PHYSICS 221
Fall 2008
Dr. Anatoli Frishman
frishman@iastate.edu
INTRODUCTION
•What is physics?
-A science
-A basic science
-The most basic science
-Discovered by several generations of science
•Measurements
•Relationship between experiments and theory
•Mathematics
•What do we measure? -Physical quantities
-Units- a unit is a measure of the quantity that is defined to be
exactly 1 (one). Examples: meter, mile, gram, kilogram.
-Standard- a reference to which all the other examples of the
quantity are compared.
-Base quantities, and their standards.
~ How many base quantities do we need?
Physics
Mechanics Thermal properties
Electromagnetism
•Electrostatic
•Electric current
•Magnetism
Condensed Mater
High energy
Optics
Atoms & particles
•Geometrical optics
•Wave optics
Biophysics
Classical physics
Quantum physics
Relativistic physics
Quantum relativistic physics
The International System of Units
Quantity Unit name Unit symbol Standard
Length Meter
m
Distance traveled by light in
1/299,792,458 second
Time
Second
s
Time required for 9,192,631,770 periods
of radiation emitted by cesium atoms
Mass
Kilogram kg
Platinum-iridium cylinder in
International Bureau of Weights and
Measures at Sevres, near Paris
CGSE: centimeter, gram, second
British engineering system has force instead of mass as one of its basic
quantities, which are feet, pounds, and seconds.
1 m = 3.281 ft; 1inch = 2.54 cm
1 kg = 0.06585 slug (Not the same as weight!)
on Earth 1 kg weighs 2.205 lb, on the Moon 1 kg weighs 0.368 lb
Multiples of Units
10-24
yocto-
y
10-21
zepto-
z
10-18
atto-
a
10-15
femto-
f
10-12
pico-
p
10-9
nano-
n
10-6
micro-

10-3
milli-
m
10-2
centi-
c
103
kilo-
k
106
mega-
M
109
giga-
G
1012
tera-
T
1015
peta-
P
1018
exa-
E
1021
zetta-
Z
1024
yotta-
Y
Conversion of units:
Multiply by the appropriate representation
of 1 to cancel the unwanted units away
Converting between metric units, is easy,
as all it involves is powers of 10.
Example: Convert 3kg into gram
1000 g
3kg  3kg
 3000 g
1kg
Example: Convert 10 mph into m/s
10
mile
mile
1h
1609 m
 10



h
h
3600 s 1 mile
 4.47 m/s
Consistency of equations. Dimensional analysis
If A=B, then A and B must have the same combination of units
Example 1: (distance)=(speed)(time)
[m] = ([m]/[s]) [s]
Dimensions of a quantity are the base units that make it up; they are
generally written using square brackets.
Example 2 (dimensions of speed): [m/s], or [L/T]
(dimensions of acceleration ): [m/s2], or [L/T2]
Quantities that are being added or subtracted must have the same
dimensions.
Example 3: The period T of a swinging pendulum depends only
on the length of the pendulum d and the acceleration of gravity, g.
Which of the following formulas could be correct?
A. T  2 dg 
2
B. T  2 dg
C. T  2 d
g
d
Measurement and uncertainty
When we measure something, there’s a limited accuracy:
result  error (or accuracy, or uncertainty)
Example 1:
2.35  0.01
Example 2:
Imagine I write: 2.35  0.1 . Does this make sense?
Wrong: 2.35  0.1
Correct: 2.4  0.1
No!
Significant figures
Example 3:
0.24630
1sf
2sf
3sf
4sf
5sf
0.2
0.25
0.246
0.2463
0.24630
Example 4:
Not so clear in some cases: 200 (1,2,3 ?)
Scientific notation is crystal clear:
2  102 (or 0.2  103)
1sf
2.0  102 (or 0.20  103)
2sf
2.00  102 (or 0.200  103)
3sf
Order of magnitude (estimations)
VECTORS
•A vector has magnitude as well as direction
•Some vector quantities: displacement, velocity, force, momentum
•A scalar has only a magnitude
•Some scalar quantities: mass, time, temperature

a
Geometric presentation:
Notations:

a
- letter with arrow;
Magnitude (length of the vector):
Some properties:
 

A  B  C

A
a – bold font

a a

B

C
Vector addition (geometric)
Two vectors:

b

c
  
a b  c

a

d
Several vectors
   
a b c  d
Subtraction
  
a b  c

b

a

b

a

c

c

b

b

b

b

a

c

c

a
Example: Which of the following arrangements will produce the largest
resultant when the two vectors of the same magnitude are added?
A
B
C
Question
A person walks 3.0 mi north and then
4.0 miles west. The length and
direction of the net displacement of the
person are ___ and ___ .
1.
2.
3.
4.
25 mi and 45˚ north of east
5 mi and 37˚ north of west
5 mi and 37˚ west of north
7 mi and 77˚ south of west
Question
Consider the following three
vectors:
 
A B
What is the correct relationship between
the three vectors?
1. C  A  B
2. C  A  B
3. C  (A  B)
4. C  (A  B)