Micro-world Macro-world Fall 2009 Instr: Stephen L. Olsen

What does “Physics” mean? Greek: φίσίσ phisis “things” “logic”  “The logic of things” or “How things work” Chinese: 物理 WU LI  Nature

What kind of “things”? Ordinary-sized objects: : apples : footballs : ukuleles : etc.

Gigantic things Earth Sun galaxies etc.

Very tiny things molecules atoms cells quarks

Galileo Galilei st “modern” scientist

Example of how Galileo thought: His experiments on motion on inclined planes

What if the 2 nd track was horizontal?

Aristotle ~300 BC Forces are needed to keep objects in motion. A constant force will have a constant motion. Physics book VII chapter 5

Galileo’s big idea Forces are not needed to keep objects in motion. They are only needed to change the state of motion.

Actually, Galileo wasn’t first Mozi ( 墨子 ) ~400BC If there is no opposing force, motion will never stop. before Aristotle!

Object sliding on a surface If there were no friction the book would keep sliding.

Angels are not needed to keep planets moving in their orbits

Law of inertia Discovered by Galileo, but as written by Newton: Every object perseveres in its state of rest, or of uniform motion in a straight line, unless it is so compelled to change that state of motion by forces impressed thereon.

Comment on the Law of Inertia No mention of balls & tracks Applies to ordinary objects & also huge objects like galaxies and tiny objects like atoms & quarks Galileo knew nothing of galaxies or quarks, but, thanks to his imagination, his insight, based on observations of ordinary objects, proved to be a universal truth.

Units & Reference Frames Required for measurements/observations

English units King’s foot 12 inches = 1ft 5280 ft = 1 mile

One meter of length 1 meter = 40 inches (1.1 yds) 10,000,000m  1m About the distance from nose to the outstretched finger (of a pretty big guy).

Centimeters & millimeters 1 centimeter = 1/100 meters (  radius of a penny) 1 millimeter = 1/1000 meters (  thickness of a dime)  means “approximately equal to” 1cm 1mm

1km  5/8 mile 1000 m = 1 kilometer (km)

Motion depends on the circumstances of the observer (I) To us, both the red & blue cars move forward To someone in the red car, the blue car is going backwards backwardforward

Reference frames aka Coordinate systems x z y To describe the position of an object: You need to define a coordinate system (reference frame) & determine 3 coordinates e.g. x, y & z Choices of location of the coordinate system & directions of the axes are up the observer. z x-axis (e.g. the 1 st baseline) y-axis (e.g. the 3 rd baseline) z-axis (vertical) reference point (e.g. home plate)

Motion depends on the circumstances of the observer (II) In the fish’s reference frame, it is the ground that is moving x z y

Some distances Nose to finger~1m Back of the room~10m SNU library~100m SNU main gate~1000m Incheon ~50000m Center of the Earth~ m Earth to Moon ~ m Earth to Sun ~ m Next star ~ m We are only out to the next star & we are already running out of space

Some small sizes radius of a 10 won coin~0.01 m thickness of a 10 won coin~0.001 m thickness of a human hair ~ m diameter of a red blood cell~ m size of hydrogen atom ~ m size of hydrogen nucleus ~ m Again, the numbers become unwieldy & use up a lot of space

Exponential logic

Exponential logic powers of “2”

Area of a sheet of paper 28 cm 21.5 cm Area =28 cm x 21.5 cm = 600 cm 2 Cut it in half

Area of the remaining sheet after 1 cut 28 cm 21.5 cm Area =28 cm x 21.5 cm Cut it in half again = 600 cm 2 =300 cm 2 = 600 (1/2)cm 2

Area of the remaining sheet after 2 cuts 28 cm 21.5 cm Area =28 cm x 21.5 cm Cut it in half again = 600 cm 2 2x2 =600 cm 2 x(1/2) 2 =150cm 2

Area of the remaining sheet after 3 cuts 28 cm 21.5 cm Area =28 cm x 21.5 cm Cut it in half again 2 2x = 600 cm 2 2x2x2 =600 cm 2 x(1/2) 3 =75cm 2

After 12 times 28 cm 21.5 cm Area =28 cm x 21.5 cm =0.15 cm 2 Smallest piece of paper I can handle = 600 cm x2 6 =600 cm 2 x(1/2) 12 = 600 cm

Imagine we did it 70 times 28 cm 21.5 cm Area =28 cm x 21.5 cm ,180,591,620,717,411,3??,??? = 5.1 x cm 2 (about ½ the area of an atom) =600 cm 2 x = 600 cm x2 35 =600 cm 2 x(1/2) 70 1

1 1,180,591,620,717,411,3??,??? is unwieldy ( 다루기 힘든, 꼴 사나운 ) (1/2) 70 Is more tractable Difficult to manage Tractable easy to manage

Powers of 10 $ = $ x 10 Move the decimal point 1 place to the left multiply by 10 & $0.01 = $00.1 x 1/10 Move the decimal point 1 place to the right divide by 10 & “penny” “dime”

Positive powers of x x x x x x x x x x x = 1.5 x x 10x10 11 jumps to the left  +11 in the exponent

Some distances Nose to finger~1 m The back of the room~10 m SNU Library ~100 m SNU Main gate~1000 m Incheon~50000 m Center of Earth ~ m Earth to Moon ~386,000,000 m Earth to Sun~ m Next star~ m ~5.0 x 10 4 m ~6.4 x 10 6 m ~3.86 x 10 8 m ~1.5 x m ~4.0 x m

nomenclature 10 3 = 1 thousand (kilo) 10 6 = 1 million (mega) 10 9 = 1 billion (giga) = 1 trillion (tera)

Small numbers 1 penny = $ dime= $ penny = 1/10 th of a dime $0.01 = $0.10x(1 /10) Moving the decimal point 1 place to the right is the same as multiplying by 10

Negative powers of x 1/ x x x x x x x x x x x x x = 6.0 x jumps to the right  -11 in the exponent

Some small sizes radius of a 10 won coin~0.01 m thickness of a 10 won coin~0.001 m thickness of a human hair ~ m diameter of a red blood cell~ m size of hydrogen atom ~ m size of hydrogen nucleus ~ m ~7.5 x m ~2.0 x m ~6.0 x m ~1.0 x m

More nomenclature = centi = milli = micro- (  ) = nano = pico-

exponents are very important Distance to the Sun: 1.5 x m Distance to next star: 4 x m Exponents differ by 5 Light from the sun takes 8 minutes to get here Light from the next star takes 4 years to get here 8 minutes from now 4 years from now?

Multiplying large numbers Use the rule: (A x 10 a ) x (B x 10 b ) Number of people In the US x average income/person 301,000,000 x $ 40,100 = 3.01x10 8 x $ 4.01x10 4 = (3.01 x $4.01) x (10 8 x 10 4 ) = $12.1 x =$12.1 x = $1.21 x on 1/8/07 10 a+b =(AxB) x (10 a x 10 b ) Sum of everyone’s income

dividing large numbers = US national debt # of persons in US on 818/07 Ax10 a Bx10 b x= = x 10 a x 10 -b $8,598,000,000, ,000,000 persons $8.60 x x 10 8 persons == $ persons x = $2.86 x / person move b upstairs = $2.86 x 10 4 / person (= $28,600/ person ) ABAB ABAB 10 a 10 b & change sign 10 a-b

$28,600 (ladies not included)