Physics 201
General Physics
Prof. Susan Coppersmith
Prof. Albrecht Karle
Course information
•  Course homepage
•   Let’s have a quick look:
•  http://www.physics.wisc.edu/undergrads/courses/
fall09/201/
–  Find there detailed information on syllabus, homework,
exams, grading, discussion, labs
•  Syllabus:
–
–
–
–
Lectures: Typically 1 Chapter per week from textbook
Two discussion sessions
One Lab
One homework (Always due on Thursdays)
Textbook, e-book and WebAssign
•  The textbook is available in electronic form as an
e-book.
–  Paul Tipler and Gene Mosca, Physics for Scientists and
Engineers, 6th ed.
–  You can read it from any computer with access to
internet.
http://webassign.net/login.html.
–  This is by far the cheapest solution. If you like to buy it in real
paper, it is also available as softcover in 2 volumes (this course covers
the 1st volume)
•  WebAssign:
–  This is our homework assignment system. Problems are taken from the
textbook but numbers are randomized.
–   Let’s have a quick look into WebAssign:
–  Intro to WebAssign
–  Student Guide to WebAssign
©2008 by W.H. Freeman and Company
Discussions, Labs
TA’s
Our team of Teaching Assistants will be your instructors in
discussions and labs:
Sections
Your TA
-------------------------------•  301 302
Eunsong Choi
•  303 309
Jialu Yu
•  304 310
Jared Schmitthenner
•  305 311
Andrew Long
•  306 307
Daniel Schroeder
Office hours
Monday
4:20pm – 5:10pm
Tuesday
10:45am – 11:45am
11am – 11:50am
1:20pm – 2:10pm
2:25pm – 3:15pm
Wednesday
11am – 11:50am
4:20pm – 5:10pm
Thursday
11am – 11:50am
12:05pm – 12:55pm
1:20pm – 2:10pm
2:25pm – 3:15pm
2:30pm – 3:30pm
(best by appointment)
Jared
Prof. Coppersmith
Dan
Jialu
Andrew
Eunsong
Jared
Dan Eunsong
Jialu
Andrew
Prof. Karle
Nature of Science Theory and observation
Theories are made to explain observations.
Theories will make predictions, (so that they are
testable).
Observations and experiments are used to test if the
prediction is accurate.
The cycle continues.
In history, physics and astronomy, have set the ground
rules of modern science.
©2008 by W.H. Freeman and Company
Example: Determination of the Earth diameter by
Eratosthenes (276 BC– 195 BC)
•  Eratosthenes wanted to determine
the diameter of the Earth. (Yes,
the standard model at the time
was that the Earth was round –
that was not the question.)
•  He observed the angle of the sun
at the same time in Alexandria and
some 800km South of Alexandria
(Syene= Aswan)
•  From the difference, he was able
in inclination, records indicate
that he was able to determine the
Earth’s diameter to within 2%
precision.
•  An example of great science!
Example: Determination of the Earth diameter by
Eratosthenes (276 BC– 195 BC)
•  Eratosthenes wanted to determine
the diameter of the Earth. (Yes,
the standard model at the time
was that the Earth was round –
that was not the question)
•  He observed the angle of the sun
at the same time in Alexandria and
some 800km South of Alexandria
(Syene= Aswan)
•  From the difference, he was able
in inclination, records indicate
that he was able to determine the
Earth’s diameter to within 2%
precision.
•  An example of great science!
Units
•  Physical quantities have units!
•  Example: Unit of length
–  Eratosthenes used the unit stadion. (The hellenic stadion was
pretty big: 185m)
–  In the Middle ages many kingdoms had different definitions of
a foot, etc.
•  Today, the scientific community uses the SI system of
units. There are 7 basic units, such as
–  Length: Meter (Based on the speed of light: length of path
traveled by light in 1/299,792,458s)
–  Mass: kg (Platinum cylinder in International Bureau of Weights
and Measures, Paris)
–  Time: s (Time required for 9,192,631,770 periods of
radiation emitted by cesium atoms.)
SI Units
SI Base quantities
Length
meter
m
Time
second
s
Mass
kilogram
kg
Electric Current
ampere
A
Temperature
kelvin
K
Amount of substance
mole
mol
Luminous intensity
candela
cd
©2008 by W.H. Freeman and Company
Prefixes
•  Depending on the
scale one often likes
to use prefixes.
•  Example, for length it
is convenient to use
km = 1000m when
traveling by car, or
nm=10^-9m when
discussing molecular
scale objects.
Conversions
•  Conversions between units are very helpful. The use of different
units has again and again lead to errors, sometimes with bad
consequences.
•  The conversion of units is also a frequent source of errors
engineering and science (and exams).
•  But it is easy to avoid.
•  Avoid skipping the units (for example because it is less writing
time)
–  Are all the ingredients for a problem in the same units? If not, it is good
practice to perform the conversion, before doing any algebra.
–  Basic SI units are always safe
–  It is OK to us km or nm, but need to take care that you don’t mix m and km
–  Develop good practice.
•  We will expect that you give results with units, also in exams.
Derived quantities and dimensions
m2
m3
m/s
m/s2
N=kg•m/s2
N/m2=kg/m•s2
kg/m3
…
Measurement and Significant figures
•  A measurement has a precision (or error).
•  Measurement of the distance Earth – moon with laser pulse based
on travel time of light. Error: a few cm! (position of the mirror)
•  What is the relative error?
The Greek astronomer Hipparch, ~200BC determined the distance
of the moon to about ~70 Earth diameters, 5% precision, not too
bad.
Measurement and Significant figures
•  A measurement has a precision (or error).
•  Significant figures reflect the precision of the measurement.
Example:
•   Pocket calculator
•   whiteboard,
•   WebAssign intro
Measurement and Significant figures
•  Calculators will not give you the right
number of significant figures; they
usually give too many but sometimes
give too few (especially if there are
trailing zeroes after a decimal point).
•  The top calculator shows the result
of 2.0 / 3.0.
•  The bottom calculator shows the
result of 2.5 x 3.2.
The universe by orders of
magnitude
©2008 by W.H. Freeman and Company
Order of magnitude: Rapid Estimating
A quick way to estimate a calculated
quantity is to round off all numbers to
one significant figure and then
calculate. Your result should at least be
the right order of magnitude; this can
be expressed by rounding it off to the
nearest power of 10.
Such back on the envelope estimates
are very helpful for double checking a
result of a calculation.
Diagrams are also very useful in making
estimations.
Tire treads
©2008 by W.H. Freeman and Company
©2008 by W.H. Freeman and Company
©2008 by W.H. Freeman and Company
©2008 by W.H. Freeman and Company
How many grains of sand on a beach?
©2008 by W.H. Freeman and Company
Phys
201
Fall
2009
Tuesday,
September
8,
2009
Chapter
1:
Measurement
and
vectors
Review
from
last
Dme:
converDng
units
•  Units
in
every
equaDon
have
to
match!
It
is
a
very
good
idea
to
keep
units
as
well
as
numbers
when
solving
equaDons.
The
density
of
seawater
was
measured
to
be
1.07
g/cm3.
This
density
in
SI
units
is
A.
B.
C.
D.
E.
1.07
kg/m3
(1/1.07)
×
103
kg/m3
1.07
×
103
kg
1.07
×
10–3
kg
1.07
×
103
kg/m3
The
density
of
seawater
was
measured
to
be
1.07
g/cm3.
This
density
in
SI
units
is
A.  1.07
kg/m3
B.  (1/1.07)
×
103
kg/m3
C.  1.07
×
103
kg
D.  1.07
×
10–3
kg
E.  1.07
×
103
kg/m3
If
K
has
dimensions
ML2/T2,
the
k
in
K
=
kmv2
must
A.
B.
C.
D.
E.
have
the
dimensions
ML/T2.
have
the
dimension
M.
have
the
dimensions
L/T2.
have
the
dimensions
L2/T2.
be
dimensionless.
If
K
has
dimensions
ML2/T2,
the
k
in
K
=
kmv2
must
A.  have
the
dimensions
ML/T2.
B.  have
the
dimension
M.
C.  have
the
dimensions
L/T2.
D.  have
the
dimensions
L2/T2.
E.  be
dimensionless.
Vectors
•  In
one
dimension,
we
can
specify
distance
with
a
real
number,
including
+
or
–
sign.
•  In
two
or
three
dimensions,
we
need
more
than
one
number
to
specify
how
points
in
space
are
separated
–
need
magnitude
and
direcDon.
Madison, WI and
Kalamazoo, MI
are each about
150 miles from
Chicago.
DenoDng
vectors
•  Two
of
the
ways
to
denote
vectors:
– Boldface
notaDon:
A

