sy15_oct24_07hc - Physics 207, Lecture 15, Oct. 24 Physics...

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Physics 207: Lecture 15, Pg 1 Physics 207, Physics 207, Lecture 15, Oct. 24 Lecture 15, Oct. 24 Agenda: Chapter 11, Finish, Chapter 13, Just Start Agenda: Chapter 11, Finish, Chapter 13, Just Start Assignment: For Monday read Chapter 13 carefully (you may Assignment: For Monday read Chapter 13 carefully (you may skip the parallel axis theorem and vector cross products) skip the parallel axis theorem and vector cross products) MP Homework 7, Ch. 11, 5 problems, available today, MP Homework 7, Ch. 11, 5 problems, available today, Due Wednesday at 4 PM Due Wednesday at 4 PM MP Homework 6, Due tonight MP Homework 6, Due tonight Chapter 11: Variable forces Conservative vs. Non-conservative forces Power Start Chapter 13 Rotation Torque
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Physics 207: Lecture 15, Pg 2 Lecture 15, Lecture 15, Exercise 1 Exercise 1 Work in the presence of friction and non-contact forces Work in the presence of friction and non-contact forces A. 2 B. 3 C. 4 A box is pulled up a rough ( μ > 0 ) incline by a rope-pulley- weight arrangement as shown below. How many forces are doing work on the box ? Of these which are positive and which are negative ? Use a Force Body Diagram Compare force and path v
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Physics 207: Lecture 15, Pg 3 Lecture 15, Lecture 15, Exercise 1 Exercise 1 Work in the presence of friction and non-contact forces Work in the presence of friction and non-contact forces A box is pulled up a rough ( μ > 0 ) incline by a rope-pulley-weight arrangement as shown below. How many forces are doing work on the box ? And which are positive and which are negative? Use a Force Body Diagram (A) (A) 2 (B) (B) 3 is correct (C) (C) 4 v f mg N T
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Physics 207: Lecture 15, Pg 4 Work and Varying Forces (1D) Work and Varying Forces (1D) Consider a varying force F(x) F x x x Area = F x x F is increasing Here W = F F · r r becomes dW = F F dx F θ = 0° Start Finish Work is a scalar, the rub is that there is no time/position info on hand = f i x x dx x F W ) ( F x
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Physics 207: Lecture 15, Pg 5 x x v o m t o F Example: Work Kinetic-Energy Theorem How much will the spring compress (i.e. x x ) to bring the object to a stop (i.e., v = 0 ) if the object is moving initially at a constant velocity (v o ) on frictionless surface as shown below ? spring compressed spring at an equilibrium position V=0 t m Notice that the spring force is opposite to the displacement. For the mass m, work is negative For the spring , work is positive
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Physics 207: Lecture 15, Pg 6 Example: Work Kinetic-Energy Theorem How much will the spring compress (i.e. x x = x = x f f - x - x i i ) to bring the object to a stop (i.e., v = 0 ) if the object is moving initially at a constant velocity (v o ) on frictionless surface as shown below ? x
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sy15_oct24_07hc - Physics 207, Lecture 15, Oct. 24 Physics...

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