Post on 20-Dec-2015
What does “Physics” mean?
Greek: φίσίσ
phisis
“things”“logic”
“The logic of things”
or “How things work”Chinese: 物理 WU LI
Nature
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!
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.
One meter of length1 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
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
backward forward
Reference frames aka Coordinate systems
x
z
y
To describe the positionof an object: You needto define a coordinate system (reference frame)& determine 3 coordinates
e.g. x, y & z
Choices of location ofthe coordinate system& directions of the axesare up the observer.
z
x-axis(e.g. the 1st baseline)
y-axis(e.g. the 3rd 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 distancesNose to finger ~1m
Back of the room ~10mSNU library ~100mSNU main gate ~1000mIncheon ~50000mCenter of the Earth~6400000mEarth to Moon ~385000000mEarth to Sun ~150000000000mNext star ~40000000000000000m
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 ~0.000075 m• diameter of a red blood cell ~0.00000002 m• size of hydrogen atom ~0.00000000006 m• size of hydrogen nucleus ~0.0000000000000001 m
Again, the numbers become unwieldy& use up a lot of space
Area of the remaining sheet after 1 cut
28 cm
21.5 cmArea =28 cm x 21.5 cm
Cut it in half again
2
2
2
= 600 cm2
=300 cm 2
= 600 (1/2)cm2
Area of the remaining sheet after 2 cuts
28 cm
21.5 cmArea =28 cm x 21.5 cm
Cut it in half again
2
22
2
= 600 cm2
2x2
=600 cm 2 x(1/2)2
=150cm2
Area of the remaining sheet after 3 cuts
28 cm
21.5 cmArea =28 cm x 21.5 cm
Cut it in half again
2
2x22
2
12
= 600 cm2
2x2x2
=600 cm 2 x(1/2)3
=75cm2
After 12 times
28 cm
21.5 cm Area =28 cm x 21.5 cm
26
2626 26
=0.15 cm2
Smallest piece ofpaper I can handle
= 600 cm2
26x26
=600 cm 2
x(1/2)12
= 600 cm2
4096
Imagine we did it 70 times
28 cm
21.5 cmArea =28 cm x 21.5 cm
235
235235
235
1,180,591,620,717,411,3??,???
= 5.1 x10-19 cm2
(about ½ the area of an atom)
=600 cm2 x
= 600 cm2
235x235
=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
Tractableeasy to manage
Powers of 10
$ 100.0 = $ 10.00 x 10
Move the decimalpoint 1 place to the
left
multiplyby 10&
$0.01 = $00.1 x 1/10
Move the decimalpoint 1 place to the
right
divide by 10&
“penny”“dime”
Positive powers of 10
150000000000150000000000.15000000000.0 x 1011500000000.00 x 102150000000.000 x 10315000000.0000 x 1041500000.00000 x 105150000.000000 x 10615000.0000000 x 1071500.00000000 x 108150.0000000000 x 10915.0000000000 x 10101.50000000000 x 1011
= 1.5 x 1011
1500000000.00 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 ~6400000 m• Earth to Moon ~386,000,000 m• Earth to Sun ~150000000000 m• Next star ~40000000000000000 m
~5.0 x 104 m ~6.4 x 106 m ~3.86 x 108 m
~1.5 x 1011 m ~4.0 x 1016 m
nomenclature
• 103 = 1 thousand (kilo)
• 106 = 1 million (mega)
• 109 = 1 billion (giga)
• 1012 = 1 trillion (tera)
Small numbers
1 penny = $0.01
1 dime= $0.10
1 penny = 1/10th 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 10
0.00000000006 00.0000000006 x 1/10 00.0000000006 x 10 -1000.000000006 x 10 -1x10-1000.000000006 x 10 -20000.00000006 x 10 -300000.0000006 x 10 -4000000.000006 x 10 -50000000.00006 x 10 -600000000.0006 x 10 -7000000000.006 x 10 -80000000000.06 x 10 -900000000000.6 x 10 -10000000000006.x 10 -11
= 6.0 x 10-11
11 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 ~0.000075 m• diameter of a red blood cell ~0.00000002 m• size of hydrogen atom ~0.00000000006 m• size of hydrogen nucleus ~0.000000000000001 m
~7.5 x 10-5 m ~2.0 x 10-8 m
~6.0 x 10-11 m ~1.0 x 10-15 m
exponents are very important
Distance to the Sun: 1.5 x 1011m
Distance to next star: 4 x 1016 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 numbersUse the rule:
(A x 10a ) x (B x 10b)
Number of people
In the USx average income/person
301,000,000 x $ 40,100
= 3.01x108 x $ 4.01x104
= (3.01 x $4.01) x (108 x 104)
= $12.1 x 108+4 =$12.1 x 1012 = $1.21 x 1013
on1/8/07
10a+b
=(AxB) x (10a x 10b)
Sum of everyone’s income
dividing large numbers
=US national debt# of persons in US
on818/07
Ax10a Bx10b
x= = x 10a x 10-b
$8,598,000,000,000301,000,000 persons
$8.60 x 1012
3.01 x 108 persons= =
$8.60 1012
3.01 108 personsx
= $2.86 x 1012- 8 /person
move b upstairs
= $2.86 x 104/person
(= $28,600/person)
AB
AB
10a 10b
& change sign
10a-b