ASTR 1200Announcements

Websitehttp://casa.colorado.edu/~wcash/APS1200/APS1200.html

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Explaining the Universe

Science: Model or ExplanationTestCorrectionIterate“Truth” -- All that’s left

Model of the Universe MythMeasurementModern Science

All That’s Left

Universe Unfolded on Itself in Space and Time

Big Bang and Expanding Universeabout 13 Billion Years ago

Formed Galaxies from gas

Formed Stars and Planets

Earth & Sun about 4.6 Billion Years Old

In this course we will explain why we think these thingsabout the universe. Plus more….

Question:Why Are Our Atoms Arranged this Way?

e.g. Why are our atoms on a planet, not in a black hole?

HISTORY IS DESTINY

Make assumption about initial state of matter and thenfollow what happens.

eg solar system was a gas cloud that collapsed due to gravitySun and Earth formed at same time

Formation of the Solar SystemUniverse is about 13 Billion Years old (4x1017s)Milky Way 10 Billion Years oldSun and Planets – 4.6 Billion Years old

Sun has done about 25 orbits of Milky Way

Stars are being formed all the time -- some young, some old

Stars are formed from clouds of gas

Starts as Thin Cloud

Ensuing High DensityShocks Is Site of Collapse

Spins Up Like Ice Skater

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-SizeBullets into Space

Lots of Stars

• 100 Billion Stars in Milky Way

• 100 Billion Galaxies in Visible Universe

• Almost an Avagodro’s Number in Universe!

Catastrophe Theory

• Theory In Vogue When I was in Grade School – (1950’s ouch!)

• Two Stars Stray Near Each Other• Close Orbit Pulled Gas from Sun• Gas Formed Planets• Predicts Planets Very Rare

(Collisions Rare)• Astronomy Advanced Hugely in 20th

Century

Question

Why was the Solar System formed from a cloud of gas?

a) The Universe started as diffuse gas

b) Old stars explode to create new gas

c) The galaxy makes new gas that must collapse

d) Stars never collide so they cannot form solar systems

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

Motion

• x = v * t

• v = a * t

• x = ½ * a * t2

Motion

12

12

tt

xxv

X1 marks position in space at time t1

X2 marks position in space at time t2

Position

VelocityChange of Positionwith time

v1 is velocity at time t1

v2 is velocity at time t2

Acceleration

12

12

tt

vva

Change of Velocitywith time

Question

A spaceship accelerates from rest at 20gees (200m/s/s) steadily for 1.2 days (105 seconds). How far has it travelled?

a) 2x107m

b) 5000m

c) 1012m

d) 104m

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 #1A 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

Note: Law #1 is special case of Law #2 (F=0 a=0)

F = Forcem = massa = acceleration

Example of Law #2

Push Your Car ---- Large m implies large F to get any a

Let m = 1000kgIf 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)

Clicker Question

A force of 250 Newtons is applied to a 100kg object. How fast does it accelerate?

a) 2.5m/s/s

b) 25m/s/s

c) 0.4m/s/s

d) 4m/s/s

Law #3For 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

GravityWeak Nuclear ForceElectromagnetismStrong Nuclear Force

In order of strengthGravity Weakest

Newton’s Law of Universal Gravitation

221

12 r

mGmF

221

21 r

mGmF

(Law #3 Satisfied)

G = 6.7x10-11 in mks units

Value of G is measured. Nobody knows why it is the value it is.

221

r

eeFe Electrical Force Equation Similar

Clicker Question

What is the weight of a 100kg person standing on the surface of the Earth. (in Newtons) M=6x1024kg. R=6000km

a) 6.7x1021N

b) 1.6x108N

c) 1000N

d) 6.7x10-3N

But What is a Force?

Newtons Laws Give Value, and Show what to do.

Force is an Exchange of Virtual Particles