Your Comments. Mechanics Lecture 19, Slide 1

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Barnard McLaughlin
6 years ago
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Your Comments i studied so hard for exam... and i did so bad :'( why physics u no love me. When we say a system will conserve angular momentum, does the solar system count?

1 Your Comments i studied so hard for exam... and i did so bad :'( why physics u no love me. When we say a system will conserve angular momentum, does the solar system count? Say the sun suddenly expands in volume when it nears its death, then it should slow down its rotation, but will the planets also change rotational speed? Or if Uranus shrinks, will Neptune speed up? I never really liked this topic in high school. In fact I really hated it and didn't really understand it. Hopefully it'll be different in college and in lecture because that seems to be the trend. The merry-go-round derivation was very difficult to follow... BUT, did you know that the merry-go-rounds are now banned from school playgrounds because young children have been severely hurt from increasing their omegas too much and then flying off? A kid almost died doing this at my elementary school, then a huge machine/crane thingy uprooted the merry-go-around. Can we go over the "Block on Spinning Disk a"? I have a feeling that the total angular momentum may be equal to the initial angular momentum of the disk, but I don't know could you go over the spinning disks More examples in class would be helpful. Were you a good student before you became a professor? m1 m Mechanics Lecture 19, Slide 1

2 What was the average for the exam and is there a curve. I know some people who did really well and some that didn't. I was among the ones that didn't do as well, but I left the exam feeling very confident. I thought I knew what I was doing- but I guess I don't. I go to office hours, do all the practice exams and I feel like I understand the material. What else should I be doing to do well on the exams? Average: 75.4% (no curve) Mechanics Lecture 19, Slide

3 Your grade so far Prelectures + Checkpoints + Lectures 100 Labs (9) 150 Quizzes (9) 100 Homework (6) 150 Hour exams (3 x 100 each) 300 Final Exam 00 Prelectures: 50 Preflight's: 5 Lecture participation: 5 You can miss up to 3 of each The lowest Quiz is dropped The lowest Homework is dropped To get lecture participation credit you need to vote on at least half of the questions in a lecture. (i.e. it is possible to get part of a bonus point but no participation point for a lecture) Bonus Points: You can earn up to 1 extra bonus point in every lecture (for a maximum of 5 bonus points for the semester) by getting the right answers to at least 5 of the clicker questions. At the end of the semester your bonus points are added to your HW & Quiz scores (max 50) Your total score out of 1000 points determines your final grade. Its just a simple formula the computer calculates it. Your grade is determined entirely by the your performance on the components of the course as described above. There is no other extra credit possible. A+ (950), A (90), A- (900), B+ (880), B (860), B- (835), C+ (810), C (780), C- (750), D+ (70), D (690), D- (610), and F (<610).

4 Is there a way we can check our physics grade in the class as a whole? I know I could calculate it from the grade book, but that would take a lot of averaging, weighting, and dropping of lowest scores, and I'd much rather spend all my free time studying physics! Yes I made a spreadsheet for this yesterday.

5 I will post it someplace (use at your own risk)

6 Physics 11 Lecture 19 Today s Concepts: Angular Momentum Mechanics Lecture 19, Slide 6

7 Linear and Rotation Mechanics Lecture 19, Slide 7

8 Angular Momentum We have shown that for a system of particles F ext dp dt Momentum is conserved if F ext 0 r What is the rotational version of this? Well to start with we know this: rf r ext F ext d r L dt p Mechanics Lecture 19, Slide 8

9 Torque & Angular Momentum EXT dl dt ext rf L r p EXT L I In the absence of external torques 0 EXT dl dt Total angular momentum is conserved Mechanics Lecture 19, Slide 9

10 Example Disk dropped on Disk top, initial o top, final bottom, final f bottominitial, 0 Mechanics Lecture 19, Slide 10

11 Example Disk dropped on Disk top, initial o top, final bottom, final f bottominitial, Linitial MR o 0 Lfinal MR f MR f 1 MR o 1 MR f f 0 Mechanics Lecture 19, Slide 11

12 What about Kinetic Energy? top, initial o top, final bottom, final f bottominitial, 0 K 1 I L I K K before after L I L I before after same x Mechanics Lecture 19, Slide 1

13 CheckPoint In both cases shown below a solid disk of mass M, radius R, and initial angular velocity o is dropped onto an initially stationary second disk having the same radius. In Case the mass of the bottom disk is twice as big as in Case 1. If there are no external torques acting on either system, in which case is the final kinetic energy of the system biggest? A) Case 1 top, initial o top, initial o B) Case M M C) Same M bottominitial, 0 bottominitial, 0 M Same initial L Case 1 Case Mechanics Lecture 19, Slide 13

14 CheckPoint In which case is the final kinetic energy of the system biggest? A) Case 1 B) Case C) Same M M A) K=L/I, in case 1 the system has the smaller I Case 1 Case B) if two objects have the same velocity, the one with greater mass will have greater kinetic energy K M M L I C) Kinetic energy is the same because they start and end with the same L Mechanics Lecture 19, Slide 14

