Transcript Powerpoint for today

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.
Kepler (1571-1630)
Used Tycho Brahe's precise data on
apparent planet motions and relative
distances.
Deduced three laws of planetary
motion.
Kepler's First Law
The orbits of the planets are elliptical (not circular)
with the Sun at one focus of the ellipse.
Ellipses
distance between foci
eccentricity =
major axis length
(flatness of ellipse)
Kepler's Second Law
A line connecting the Sun and a planet sweeps out equal areas
in equal times.
slower
Translation: planets move faster
when closer to the Sun.
faster
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
or
P2  a3
(for circular orbits, a=b=radius).
Translation: the larger a planet's orbit,
the longer the period.
a3
a
b
Solar System Orbits
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, and
a in AU,
P2 = a3
(for circular orbits, a=radius).
Translation: the larger a planet's orbit,
the longer the period.
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.
Orbits of some planets (or dwarf planets):
Planet
a (AU)
Venus
Earth
Pluto
0.723
1.0
39.53
P (Earth years)
0.615
1.0
248.6
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
Copernican model was a triumph of the Scientific Method
Scientific Method:
a)
b)
c)
d)
e)
Make high quality observations of some natural phenomenon
Come up with a theory that explains the observations
Use the theory to predict future behavior
Make further observations to test the theory
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
Prediction
Observation
Theory
Characteristics of Scientific Theories
Scientific Theories:
a) Must be testable
b) Along with their consequences, must be continually tested
c) Should be simple (Occam’s Razor) and no more complex than
necessary
d) 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
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.
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.
Newton's Zeroeth Law of Motion
DEMO - Pushing the cart on track
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.
Newton's First Law of Motion
DEMO - Air Puck motion
DEMO - Smash the HAND
DEMO - Tablecloth
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.
Fnet
a=
m
acceleration is a change in speed or a change in direction of speed.
Newton's Second Law of Motion
Demo - Force and Acceleration with
fan carts
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.
Newton's Third Law of Motion
DEMO: CART
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
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.
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.
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
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.
Escape Velocity
Velocity needed to completely escape the gravity of a planet.
The stronger the gravity, the higher the escape velocity.
Examples:
Earth
Jupiter
Deimos (moon of Mars)
11.2 km/s
60 km/s
7 m/s = 15 miles/hour
Timelines of the Big Names
Galileo
Copernicus
1473-1543
1564-1642
Brahe
1546-1601
Kepler
1571-1630
Newton
1642-1727
Electromagnetic Radiation
(How we get most of our information about the cosmos)
Examples of electromagnetic radiation:
Light
Infrared
Ultraviolet
Microwaves
AM radio
FM radio
TV signals
Cell phone signals
X-rays
Radiation travels as waves.
Waves carry information and energy.
Properties of a wave
wavelength (l)
crest
amplitude (A)
trough
velocity (v)
l is a distance, so its units are m, cm, or mm, etc.
Also, v = l n
Period (T): time between crest (or trough) passages
Frequency (n): rate of passage of crests (or troughs), n =
(units: Hertz or cycles/sec)
1
T
 = hn
Waves
Demo: making waves - wave table
Demo: slinky waves
Radiation travels as Electromagnetic waves.
That is, waves of electric and magnetic fields traveling together.
Examples of objects with magnetic fields:
a magnet
the Earth
Clusters of galaxies
Examples of objects with electric fields:
Power lines, electric motors, …
Protons (+)
"charged" particles that
make up atoms.
Electrons (-)
}
Scottish physicist James Clerk Maxwell showed in 1865
that waves of electric and magnetic fields travel together =>
traveling “electromagnetic” waves.
The speed of all electromagnetic waves is the speed of light.
c = 3 x 10 8 m / s
or c = 3 x 10 10 cm / s
or c = 3 x 10 5 km / s
light takes 8 minutes
Earth
Sun
c= ln
or, bigger l means smaller n
The Electromagnetic Spectrum
1 nm = 10 -9 m , 1 Angstrom = 10 -10 m
c= ln
A Spectrum
Demo: white light and a prism
Refraction of light
All waves bend when they pass through materials of different densities.
When you bend light, bending angle depends on wavelength, or color.
Clicker Question:
Compared to ultraviolet radiation, infrared
radiation has greater:
A: energy
B: amplitude
C: frequency
D: wavelength
Clicker Question:
The energy of a photon is proportional to its:
A: period
B: amplitude
C: frequency
D: wavelength
Clicker Question:
A star much colder than the sun would
appear:
A: red
B: yellow
C: blue
D: smaller
E: larger
Rainbows
rred orange yellow green blue violet
What's happening in the cloud?
raindrop
42o
40o
Double Rainbows