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sy06_sept24_07hc

sy06_sept24_07hc - Physics 207 Lecture 6 Physics 207...

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Page 1 Physics 207 – Lecture 6 Physics 207: Lecture 6, Pg 1 Physics 207, Physics 207, Lecture 6, Sept. 24 Lecture 6, Sept. 24 Agenda: Assignment: For Wednesday read Chapter 7 Assignment: For Wednesday read Chapter 7 circle6 MP Problem Set 3 due Wednesday MP Problem Set 3 due Wednesday circle6 MP Problem Set 4 available soon MP Problem Set 4 available soon circle6 MidTerm MidTerm Thursday, Oct. 4, Chapters 1 Thursday, Oct. 4, Chapters 1- 7, 90 minutes, 7:15 7, 90 minutes, 7:15- 8:45 PM 8:45 PM circle6 Chapter 5 Chapter 5 xrhombus Friction (a external force that opposes motion) Chapter 6 (Dynamics II) xrhombus Motion in two (or three dimensions) xrhombus Frames of reference Physics 207: Lecture 6, Pg 2 Friction... circle6 Friction is caused by the “microscopic” interactions between the two surfaces: Physics 207: Lecture 6, Pg 3 A special contact force, friction circle6 What does it do? xrhombus It opposes motion ! xrhombus Parallel to a surface xrhombus Perpendicular to a surface Normal force circle6 How do we characterize this in terms we have learned? xrhombus A resulting force in a direction opposite to the direction of motion (actual or implied )! m a F APPLIED f FRICTION m g N i j Physics 207: Lecture 6, Pg 4 Sliding Friction (i.e., with motion) circle6 Direction: A force vector to the normal force vector N circle6 Magnitude: | f K | is proportional to the magnitude of | N | xrhombus | f K | = μ K | N N | ( = μ K | m g | in the previous example) circle6 The constant μ K is called the “coefficient of kinetic friction” circle6 As the normal force varies so does the frictional force Physics 207: Lecture 6, Pg 5 Case study ... big F Case study ... big F circle6 Dynamics: x-axis i : ma x = F - μ K N y-axis j : ma y = 0 = N – mg or N = mg so F - μ K mg = m a x m a x F m g N i j μ K mg v f k f k Physics 207: Lecture 6, Pg 6 Case study ... little F Case study ... little F circle6 Dynamics: x-axis i : ma x = F - μ K N y-axis j : ma y = 0 = N – mg or N = mg so F - μ K mg = m a x m a x F m g N i j j μ K mg v f k f k

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Page 2 Physics 207 – Lecture 6 Physics 207: Lecture 6, Pg 7 Static Friction... F applied m g N i j j f S circle6 So far we have considered friction acting when something has a non-zero velocity xrhombus We also know that it acts in fixed or “static” systems: circle6 In these cases, the force provided by friction depends on the forces applied on the system (magnitude: f s μ s N) circle6 Opposes motion that would occur if μ s were zero Physics 207: Lecture 6, Pg 8 Static Friction... circle6 Just like in the sliding case except a = 0 . i : F applied - f S = 0 j : N = mg F applied m g N i j j f S circle6 While the block is static: f S = F applied (unlike kinetic friction) Physics 207: Lecture 6, Pg 9 Static Friction... F m g N i j f S circle6 The maximum possible force that the friction between two objects can provide is f MAX = μ S N , where μ s is the “coefficient of static friction”.
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sy06_sept24_07hc - Physics 207 Lecture 6 Physics 207...

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