sy10_oct08_07hc

sy10_oct08_07hc - Physics 207 Lecture 10 Oct 8 Physics...

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Physics 207: Lecture 10, Pg 1 Physics 207, Physics 207, Lecture 10, Oct. 8 Lecture 10, Oct. 8 Agenda Exam I Newton’s Third Law Pulleys and tension revisited Assignment: Assignment: MP Problem Set 4A due Oct. 10,Wednesday, 11:59 PM MP Problem Set 4A due Oct. 10,Wednesday, 11:59 PM For Wednesday, read Chapter 9 For Wednesday, read Chapter 9 MP Problem Set 5 (Chapters 8 & 9) available soon

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Physics 207: Lecture 10, Pg 2 Exam I results Exam I results Exams should be returned in your next discussion section Regrades: Write down, on a separate sheet, what you want regraded and why. 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’s Laws 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 F NET = Σ F F = m a a Law 3 : Forces occur in pairs: F F A , B = - F F B , A (For every action there is an equal and opposite reaction.) Read: Force of B on A

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Physics 207: Lecture 10, Pg 4 Newton’s Second Law 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. This expression is vector expression: F x , F y , F z
Physics 207: Lecture 10, Pg 5 Newton’s Third Law: 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”

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Physics 207: Lecture 10, Pg 6 Example (non-contact) 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
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

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Physics 207: Lecture 10, Pg 8 Example 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

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Physics 207: Lecture 10, Pg 10 Force Pairs Force Pairs Newton’s 3rd law concerns force pairs : Two members of a force pair cannot act on the same object. Don’t mix gravitational (a non-contact force of the Earth on an
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sy10_oct08_07hc - Physics 207 Lecture 10 Oct 8 Physics...

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