ASTR 1040 Honors Astronomy: Stars and Galaxies Section 010 G131 TR 9:30-10:45 Instructor: Webster Cash TA: Dennis Tilipman http://casa.colorado.edu/~wcash/APS1040/APS1040.html

Announcements Website http://casa.colorado.edu/~wcash/APS1040/APS1040.html First Homework Due January tbd First Observatory September 26 at 8pm

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

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

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=0 a=0)

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

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 (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

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

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