Welcome to Physics 101!
Lecture 01: Introduction to Forces
“I must take this course to fulfill my Pre-med prereqs, and everyone
I know has opted to take this course outside of the U of I. ”
“I am not happy; and from the way the prelecture
was going, I don't have very high hopes for this
semester.”
“so I'm thinking that this class won't be as terrible as I was
thinking it might be.”
My biggest fear is that, while braving the steep stairs of
the Loomis lecture hall, gravity will get the better of me
and I will tumble 30 feet, landing in an embarrassed heap.
Tim Stelzer
http://online.physics.uiuc.edu/courses/phys101/spring07/index.html
Physics 101: Lecture 27, Pg 1
05
Meet the Lecturer
 Tim
Stelzer
–
tstelzer@uiuc.edu
Subject: Physics 101
 Office
Hours Monday 3-4
 Research
High Energy Particle Theory
Physics Education
 P101-102
at UIUC are my
favorite!
•http://online.physics.uiuc.edu/courses/phys101/Fall06/index.html
Physics 101: Lecture 27, Pg 2 05
06
Course Format
(Spiral Learning)
Lecture Preflights
 Lecture Participation
 Homework
 Lab
 Discussion
25
25
100
150
100

clickers
quizzes; drop lowest 1
Hour Exams (3 x 100)
 Final Exam
300
300

“I'm excited that people have told me this class
won't be extremely difficult, and my biggest fear is
that they are all dirty rotten liars. :)”
1000
Physics 101: Lecture 27, Pg 3
07
Grading Scale
950-1000
 920-949
 900-919
 880-899
 860-879
 835-859

A+
A
AB+
B
B-
810-834
 780-809
 750-779
 720-749
 690-719
 610-689
 <610

C+
C
CD+
D
DF
I'm worried because it is a five hour
course and can so greatly affect your GPA.
Physics 101: Lecture 27, Pg 4
8
Reading & Lecture Preflight
I am concerned because I did not check and receive my e-mail until
late last night and just finished the pre-lecture and tried to do the
preflight and it was closed buy I did not know it had to be done by
6am this morning otherwise I would have done it earlier. Is there
anything I can go to get those points because I do not want to start
the course out by missing points?
 Need
to complete PreLecture for Preflight
 Answer preflights 25/1000 points
Due 6:00 am day of lecture.
1 points for honest attempt at preflight.
No EX, 28 Lectures can miss three and still get all 25
points.
 Everyone
gets 1 points for today!
Physics 101: Lecture 27, Pg 5
11
P101 Lectures

Participation is key!
“However, I am very excited to meet
new people and explore new concepts”.
Come to lecture prepared!
1 point for each lecture using iclicker
» No EX, 28 Lectures can miss three and still get all 25 points.
» Available at bookstore---register in gradebook

Not everything you need for exams!
Concepts, Connections, Motivation
Comprehensive Text
Calculations
Homework + Discussion
Hands-On Lab

Taking Notes
Lecture notes available for lecture.
Some key pieces for you to fill in.
Physics 101: Lecture 27, Pg 6
13
P101 Homework
 All
web based, immediate feedback
 100% if done before 6:00 am deadline
 90% credit until following Tuesday
 0% after that
 Always keep 5 significant figures!
 First one is due Tuesday Jan 30th !
 Second one is due Tuesday Jan 30th !
Physics 101: Lecture 27, Pg 7
15
P101 Labs
“I am most excited for labs and working with
experiments.”
 Director:
Professor Alexey Bezryadin
 First
is Monday!
 No “dropped” labs….. Don’t miss one!
Physics 101: Lecture 01, Pg 8
18
Discussion Sections
“I went to discussion today and was completely lost and then going
through the prelecture i was also completely lost. ”
 Director:
Professor Peter Abbamonte
 Start: Tuesday
 Quiz during last 20 minutes of section
 10 minutes late 
Physics 101: Lecture 27, Pg 9
21
Physics Philosophy
“i fear that since i have heard from others
that it is an easy class, i might become
complacent.”

Problem: You are too smart!

Physics is DIFFERENT

Describe large number of “complicated”
observations with a few simple ideas

Exams don’t have same problems, but do have
same IDEAS
Physics 101: Lecture 27, Pg 10 23
Newton’s Laws of Motion
1.
If the sum of all external forces on an
object is zero, then its speed and direction
will not change. Inertia
2.
If a nonzero net force is applied to an
object its motion will change F= ma
3.
In an interaction between two objects, the
forces that each exerts on the other are
equal in magnitude and opposite in
direction.
Physics 101: Lecture 27, Pg 11
30
Forces in P101
 Non-Contact
---- Gravity (F = G m M /r2 )
G = 6.7x10-11 m3 / (kg s2)
Earth: Mass = 6x1024 kg, radius = 6.4 x106 m.
 Contact
(fundamentally E+M)
Normal: Perpendicular to surface
Friction: Parallel to surface
Anything touching the object
» Rope: Tension
» Spring F = -kx
Physics 101: Lecture 27, Pg 12 31
Example Weight of Object
 Calculate
the gravitational force on a 3 kg book
held 1 meter above the surface of the earth.
F = G M m / r2
= (6.7x10-11 m3 / (kg s2)) (6x1024 kg) (3 kg)/ (6.4x106 + 1)2 m2
= 29.4 kg m/s2
Physics 101: Lecture 27, Pg 13 33
Gravitational ACT

A)
If the book is raised 10 meters above the
surface of the earth, the gravitational force
on the book will
100 times stronger
C) Same
D) 10 times weaker
B) 10 times stronger
E) 100 times weaker
F = G M m / r2
= (6.7x10-11 m3 / (kg s2)) (6x1024 kg) (3 kg)/ (6.4x106 + 10)2 m2
= 29.4 kg m/s2
Near surface of earth r = 6.4x106 m
F = m (G M /r2) = m (9.8 m/s2)
Physics 101: Lecture 27, Pg 14 37
Contact Forces: Friction
 Magnitude
of frictional force (parallel to
surfaces) is proportional to the normal force.
fkinetic = mk N
mk coefficient of Kinetic friction
fstatic <= ms N
ms coefficient of Static friction
 Be
Careful!
Static friction <=, can be any value up to msN
Direction always opposes motion
Physics 101: Lecture 27, Pg 15 42
Free Body Diagrams
 Choose
Object (book)
 Label coordinate axis
 Identify All Forces
Hand (to right)
Gravity (down)
Normal (table, up)
Friction (table, left)
Physics
y
x
Physics 101: Lecture 27, Pg 16 48
Summary

Newton’s Laws of Motion
Inertia
F=ma
Pairs

Forces:
Non-Contact: Gravity
Contact: Friction and Normal

Free Body Diagrams
Each direction is independent

Friction opposes motion, parallel to surface
Kinetic f = mk N
Static f <= ms N
Physics 101: Lecture 27, Pg 17
To Do

Discussions and Labs start next week!

Do lecture preflight before 6:00 am
Monday

Have a GREAT WEEKEND!
50
Physics 101: Lecture 27, Pg 18