Physics 101: Lecture 27, Pg 1 Welcome to Physics 101! Lecture 01: Introduction to Forces l Forces l Kinematics l Energy/Momentum l Rotations l Fluids l Waves/Sound l Thermodynamics

Physics 101: Lecture 27, Pg 2 Meet the Lecturer l – Subject: Physics 101 l Office Hours __________________ l Physics 101 URL: courses/phys101/fall07

Physics 101: Lecture 27, Pg 3 Course Format (Spiral Learning) l Lecture Preflights 50 l Homework100 l Lab 150 l Discussion 100 è quizzes; drop lowest 1 l Hour Exams (3 x 100)300 l Final Exam

Physics 101: Lecture 27, Pg 4 Grading Scale l A+ l A l A- l B+ l B l B- l C+ l C l C- l D+ l D l D- l <610 F 8

Physics 101: Lecture 27, Pg 5 Reading & Lecture Preflight l Need to complete PreLecture for Preflight l Answer preflights 50/1000 points è Due 6:00 am day of lecture. è 1 points for honest attempt at preflight. è Everyone gets 2 points for today! 11

Physics 101: Lecture 27, Pg 6 P101 Lectures l Participation is key! è Come to lecture prepared! l Not everything you need for exams! è Concepts, Connections, Motivation è Comprehensive Text è Calculations Homework + Discussion è Hands-OnLab l Taking Notes è Lecture notes available for lecture. è Some key pieces for you to fill in. 13

Physics 101: Lecture 27, Pg 7 P101 Homework l All web based, immediate feedback l 100% if done before 6:00 am deadline l 90% credit until following Tuesday l 0% after that l To be safe, keep 5 significant figures! l First one is due __________. l Second one also due _________ ! 15

Physics 101: Lecture 01, Pg 8 P101 Labs l Director: ____________ l First lab is ____________ ! l No “dropped” labs….. Don’t miss one! 18

Physics 101: Lecture 27, Pg 9 Discussion Sections l Director: ______________ l Start: Tuesday Aug 28 ! l Quiz during last 20 minutes of section l 10 minutes late:  l Drop lowest quiz 21

Physics 101: Lecture 27, Pg 10 Physics Philosophy l Problem: You are too smart! l Physics is DIFFERENT l Describe large number of “complicated” observations with a few simple ideas l Exams don’t have same problems, but do have same IDEAS 23

Physics 101: Lecture 27, Pg 11 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. 30

Physics 101: Lecture 27, Pg 12 Forces in P101 l Non-Contact ---- Gravity (F = G m M /r 2 ) è G = 6.7x m 3 / (kg s 2 ) è Earth: Mass = 6x10 24 kg, radius = 6.4 x10 6 m. l Contact (fundamentally E+M) è Normal: Perpendicular to surface è Friction: Parallel to surface è Anything touching the object »Rope: Tension »Spring F = -kx 31

Physics 101: Lecture 27, Pg 13 Example Weight of Object l Calculate the gravitational force on a 3 kg book held 1 meter above the surface of the earth. F = G M m / r 2 = 33

Physics 101: Lecture 27, Pg 14 Gravitational ACT l If the book is raised 10 meters above the surface of the earth, the gravitational force on the book will A) 100 times stronger B) 10 times stronger C) Same D) 10 times weaker E) 100 times weaker 37

Physics 101: Lecture 27, Pg 15

Physics 101: Lecture 27, Pg 16 Contact Forces: Friction l Magnitude of frictional force (parallel to surfaces) is proportional to the normal force.  f kinetic =  k N  k coefficient of Kinetic friction  f static <=  s N  s coefficient of Static friction l Be Careful!  Static friction <=, can be any value up to  s N è Direction always opposes motion 42

Physics 101: Lecture 27, Pg 17 Free Body Diagrams l Choose Object (book) l Label coordinate axis l Identify All Forces è Hand (to right) è Gravity (down) è Normal (table, up) è Friction (table, left) Physics y x 48

Physics 101: Lecture 27, Pg 18Summary l Newton’s Laws of Motion è Inertia è F=ma è Pairs l Forces: è Non-Contact: Gravity è Contact: Friction and Normal l Free Body Diagrams è Each direction is independent l Friction opposes motion, parallel to surface  Kinetic f =  k N  Static f <=  s N