UW-Madison PHYSICS 207 - Physics 207 – Lecture 9 (12 pages)

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Physics 207 – Lecture 9



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Physics 207 – Lecture 9

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Lecture Notes


Pages:
12
School:
University of Wisconsin, Madison
Course:
Physics 207 - General Physics
General Physics Documents

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Physics 207 Lecture 9 Lecture 9 l Goals v Describe Friction in Air Ch 6 not on 1st Exam v Differentiate between Newton s 1st 2nd and 3rd Laws v Use Newton s 3rd Law in problem solving 1st Exam Thurs Oct 6th from 7 15 8 45 PM Chapters 1 6 7 light direct applications of the third law Rooms 2103 302 303 306 309 310 313 2141 304 307 308 312 2223 311 Chamberlin Hall plus quiet room Physics 207 Lecture 9 Pg 1 Friction in a viscous medium Drag Force Quantified l l With a cross sectional area A in m2 coefficient of drag of 1 0 most objects sea level density of air and velocity v m s the drag force is D C A v2 c A v2 in Newtons c kg m3 In falling when D mg then at terminal velocity Example Bicycling at 10 m s 22 m p h with projected area of 0 5 m2 exerts a force of 30 Newtons v At low speeds air drag is proportional to v but at high speeds it is v2 v Minimizing drag is often important Physics 207 Lecture 9 Pg 2 Page 1 Physics 207 Lecture 9 Newton s Third Law If object 1 exerts a force on object 2 F2 1 then object 2 exerts an equal and opposite force on object 1 F1 2 F1 2 F2 1 For every action there is an equal and opposite reaction IMPORTANT Newton s 3rd law concerns force pairs which act on two different objects not on the same object Physics 207 Lecture 9 Pg 3 Force Pairs vs Free Body Diagrams Consider the following two cases a falling ball and ball on table Compare and contrast Free Body Diagram and Action Reaction Force Pair sketch Physics 207 Lecture 9 Pg 4 Page 2 Physics 207 Lecture 9 Forces just on a single body 1st and 2nd Laws only mg FB T N mg Ball Falls For Static Situation N mg Physics 207 Lecture 9 Pg 5 Force Pairs 3rd Law 1st and 2nd Laws Free body diagram Relates force to acceleration rd 3 Law Action reaction pairs Shows how forces act between objects FB E mg FB T N FT B N FE B mg FB E mg FE B mg Physics 207 Lecture 9 Pg 6 Page 3 Physics 207 Lecture 9 Example non contact Consider the forces on an object undergoing projectile motion FB E mB g FB E mB g FE B mB



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