Transcript ASTR 1120 General Astronomy: Stars and Galaxies

ASTR 1020
General Astronomy: Stars and Galaxies
Section 010
G1B20 TR 11:00-12:15
Instructor: Webster Cash
TA: David Schenck
http://casa.colorado.edu/~wcash/APS1020/APS1020.html
Announcements
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Website
http://casa.colorado.edu/~wcash/APS1020/APS1020.html
First Homework Due January tbd
First Observatory January 31 at 7pm
Swirling Gas
Falls into “Disk Stability”
99.9% Ended in Sun (0.1% in Jupiter)
Probably the Same Around All Stars -- Planets are Common
Shoots Planet-Size
Bullets into Space
Some Physics
• Scientific Method
• Hypothesis and Experiment
• THERE IS A TRUTH!!!
“I Think Therefore I Am”
• If Experiment Shows Theory Wrong Then
Its Wrong
• Doesn’t Mean Theory is Right
Galileo
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First Great Modern Scientist
Quantitative Analysis
Tenacious Pursuit of Truth
Challenged Aristotle and Church
Used Telescopes to Demolish Old Solar
System View
• Persecuted In Italy – Celebrated
Elsewhere
Isaac Newton (1643-1727)
• Greatest Scientist of All Time
• Quantitative Prediction
• Mechanics, Optics, Math – Theory & Observation
Two Basic Concepts
Mass – Amount of Matter (Not Weight)
Inertia – Tendency of Objects to Have Unchanged Motion
Newton’s Laws
• Law #1: A body at rest or in motion
remains that way unless acted upon by an
outside force.
• Law #2: a=F/m
• Law #3: For every action there is an equal
and opposite reaction.
Law #1
A body in motion remains that way unless acted upon by an outside force.
No Forces
Contact Forces (electrical)
Gravity
Question
A hockey rink is a frictionless surface. If a hockey
player falls down, how is it that he is not
moving in a straight line at constant velocity?
a) The Earth’s gravity is acting as an outside
force.
b) Electrical forces in the rink force him off the
straight line
c) He must push off against another player
d) He IS moving along a straight line.
Law #2
F=ma
a=F/m
F = Force
m = mass
a = acceleration
• Have a mass
• Apply a force
• Acceleration Results
Note: Law #1 is special case of Law #2 (F=0
a=0)
Question
If there is no force on an object how is that
the object can keep moving?
a) Velocity is conserved in the Universe
b) Momentum cannot be changed
c) Velocity is independent of accleration
d) The object needs energy to keep moving
Motion
Position
X1 marks position in space at time t1
X2 marks position in space at time t2
Velocity
Acceleration
x2  x1
v
t 2  t1
Change of Position
with time
v1 is velocity at time t1
v2 is velocity at time t2
v2  v1
a
t 2  t1
Change of Velocity
with time
Example of Law #2
Push Your Car ---- Large m implies large F to get any a
Let m = 1000kg
If you apply 20N (Newtons is the unit of force)
a=20/1000=.02m/s2 ---- accelerates .02m/s for every second you push
Push for 10 seconds --- car will be moving .02m/sx10s= 0.2m/s
(if on ice so no friction)
(friction is yet another force)
Law #3
For every action there is an equal and opposite reaction.
For every ma there is an equal and opposite ma.
Big one doesn’t move much
Equal Mass Implies Equal Acceleration
But What is F?
a=F/m not enough because we don’t know what F is.
Need a new law to describe forces
There are Four KNOWN Forces
Gravity
Weak Nuclear Force
Electromagnetism
Strong Nuclear Force
In order of strength
Gravity Weakest
Newton’s Law of Universal Gravitation
Gm1m2
F21 
r2
Gm1m2
F12 
r2
(Law #3 Satisfied)
G = 6.7x10-8 dyne.cm2/g2 in cgs units
Value of G is measured. Nobody knows why it is the value it is.
Electrical Force Equation Similar
e1e2
Fe  2
r
But What is a Force?
Newtons Laws Give Value, and Show what to do.
Force is an Exchange of Virtual Particles