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.
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
Newton's Laws of Motion
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 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 an unbalanced force.
Newton’s 2nd 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
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.
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.
Throwing a ball into Orbit
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 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
We form a "spectrum" by spreading out radiation according to
its wavelength (e.g. using a prism for light).
What does the spectrum of an astronomical object's radiation
look like?
Many objects (e.g. stars) have roughly a "Black-body"
spectrum:
• Asymmetric shape
Brightness
• Broad range of wavelengths
or frequencies
• Has a peak
Frequency
also known as the Planck spectrum or Planck curve.
Approximate black-body spectra of astronomical objects
demonstrate Wien's Law and Stefan's Law
cold dust
hotter star (Sun)
“cool" star
very hot stars
frequency increases,
wavelength decreases
Laws Associated with the Black-body Spectrum
Wien's Law:
lmax energy a
1
T
(wavelength at which most energy is radiated is longer for cooler objects)
Stefan's Law:
Energy radiated per cm2 of area on surface every second a T 4
(T = temperature at surface)
1 cm2
Betelgeuse
Rigel
Betelgeuse
The total energy radiated from entire surface every second is called the
luminosity. Thus
Luminosity = (energy radiated per cm2 per sec) x (area of surface in cm2)
For a sphere, area of surface is 4pR2, where R is the sphere's radius.
The "Inverse-Square" Law Applies to Radiation
Each square gets 1/4
of the light
Each square gets 1/9
of the light
apparent brightness a 1
D2
D is the distance between
source and observer.
The Doppler Effect
Applies to all kinds of waves, not just radiation.
at rest
velocity v1
velocity v2
velocity v1
velocity v1
velocity v3
you encounter
more wavecrests
per second =>
higher frequency!
fewer wavecrests
per second =>
lower frequency!
Doppler Effect
Demo: buzzer on a moving arm
Demo: The Doppler Ball
The frequency or wavelength of a wave depends on the
relative motion of the source and the observer.
Things that waves do
1. Refraction
Waves bend when they pass through material of different densities.
air
water
swimming pool
prism
air
glass
air
2. Diffraction
Waves bend when they go through a narrow gap or around a corner.
3. Interference
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Waves can interfere with each other
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