Your Comments Mechanics Lecture 9, Slide 1 Just a thought, in the prelectures, when whoever is narrating them says "this all makes sense", it makes me increasingly more frustrated because apparently I'm too dumb to understand whatever it is that "makes sense". Prof. Selen said it a few times last week. Please don't say it anymore. I don't understand.....this. Can we just shoot monkeys and make rocket carts instead of learning things? Like the good ol' days? I want help with signs!!!!!!!!!!!!! With today being Valentines Day and all, I was hoping you could give us some physics pick up lines. I don't understand why we are already working on hour exam two and do far into the section before we've taken the first exam. Everything is getting overlapped and I'm struggling on the new homework assignments. Happy Valentine's Day everyone!! especially my Peanut, I love you sweetie from your Jelly Bear. Ramp problems with springs! Also, how many physics examples are made only for physics learning but have no application in real life? Why do we need to understand the microscopic trends of friction? Also, you really need to use the pun "work of friction/ work of fiction" in this lecture. How much potential energy is in a Pikachu (electricity but still just wondering)? I am lost. I am not sure what the graph is graphing, or exactly what the difference is between macroscopic work and the work that we studied before. I only understand what was shown straight out in the prelecture. It would be very beneficial to go over everything during lecture, please. Lots of music requests

Exam Info: Covers Lectures 1-6 only (No “work & energy” on the exam) We offer a conflict exam from 5:15 – 6:45 on the night of the exam. You can sign up for this in your grade-book before Feb/18. If you have a serious conflict with both exams, please get in touch with Dr. Paco Jain immediately Exam review in class next Tuesday – we will work through the first hour exam from Fall/2010 The old exams are a fantastic way to practice. We have put hundreds of great problems online (with formula sheet)… Mechanics Lecture 9, Slide 2

Physics 211 Lecture 9 Today's Concepts: a) Energy and Friction b) Potential energy & force Mechanics Lecture 9, Slide 3 We will work our 3 HW problems in class today “Are we ever going to go over the homework in class?.”

Macroscopic Work: This is not a new idea – it’s the same “work” you are used to. Applied to big (i.e. macroscopic) objects rather than point particles (picky detail) We call it “macroscopic” to distinguish it from “microscopic”. You will deal with this in Physics 213 Mechanics Lecture 9, Slide 4 “Calculating total work is confusing, and "macroscopic" throws me off..” Feel free to ignore this word for now…

Mechanics Lecture 9, Slide 5 Macroscopic Work done by Friction

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Do spinning Heat Demo “Heat” is just the kinetic energy of the atoms! Mechanics Lecture 9, Slide 9

H N1N1 mg N2N2   mg must be negative m Mechanics Lecture 9, Slide 10 “i get tricked by when something is supposed to be negative or positive.”

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  k. What is the macroscopic work done on the block by friction during this process? A) mgH B) –mgH C)  k mgD D) 0 D m CheckPoint Mechanics Lecture 9, Slide 11 H

What is the macroscopic work done on the block by friction during this process? A) mgH B) –mgH C)  k mgD D) 0 B) The work done by friction is the negative value of the initial potential energy since that is how much that needs to be applied to make it rest. C) work done = force x distance. D H m Mechanics Lecture 9, Slide 12 CheckPoint You can ignore this word…it just tells us we don’t need to worry about individual atoms

What is the total macroscopic work done on the block by all forces during this process? A) mgH B) –mgH C)  k mgD D) 0 D m Mechanics Lecture 9, Slide 13 CheckPoint H

Potential Energy vs. Force Mechanics Lecture 9, Slide 14 Perhaps we can go over how the derivative of the potential energy is the force on the object, because this is new to me.

Demo Potential Energy vs. Force Mechanics Lecture 9, Slide 15

Suppose the potential energy of some object U as a function of x looks like the plot shown below. Where is the force on the object zero? A) (a) B) (b) C) (c)D) (d) U(x)U(x) x (a)(b) (c)(d) Clicker Question Mechanics Lecture 9, Slide 16 “How will this stuff be useful in Chemical Engineering? I mean mechanical and aero yes I see the importance. But where would this be useful in Chemical Engineering?”

Suppose the potential energy of some object U as a function of x looks like the plot shown below. 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 U(x)U(x) x (a)(b) (c)(d) Clicker Question Mechanics Lecture 9, Slide 17

Suppose the potential energy of some object U as a function of x looks like the plot shown below. Where is the force on the object biggest in the –x direction? A) (a) B) (b) C) (c)D) (d) U(x)U(x) x (a)(b) (c)(d) CheckPoint Mechanics Lecture 9, Slide 18

Mechanics Lecture 9, Slide 19 “The interactive example for the homework due today was terrible. I entered mg(x+d)sin(30) for the change in potential energy about 6 times, and never got it right. (even though I successfully used that solution to find the correct answer to the overall problem). stick to multiple choice questions if you can't get smartphysics to recognize the correct answer.” HW system likes angles in radians!!

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Mechanics Lecture 9, Slide 21 a b c

L Mechanics Lecture 9, Slide 22

L Mechanics Lecture 9, Slide 23 Will the tension be (A) bigger or (B) smaller than mg?

L mg T Mechanics Lecture 9, Slide 24

mg T Mechanics Lecture 9, Slide 25

Conserve Energy from initial to final position h h Mechanics Lecture 9, Slide 26

h mgT r Mechanics Lecture 9, Slide 27

Mechanics Lecture 9, Slide 28 mgN Just barely making it means N = 0 v

Mechanics Lecture 9, Slide 29 v h