How Things Work

1
How Things Work
THINK instead of memorize DONT get behind on
reading or assignments DONT be afraid to ask
questions (youre paying me for the time)
2
Administrivia

• Final Exam will be 8am, Dec 9 (Saturday)
• No exceptions except via the Deans Office
• Final 35
• Midterm Exams (Sept 27, Oct 20, Nov 17) 40
• GRADING WILL BE PICKY ON EXAMS!
• Use the best 2 out of the 3 -- BUT NO MAKE-UP
  EXAMS.
• HW ( pop quizes!) 25
• Grading will be lax on these
• TAs Laurel Averett (SI Center) Haw Cheng
  (Demos) Samantha Hammond (grading)

3
Words from the wise(?)

• Intelligence is knowing what to do when you do
  not know what to do. (John Holt)
• The one real object of an education is to leave
  a person in the condition of continually asking
  questions. (Bishop Mandell Creighton)
• Im here to guide you and to challenge you to
  think. (LH Greene, HTW instructor at Illinois)
• Thinking clearly is just as hard as hitting a
  jump shot in traffic -- IT TAKES LOTS OF PRACTICE

4
What Physics is -- not

• Magic
• Art
• Astrology
• Religion
• Engineering
• Philosophy
• Math

5
OK so what IS physics?

• Collection of observations of the world from
  which theoretical rules or laws are inferred
  (philosophy math)
• Intellectual structure that houses the
  theoretical understanding of the observations
  (philosophy)
• Ongoing effort to refine the observations and the
  understandings (engineering and philosophy)

6
Physicist Person who can make rainbows and sea
shores boring
7
Who cares?

• Daily life (microwaves, cars, TVs, PCs, cell
  phones, etc)
• Your and your childrens lives (global warming,
  energy reserves, etc)
• And of course you might just be curious about how
  to fix it when the coffee pot refuses to behave.

8
Wake up call
Eat a live frog first thing in the morning
and nothing worse will happen to you for the rest
of the day!
9
Just accept these

• Distance, Mass, Charge, and Time are the starting
  concepts
• All are common sense to us, but there are precise
  definitions from NIST, dictionaries if you need
  them.
• There are no deeper rules that underlie these
  basic concepts

10
For what is time?St. Augustine (354-430),
Confessions, Book 11, A.C. Outler, trans.

• For what is time? Who can easily and briefly
  explain it? Who can even comprehend it in
  thought or put the answer into words? Yet is it
  not true in conversation we refer to nothing more
  knowingly than time? And surely we understand it
  when we speak of it we understand it also when
  we hear another speak of it.
• What, then, is time? If no one asks me I know
  what it is. If I wish to explain it to him who
  asks me, I do not know. Yet I say with
  confidence that if nothing passed away, there
  would be no past time, and it nothing were still
  coming, there would be no future time and if
  there were nothing at all, there would be no
  present time.
• 1. Why is time more difficult to define than
  other words?
• 2. Do you agree that time would not exist
  without objects and events? Why?

11
UNITS !!!!!

• Mass kilogram, gram, stone
• Distance meter, centimeter, inch
• Time seconds, seconds, seconds (thank heavens!)
• Charge Coulombs, , _at_ (you dont ever use
  those anyway)

12
Algebra with units

• Turns out that

39.4 in 1 m
So for example
72 in
1.8 m
13
More algebra with units
A baby has a mass of 3.2 kg at birth and grows to
8.6 kg 9 months later. What is its average
annual growth rate?
10
8
mass (kg)
6
slope
7.2 kg/yr
4
2
0
0
1
time (yr)
14
Distance, velocity, speed and acceleration

• Tortoise and hare racing

finish
tortoise
Distance (m)
hare
start
t2
t3
t1
Time (sec)
hare rests
tortoise finishes
hare finishes
15
Speed from the graph
D distance
Speed
slope of graph!
D time
distance
D d
D t
16
Averages are never the whole story
Average velocity 0
17
Instantaneous vs average speed
distance
D d

• time

D t
18
Graphical analysis of motion
distance
speed
area distance
tortoise
time
t2
t3
t1
19
Complicated motion person hiking with a dog
negative slope
d
t
v
t
negative area Dd lt 0
negative velocity
20
Acceleration
Waiting at a traffic light that turns green
slope velocity
d
t
slope acceleration
v
area distance
t
a
area velocity
t
21
Pop quiz
Cars are strobe-photographed at 1 second
intervals
What is v for the car on top?
What is a for the car on top?
What is v for the car on the bottom?
What is a for the car on the bottom?
22
What is inertia?

• Aristotle got it all wrong (350BC)
• (natural state is NOT at rest)
• He messed up on friction dissipation
• Until Leonardo, Galileo (1800 years later)
• Galileos version still obtains today
• Objects AT REST remain so
• Objects IN MOTION remain so
• Only understood (sort of) by
• Einstein (1915)

23
You knew this already

• Skaters do it smoothly
• Straight lines at constant speed until pushed

24
Why is ice slippery?

• Always (entropy) have a thin liquid layer on top
• Entropy (Gabor Somorjai) says so even though your
  
• tongue sticks to the flagpole in winter

25
Fidos inertia technology

• And of course you know inertia too
• Slow down if the turn isnt bankedand DONT
  forget to buckle up!

26
Would you lift this for me?

• Weight is like inertia (aka mass)
• Weight equals (mass)?(gravity acceleration)
• Also written as W mg

On earth g 9.8 m/sec2 (in simple
estimates, use g 10 m/sec2) Everything falls
at the same rate (see, however, RG Are Dead)
27
Take home messages for today

• Units algebra can be a sanity check on answers
• Inertia tendency to maintain a state of motion
• Tendency to stay still
• Tendency to keep going
• Intimately related to weight
• Think about all the details or people will laugh
  at you later
• (just ask Aristotle)