Transcript WK6revisedtwoweeks

Phys 181-701
Astronomy
Galileo’s Apparatus – Deutches Museum, Munchen, Germany
“To command the professors of astronomy to confute their
own observations is to enjoin an impossibility, for it is to
command them to not see what they do see, and not to
understand what they do understand, and to find what they
do not discover.”
Galileo Galilei – In Science
“Numero pondere et mensura Deus omnia condidit.”
Sir Isaac Newton – Principia Mathematica
“If I have been able to see further, it was only because I stood
on the shoulders of giants.”
Newton, in a letter to Robert Hooke
REVIEW:
Kepler Develops Three Laws:
•Law of Ellipses
•Law of Areas
•Harmonic Law
P2
 k  constant
a3
We now understand HOW the planets move…
but not WHY they move.
Review
Galileo: The Death of the Earth
Centered Universe
•Contemporary of Kepler
•Demonstrated that all objects are accelerated by
gravity by the same amount
•Moving objects remain in motion
•Built a telescope in 1609* and observed the Sun,
Moon, Milky Way, Moons of Jupiter and the phases
of Venus.
1564-1642
*Hans
Lippershey invented the telescope in 1608
If we assume (incorrectly) that the Tower of Pisa is
20m tall, the ball will take 2s to hit the ground.
Even if the ball is thrown horizontally from the tower,
the acceleration toward the earth is still 10m/s2.
As a result, the ball that is dropped and the ball that is
thrown both hit the ground after 2 seconds!!!
We will return to this essential idea in a few slides…
Sir Isaac Newton
Newton’s Laws:
1.
All objects at rest shall remain at rest and all objects
in motion shall Remain in motion in a straight line,
unless compelled by a FORCE to do otherwise.
2.
The ACCELERATION of any object is directly
proportional to the FORCE applied to it and
inversely proportional to its MASS.
3.
For every force applied to an object, there is an
equal and opposite force applied by the object on the
actor.
1642-1727
N
e
w
t
o
n
v
.
s
Newton’s Laws Relative to Galileo’s Experiment:
1.
When the ball is dropped it ceases to be at rest.
Therefore there must be a force, directed downward,
to cause the acceleration.
2.
The acceleration will be equal to the force that gravity
exerts on the ball divided by the mass of the ball, that
is, the acceleration is equal to the force per unit mass.
3.
If the Earth exerts a gravitational force on the ball, the
ball must exert an equal and opposite force on the
Earth!!!!
Law of Universal Gravitation
Newton knows that the more mass an object has, the greater
the force of Gravity on it.
FG= m g
Where “g” is the special name given to the acceleration that
is caused by gravity. 10 m/s2
The inverse square law…
Inverse square
The Law
Example:
Example
“Weighing” the Earth…
A & B have equal masses and therefore equal weights.
The rod is balanced.
The very small mass is needed to
balance the gravitational force of
the very large mass.
“G” can be calculated!
Knowing G and Kepler’s Law’s allows us to calculate the mass of the Earth,
Sun and all of the planets moons and asteroids in the solar system
Return to Pisa…The earth is not flat…
Return to Pisa
Curved Earth
After one second the projectile has fallen five meters…
But the earth has curved away.
Question…
If the earth is curved such that it “curves away” 5 meters
for every 8000 meters traveled, how fast would the
projectile need to be going so that, after falling 5 meters, it
was still 5 meters above the earth?
Q
u
e
s
t
a3
k
P2
8000 m/s!!!
•Newton invents calculus
Summary
•Newton Proves Kepler’s Laws
•Universal Gravitation used to determine the mass
of the earth
•Tides understood
•Moon “lock” understood
•Satellite motion possible
•Solar system travel made possible
S
u
m
m
Electromagnetic Radiation
Chapter 3 (3.3, 3.4, 3.5)
Chapter 4 (4.2, 4.4)
The frequency is measured in Hertz (Hz) and is the number of
waves which pass a given point in one second.
Time period of mars
NEXT TIME:
Two Laws of
RADIATION
E
Phys 181-701
Astronomy
Garment of Brightness
May the warp be the white light of morning,
May the weft be the red light of evening,
May the fringes be the falling of the rain,
May the border be the standing rainbow.
Thus weave for us a garment of brightness.
Song of the Sky Loom (Native American Traditional)
•Light is a form of Electromagnetic Radiation
•Radiation may be described as waves
Review
•Waves are described by their frequency, wavelength, amplitude
and speed.
The Interaction of Matter and
IntroRadiation
Electromagnetic
(Light)
Understanding This is an Important Tool of Astronomy
Light Interacts with matter in several ways…
•Reflection
•Refraction
•Diffraction
•Absorption
•Emission
•Gravitation!
Reflection
Reflection
Light simply scatters off the dust and gas…
Reflection Nebulae
Refraction
Refraction
Light bends when moving from one substance to another…
r
a
c
t
i
o
n
Light bends around obstacles…
Diffraction
Absorption and Emission
Absorption&Emission
O
b
s
e
r
v
a
Thermal (Blackbody) Radiation Spectra
T
h
e
r
m
a
l
R
273 Kelvin = 0 ºC
Two Laws of Thermal Radiation:
Two
Law
s
1.
Stefan-Boltzmann Law: Hotter objects emit more power per unit area.
2.
Wien’s Law: Hotter objects emit light that has a higher average energy.
Change in Brightness
Practical Example
Change in Color
C
a
l
c
u
l
a
t
i
o
Example:
Consider a perfect thermal emitter with a temperature of 15000 K. How much
power does it emit per unit area? What is the wavelength of maximum intensity?
Putting it Together…
Putti
ng it
A
n
a
l
y
D
o
p
p
l
e
S
h
i
f
t
e
d
S
Doppler Effect Formula:
r
F
o
r
m
u
l
a
Next Time……
STARS,
A Mass of
Incandescent Gas
E
N