1. ASTR 1040 Honors Astronomy: Stars and Galaxies
Section 010
G131 TR 9:30-10:45
Instructor: Webster Cash
TA: Dennis Tilipman

2. Announcements Website
First Homework Due January tbd
First Observatory September 26 at 8pm

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. 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.

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

6. 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)

7. 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

8. 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)

9. 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

10. 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

11. 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

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

13. Question
A force of 250 Newtons is applied to a 100kg object. How fast does it accelerate?
2.5m/s/s
25m/s/s
0.4m/s/s
4m/s/s