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Lect09 - Physics 211 Lecture 9 Today's Concepts Energy and...

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Unformatted text preview: Physics 211 Lecture 9 Today's Concepts: Energy and Friction Potential energy & force Physics 211 Lecture 9, Slide 1 Physics some comments about the course Can I get extra credit for saying it's awesome? Seriously, though, it's a great class to be in and the preflights/prelectures coupled with the in-class coupled examples make it an enjoyable experience. would like to see more examples on how to do different types of problems. I enjoy the course. I really wish there was a coursebook. It would make studying easier rather than going back and skipping around in the prelectures. Mainly, this course is a tad confusing for me because I never took physics in high school. Despite this, I feel I am catching up alright and understand the material pretty well. I just wanted you to know that there ARE some (or maybe I'm the only one) students who are taking this class for the first time. :) I keep saying in all my responses to "What topics would you like us to discuss us in lecture?" that I would like to learn about the calculus aspects of the topic we are on. One of these days I'm going to write something in here that is so interesting that it will get posted up on the PowerPoint in class... that's my goal. Oh and also to get an A... but getting on the PowerPoint is more important. Whoever wanted to throw the pudding/pizza party, can you tell me what what time and where you'll do it? It is sort of hard for me to go there if I don't Physics 211 Lecture 9, Slide Physics know where it is. Love you lots! 2 Physics 211 Lecture 9, Slide 3 Physics Macroscopic Work: This is not a new idea – it’s the same “work” you are used to. it you b W = ∫ F ⋅ dl Applied to big (i.e. macroscopic) objects a We call it “macroscopic” to distinguish it from “microscopic”. to You will deal with this in Physics 213 Physics 211 Lecture 9, Slide 4 Physics Physics 211 Lecture 9, Slide 5 Physics Physics 211 Lecture 9, Slide 6 Physics f f Physics 211 Lecture 9, Slide 7 Physics “Heat” is just the kinetic energy of the atoms ! is Do Stupid Spinning Heat Demo Physics 211 Lecture 9, Slide 8 Physics ΔK = Wtot = Wgravity + W friction 0 = Wgravity + W friction 0 = mgH + W friction must be negative N1 m H N2 mg μmg mg Physics 211 Lecture 9, Slide 9 Physics Preflight A block of mass m, initially held at rest on a frictionless ramp a vertical distance H above the floor, slides down the ramp and onto a floor where friction causes it to stop a distance D from the bottom of the ramp. The coefficient of kinetic friction between the box and the floor is μk. What is the macroscopic work done on the block by friction during this process? A) mgH B) –mgH C) μkmgD D) 0 m H D Physics 211 Lecture 9, Slide 10 Physics … What is the macroscopic work done on the block by friction during this process? B) –mgH Gravity invests mgH Joules of energy as the box goes down the ramp, and so Joules the box has mgH Joules of kinetic energy at the bottom. Therefore, the friction Joules force must do -mgH Joules of work on the box to bring it to a stop. force Joules since the block slows down, the work done by friction must be negative. B is the only negative choice; C would also be correct if it were negated. m Great answers! H D Physics 211 Lecture 9, Slide 11 Physics Preflight A block of mass m, initially held at rest on a frictionless ramp a vertical distance H above the floor, slides down the ramp and onto a floor where friction causes it to stop a distance D from the bottom of the ramp. The coefficient of kinetic friction between the box and the floor is μk. What is the total macroscopic work done on the block by all forces during this process? A) mgH B) –mgH C) μkmgD D) 0 m H D Physics 211 Lecture 9, Slide 12 Physics … What is the total macroscopic work done on the block by all forces during this process? D) 0 The kinetic energy is the same at both the beginning and end of its journey. delta K = Work = 0 I feel like there is some sort of trickery where mgh-ukmgD cancels out. cancels Otherwise I'm not too sure. m H D Physics 211 Lecture 9, Slide 13 Physics Potential Energy vs Force Force b U b − U a = −Wa →b = − ∫ F ⋅ dl a Demo dU ( x) F ( x) = − dx Physics 211 Lecture 9, Slide 14 Physics Act Suppose the potential energy of some object U as a function as of x looks like the plot shown below. looks Where is the force on the object zero? A) (a) B) (b) C) (c) D) (d) (a) B) (b) C) (c) D) (d) U(x) x (a) (b) (c) (d) Physics 211 Lecture 9, Slide 15 Physics Act Suppose the potential energy of some object U as a function as of x looks like the plot shown below. looks Where is the force on the object in the +x direction? A) To the left of (b) B) To the right of (b) C) Nowhere To B) To C) Nowhere U(x) x (a) (b) (c) (d) Physics 211 Lecture 9, Slide 16 Physics Preflight Suppose the potential energy of some object U as a function as of x looks like the plot shown below. looks Where is the force on the object biggest in the –x direction? A) (a) B) (b) C) (c) D) (d) (a) B) (b) C) (c) D) (d) U(x) x (a) (b) (c) (d) Physics 211 Lecture 9, Slide 17 Physics ...
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