Physics 211 Lecture 9

Transcription

1 Physics 211 Lecture 9 Today's Concepts: a) Energy and Friction b) Potential energy & force Mechanics Lecture 9, Slide 1

2 Hour Exam One on THIS Thursday This is a 50 minute exam. Most of the practice exams online were 90 minute exams. Don t wait until Wednesday. But bring questions to Wednesday s review. Wednesday s review will be working on homework questions that you raise. Mechanics Lecture 8, Slide 2

3 Macroscopic Work done by Friction Mechanics Lecture 9, Slide 3

4 Macroscopic Work: do not understand the concept of macroscopic work. Kindaa confused. Help me please??/ This is not a new idea it s the same work you are used to. W b a F dl 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 Macroscopic work changes only macroscopic (mechanical) energy, not heat. Mechanics Lecture 9, Slide 4

5 Mechanics Lecture 9, Slide 5

6 Mechanics Lecture 9, Slide 6

7 f f Mechanics Lecture 9, Slide 7

8 Heat is just the kinetic energy of the atoms! Demo: cork popper Mechanics Lecture 9, Slide 8

9 Checkpoint/ACT 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 m k. What is the total macroscopic work done on the block by all forces during this process? A) mgh B) mgh C) m k mgd D) 0 H m K W tot D Mechanics Lecture 9, Slide 9

10 What is the total macroscopic work done on the block by all forces during this process? A) mgh B) mgh C) m k mgd D) 0 B) work= change in potential energy C) The only work being done on the object is by the friction force. D) total work is equal to the change in kinetic energy. since the box starts and ends at rest, the change in kinetic energy is zero. m CheckPoint A) the only work there is the potential energy initially H K W tot Mechanics Lecture 9, Slide 10 D

11 CheckPoint/ACT 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 m k. What is the macroscopic work done on the block by friction during this process? A) mgh B) mgh C) m k mgd D) 0 H m D Mechanics Lecture 9, Slide 11

12 CheckPoint/ACT What is the macroscopic work done on the block by friction during this process? A) mgh B) mgh C) m k mgd D) 0 B) All of the potential energy goes to kinetic as it slides down the ramp, then the friction does negative work to slow the box to stop C) Since the floor has friction, the work done by the block by friction is the normal force times the coefficient of kinetic friction times the distance. Work done by friction is negative. m H D Mechanics Lecture 9, Slide 12

13 Ouch! They re perpendicular. I don t understand this. no Any better understanding by now? What forces? Mechanics Lecture 9, Slide 13

14 K W tot W gravity W friction 0 Wgravity Wfriction 0 mgh Wfriction N 1 must be negative m N 2 H mg m mg mg Mechanics Lecture 9, Slide 14

15 Potential Energy vs. Force 1 2 U( x) kx 2 du ( x) Fx ( ) dx kx Mechanics Lecture 9, Slide 15

16 Potential Energy vs. Force Fx ( ) du ( x) dx Demo: car on track Mechanics Lecture 9, Slide 16

17 ACT 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) x (a) (b) (c) (d) Fx ( ) du ( x) dx Mechanics Lecture 9, Slide 17

18 ACT 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) x (a) (b) (c) (d) Fx ( ) du ( x) dx Mechanics Lecture 9, Slide 18

19 CheckPoint BAD WORDING 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) x (a) (b) (c) (d) Fx ( ) du ( x) dx Mechanics Lecture 9, Slide 19

20 ACT BETTER WORDING 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 & of the largest magnitude? A) (a) B) (b) C) (c) D) (d) U(x) x (a) (b) (c) (d) Fx ( ) du ( x) dx Mechanics Lecture 9, Slide 20

21 ACT BETTER WORDING 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 & of the largest magnitude? A) (a) B) (b) C) (c) D) (d) U(x) x (a) (b) (c) (d) Fx ( ) du ( x) dx Mechanics Lecture 9, Slide 21

22 Your Comments Careful! A negative slope means a positive force! good At A) slope is neg so force is positive. Careful: F = - du/dx At A) slope is neg so force is positive. At C) slope is pos so force is negative. Mechanics Lecture 9, Slide 22

23 More Comments from this Morning Mechanics Lecture 9, Slide 23

24 L v Conserve Energy from initial to final position v 2 gl Mechanics Lecture 9, Slide 24

25 2 v T a L L a 2 v L v 2gL mg T mg 2 mv L T mg 2 mv L Mechanics Lecture 9, Slide 25

26 2 v T a L a 2 v L v 2gL mg T 3mg Mechanics Lecture 9, Slide 26

27 v v 2gh 2 gh h h Conserve Energy from initial to final position v 2gh Mechanics Lecture 9, Slide 27

28 v 2gh h r T mg a 2 v r F ma T mg 2 mv r T 2 mv r mg Mechanics Lecture 9, Slide 28