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 .. .. 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 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? 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.

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)

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

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