# lecture02 - Physics 101 Lecture 02 Forces Equilibrium...

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Physics 101: Lecture 2, Pg 1 Forces: Equilibrium Examples Physics 101: Lecture 02 Today’s lecture will cover Textbook Sections 2.1-2.7 Phys 101 URL: http://online.physics.uiuc.edu/courses/phys101 Read the course description & FAQ! Office hours start next Monday; see web page for locations & times.

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Physics 101: Lecture 2, Pg 2 Overview Last Lecture Newton’s Laws of Motion » Inertia » S F=ma » Pairs Free Body Diagrams » Draw coordinate axis, each direction is independent. » Simple Picture » Identify/draw all forces Friction: kinetic f = m k N ; static f m s N Gravity W = m g (near Earth’s surface!) Today Contact Force---Springs Contact Force---Tension 2-D Examples
Physics 101: Lecture 2, Pg 3 Free Body Diagrams Choose Object (book) Label coordinate axis Identify All Forces Hand (to right) Gravity (down) Normal (table, up) Friction (table, left) Physics hand Gravity Normal friction y x

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Physics 101: Lecture 2, Pg 4 Book Pushed Across Table Calculate force of hand to keep the book sliding at a constant speed , if the mass of the book is 1 Kg, m s = .84 and m k =.75. Physics hand Gravity Normal friction y x x-direction: S F=0 F hand -F friction = 0 F hand =F friction F hand = m k F Normal y-direction: S F=0 F Normal -F Gravity = 0 F Normal = F Gravity F Normal = 1 x 9.8=9.8 N Combine: F hand = m k F Normal F hand =0.75 x 9.8 N F hand = 7.3 newtons Constant Speed => S F=0
Physics 101: Lecture 2, Pg 5 Contact Force: Springs Force exerted by a spring is directly proportional to its displacement x (stretched or compressed).

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## This note was uploaded on 02/05/2012 for the course PHYS 101 taught by Professor Ha during the Spring '11 term at University of Illinois at Urbana–Champaign.

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lecture02 - Physics 101 Lecture 02 Forces Equilibrium...

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