15 Point Particle moving in a Straight Line L Rp axis R 1 R R 3 D L 1 1 p R 1 p R psin( 1 ) 1 pd L R R p psin( ) pd p L 3 R R 3 3 pd p p psin( 3 ) 3 Mechanics Lecture 19, Slide 15

16 L Rp L mvd pd axis p D Direction given by right hand rule (out of the page in this case) Mechanics Lecture 19, Slide 16

17 Playground Example Before After M R Top View m v L Before mvr L After 1 I ( MR mr ) L Before L After v R 1 1 M m Disk Kid Mechanics Lecture 19, Slide 17

18 CheckPoint The magnitude of the angular momentum of a freely rotating disk around its center is L. You toss a heavy block onto the disk along the direction shown. Friction acts between the disk and the block so that eventually the block is at rest on the disk and rotates with it. What is the magnitude of the final angular momentum of the disk-block system: A) > L B) L C) < L Top View Instead of a block, imagine it s a kid hopping onto a merry go round Mechanics Lecture 19, Slide 18

CheckPoint What is the magnitude of the final angular momentum of the disk-block system: A) > L B) L C) < L Top View A) The moment of Inertia of the system is bigger than the moment of Inertia of the

19 CheckPoint What is the magnitude of the final angular momentum of the disk-block system: A) > L B) L C) < L Top View A) The moment of Inertia of the system is bigger than the moment of Inertia of the disk. B) Since the block is tossed horizontally towards the middle of the disk, it doesn't add and momentum to the disk so the momentum of the disk before must be equal to that of the disk after. C) There is friction so it will be less Recycling answers doesn t help you Mechanics Lecture 19, Slide 19

20 Clicker Question The magnitude of the angular momentum of a freely rotating disk around its center is L. You toss a heavy block onto the disk along the direction shown. Friction acts between the disk and the block so that eventually the block is at rest on the disk and rotates with it. Is the total angular momentum of the disk-block system conserved during this? A) Yes B) No Top View There are no external torques The block skidding on the disk is an internal torque (within the system) Mechanics Lecture 19, Slide 0

21 CheckPoint The magnitude of the angular momentum of a freely rotating disk around its center is L. You toss a heavy block onto the disk along the direction shown. Friction acts between the disk and the block so that eventually the block is at rest on the disk and rotates with it. What is the magnitude of the final angular momentum of the disk-block system: A) > L B) L C) < L Top View Instead of a block, imagine it s a kid hopping onto a merry go round Mechanics Lecture 19, Slide Mechanics Lecture 19, Slide 1

CheckPoint What is the magnitude of the final angular momentum of the disk-block system: A) > L B) L C) < L A) the block in this case has an initial angular momentum.

22 CheckPoint What is the magnitude of the final angular momentum of the disk-block system: A) > L B) L C) < L A) the block in this case has an initial angular momentum.. B) Nothing really changed from 1a C) The block has an initial angular momentum which runs counter to the disk's angular momentum Top View I don't understand how a change in direction of throwing the block onto the disk causes L to not be conserved..how does the change in direction count as an outside force? Mechanics Lecture 19, Slide

23 Clicker Question A student holding a heavy ball sits on the outer edge a merry go round which is initially rotating counterclockwise. Which way should she throw the ball so that she stops the rotation? A) To her left B) To her right C) Radially outward C A B top view: initial final Bender in Space Mechanics Lecture 19, Slide 3

24 Clicker Question A student is riding on the outside edge of a merry-go-round rotating about a frictionless pivot. She holds a heavy ball at rest in her hand. If she releases the ball, the angular velocity of the merry-go-round will: A) Increase B) Decrease C) Stay the same 1 Mechanics Lecture 19, Slide 4

25 Homework Mechanics Lecture 19, Slide 5

26 m L initial I disk o m 1 0 Idisk 1 m R 1 Mechanics Lecture 19, Slide 6

27 m K 1 initial I disk o m 1 Idisk o 1 m R 1 Mechanics Lecture 19, Slide 7

28 m m 1 0 L I initial disk 0 L final f L ( I I ) L final disk rod f initial Solve for f Irod Idisk 1 ml 1 1 m1r Mechanics Lecture 19, Slide 8

29 L final L initial Mechanics Lecture 19, Slide 9

30 M f 1 ( ) K final Idisk Irod f Mechanics Lecture 19, Slide 30

31 F Just like average P t for linear momentum M f average L t for angular momentum L I ( ) rod rod f 0 Mechanics Lecture 19, Slide 31

Last 6 lectures are easier

Last 6 lectures are easier Your Comments I love you. Seriously. I do. And you never post it. I felt really bad whilst completing the checkpoint for this. This stuff is way above my head and I struggled with the concept of precession.