sy06_sept24_07hc

sy06_sept24_07hc - Page 1 Physics 207 Lecture 6 Physics...

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Unformatted text preview: 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: Agenda: Assignment: For Wednesday read Chapter 7 Assignment: For Wednesday read Chapter 7 c MP Problem Set 3 due Wednesday MP Problem Set 3 due Wednesday c MP Problem Set 4 available soon MP Problem Set 4 available soon c 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 c Chapter 5 Chapter 5 x Friction (a external force that opposes motion) Chapter 6 (Dynamics II) x Motion in two (or three dimensions) x Frames of reference Physics 207: Lecture 6, Pg 2 Friction... Friction... c Friction is caused by the microscopic interactions between the two surfaces: Physics 207: Lecture 6, Pg 3 A special contact force, friction A special contact force, friction c What does it do? x It opposes motion ! x Parallel to a surface x Perpendicular to a surface N ormal force c How do we characterize this in terms we have learned? x 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 j Physics 207: Lecture 6, Pg 4 Sliding Friction (i.e., with motion) Sliding Friction (i.e., with motion) c Direction: A force vector to the normal force vector N c Magnitude: | f K | is proportional to the magnitude of | N N | x | f K | = K | N N | ( = K | m g g | in the previous example) c The constant K is called the coefficient of kinetic friction c As the normal force varies so does the frictional force Physics 207: Lecture 6, Pg 5 Case study ... big F Case study ... big F c Dynamics: x-axis i : ma x = F- K N y-axis j : ma y = 0 = N mg or N = mg so F- K mg = m a x m a x F m g N i j j K mg v f k f k Physics 207: Lecture 6, Pg 6 Case study ... little F Case study ... little F c Dynamics: x-axis i : ma x = F- K N y-axis j : ma y = 0 = N mg or N = mg so F- K mg = m a x m a x F m g N i j j K mg v f k f k Page 2 Physics 207 Lecture 6 Physics 207: Lecture 6, Pg 7 Static Friction... Static Friction... F applied m g N i j j f S c So far we have considered friction acting when something has a non-zero velocity x We also know that it acts in fixed or static systems: c In these cases, the force provided by friction depends on the forces applied on the system (magnitude: f s s N) c Opposes motion that would occur if s were zero Physics 207: Lecture 6, Pg 8 Static Friction... Static Friction... c Just like in the sliding case except a = 0 . i : F applied- f S = j : N = mg F applied applied m g N i j j f S c While the block is static: f S = F applied (unlike kinetic friction) Physics 207: Lecture 6, Pg 9 Static Friction......
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This note was uploaded on 10/30/2011 for the course PHYS 207 taught by Professor Winnokur during the Spring '06 term at Wisconsin.

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sy06_sept24_07hc - Page 1 Physics 207 Lecture 6 Physics...

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