1. ASTR 1020 General Astronomy: Stars and Galaxies
Section 010
G1B20 TR 11:00-12:15
Instructor: Webster Cash
TA: David Schenck

2. Announcements .. .. Website
First Homework Due January tbd
First Observatory January 31 at 7pm

3. 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

4. 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

5. Galileo 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

6. 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

7. 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.

8. Law #1
A body in motion remains that way unless acted upon by an outside force.
No Forces
Contact Forces (electrical)
Gravity

9. 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?
The Earth’s gravity is acting as an outside force.
Electrical forces in the rink force him off the straight line
He must push off against another player
He IS moving along a straight line.

10. Law #2 F=ma a=F/m Have a mass Apply a force Acceleration Results
F = Force
m = mass
a = acceleration
F=ma
Have a mass
Apply a force
Acceleration Results
a=F/m
Note: Law #1 is special case of Law #2 (F= a=0)

11. Question
If there is no force on an object how is that the object can keep moving?
Velocity is conserved in the Universe
Momentum cannot be changed
Velocity is independent of accleration
The object needs energy to keep moving

12. Motion X1 marks position in space at time t1
Change of Position
with time
Velocity
v1 is velocity at time t1
v2 is velocity at time t2
Acceleration
Change of Velocity
with time

13. 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/s 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)

14. 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

15. 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

16. Newton’s Law of Universal Gravitation
(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

17. But What is a Force? Newtons Laws Give Value, and Show what to do.
Force is an Exchange of Virtual Particles