to its:A: period
B: amplitude
C: frequency
D: wavelength
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Clicker Question:
A star much colder than the sun would appear:A: red
B: yellow
C: blue
D: smaller
E: larger
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Rainbows
rred orange yellow green blue violet
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What's happening in the cloud?
Sun's ray
raindrop
42o 40o
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Double Rainbows