– “Arrow”
notaDon:
A
€
Displacement
is
a
vector
©2008
by W.H. Freeman and Company
Adding
displacement
vectors
©2008
by W.H. Freeman and
Company
“Head‐to‐tail”
method
for
adding
vectors
©2008
by W.H. Freeman and
Company
Vector
addiDon
is
commutaDve
©2008
by W.H. Freeman and
Company
Adding
three
vectors:
vector
addiDon
is
associaDve.
©2008
by W.H. Freeman and
Company
A
vector’s
inverse
has
the
same
magnitude
and
opposite
direcDon.
©2008
by W.H. Freeman and Company
SubtracDng
vectors
©2008
by W.H. Freeman and
Company
Example
1‐8.
What
is
your
displacement
if
you
walk
3.00
km
due
east
and
4.00
km
due
north?
©2008
by W.H. Freeman and
Company
Components
of
a
vector
©2008
by W.H. Freeman and
Company
Components
of
a
vector
along
x
and
y
©2008
by W.H. Freeman and
Company
Magnitude
and
direcDon
of
a
vector
©2008
by W.H. Freeman and
Company
Adding
vectors
using
components
©2008
by W.H. Freeman and
Company
Cx = Ax + Bx
Cy = Ay + By
Unit
vectors
A unit vector is a
dimensionless vector
with magnitude
exactly equal to one.
©2008
by W.H. Freeman and Company
The unit vector
along x is denoted
The unit vector
along y is denoted
The unit vector
along z is denoted
Which
of
the
following
vector
equaDons
correctly
describes
the
relaDonship
among
the
vectors
shown
in
the
figure?
A.
B.
C.
D.
  
A+ B −C =0
  
A− B +C =0
  
A− B −C =0
  
A+ B +C =0
E. None of these is correct.
Which
of
the
following
vector
equaDons
correctly
describes
the
relaDonship
among
the
vectors
shown
in
the
figure?
A.
B.
C.
D.
  
A+ B −C =0
  
A− B +C =0
  
A− B −C =0
  
A+ B +C =0
E. None of these is correct.
Can
a
vector
have
a
component
bigger
than
its
magnitude?
Yes
No
Can
a
vector
have
a
component
bigger
than
its
magnitude?
• Yes
• No
The square of a magnitude of a vector R is given in terms
of its components by
R2 = Rx2 + Ry2 .
Since the square is always positive, no component can be
larger than the magnitude of the vector.
ProperDes
of
vectors:
summary
©2008
by W.H. Freeman and
Company