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sy10_oct08_07hc

# sy10_oct08_07hc - Physics 207 Lecture 10 Physics 207...

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Page 1 Physics 207 – Lecture 10 Physics 207: Lecture 10, Pg 1 Physics 207, Physics 207, Lecture 10, Oct. 8 Agenda circle6 Exam I circle6 Newton’s Third Law circle6 Pulleys and tension revisited Assignment: Assignment: circle6 MP Problem Set 4A due Oct. 10,Wednesday, 11:59 PM MP Problem Set 4A due Oct. 10,Wednesday, 11:59 PM circle6 For Wednesday, read Chapter 9 circle6 MP Problem Set 5 (Chapters 8 & 9) available soon Physics 207: Lecture 10, Pg 2 Exam I results circle6 Exams should be returned in your next discussion section circle6 Regrades: Write down, on a separate sheet, what you want regraded and why. circle6 With only 110 scores tallied: Mean: 67.0 Median: 67 Std. Dev.: 14.5 Range: High 97 Low 25 Solution posted later today on http://my.wisc.edu Tentative (only 130 scores) 87-100 A 77- 86 A/B 67- 76 B 57- 66 B/C 40- 56 C 30- 39 D Below 30 F Physics 207: Lecture 10, Pg 3 Newton Newton’ s Laws Law 1 : An object subject to no external forces is at rest or moves with a constant velocity if viewed from an inertial reference frame. Law 2 : For any object, F NET = Σ F = m a a Law 3 : Forces occur in pairs: F A , B = - F B , A (For every action there is an equal and opposite reaction.) Read: Force of B on A Physics 207: Lecture 10, Pg 4 Newton Newton’ s Second Law The acceleration of an object is directly proportional to the net force acting upon it. The constant of proportionality is the mass. circle6 This expression is vector expression: F x , F y , F z circle6 Units The metric unit of force is kg m/s 2 = Newtons (N) The English unit of force is Pounds (lb) Physics 207: Lecture 10, Pg 5 Newton Newton’ s Third Law: If object 1 exerts a force on object 2 ( F 2,1 ) then object 2 exerts an equal and opposite force on object 1 ( F 1,2 ) F 1,2 = - F 2,1 IMPORTANT: Newton’s 3 rd law concerns force pairs which act on two different objects (not on the same object) ! For every “action” there is an equal and opposite “reaction” Physics 207: Lecture 10, Pg 6 Example (non Example (non- contact) F B,E = - m B g EARTH F E,B = m B g Consider the forces on an object undergoing projectile motion F B,E = - m B g F E,B = m B g

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Page 2 Physics 207 – Lecture 10 Physics 207: Lecture 10, Pg 7 Example Example 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 10, Pg 8 Example The Free Body Diagram m g m g F B,T = N Ball Falls For Static Situation N = m g Physics 207: Lecture 10, Pg 9 Normal Forces Normal Forces Certain forces act to keep an object in place. These have what ever force needed to balance all others (until a breaking point). F T,B F B,T Main goal at this point : Identify force pairs and apply Newton’s third law Physics 207: Lecture 10, Pg 10 Force Pairs Newton’s 3rd law concerns force pairs: Two members of a force pair cannot act on the same object.
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sy10_oct08_07hc - Physics 207 Lecture 10 Physics 207...

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