Your Thoughts. Mechanics Lecture 11, Slide 1

Transcription

1 Your Thoughts Honestly, I'm finding this class a bit difficult to keep up with especially coming straight off that midterm, although it wasn't bad. There's a lot going on and I understand that, that's part of the challenge of college, but with quiz, homework, prelecture, checkpoint, prelab, lab, discussion, discussion quiz, opfonal homework, the exam, and studying on top of 14 other credit hours and extracurriculars and drama it seems a bit much. I really like this class and am gonna sfck it out, but I just need some sleep and forgieness from Tony in the 10am lecture. Had exam tonight and a paper to write and an exam tomorrow, had to just click through the prelecture and checkpoint. Going to come back tomorrow night and watch it again and take notes. Please go oer the checkpoints in lecture. It's late and I didn't really think about my answers to the checkpoints, so a clarificafon would help. Is it true that in the center of mass reference frame momentum is ALWAYS consered, or always equals zero? Can we go oer this more in lecture? I'm going to keep menfoning this: please do something about all the disrupfe students during lecture! Maybe they will all drop auer surely doing poorly on the first exam, but it is not fair to allow these students to ruin the quality of lecture for others. Mechanics Lecture 11, Slide 1

2 Physics 211 Lecture 11 Today s Concept: Momentum ConseraFon Mechanics Lecture 11, Slide 2

3 Exam 1 We will post the grades in the next few days. We neer scale anything down. If the aerage is below 75% we will scale the exam up. You will be able to see your answers to each quesfon in the course gradebook. You will get your exam booklets back in discussion next week. We will post the exam (with detailed solufons) on smartphysics Mechanics Lecture 11, Slide 3

4 Recap r P r = m tot i i i Mechanics Lecture 11, Slide 4

5 Can you go oer energy conseration in collisions? That description in the prelecture confused me. Energy (Prelecture 8) Mechanics Lecture 11, Slide 5

6 Comparison of Momentum and Energy Concepts Δ K = Wtotal r r = F Δx net Lecture 7 (2 weeks ago) Work KineFc Energy theorem r r Lecture 14 (next week) Δ P = F Δt total net Impulse- Momentum theorem Mechanics Lecture 11, Slide 6

7 Clicker Question A wood block rests at rest on a table. A bullet shot into the block stops inside, and the bullet plus block start sliding on the fricfonless surface. The horizontal momentum of the bullet plus block remains constant Before AUer A) Before the collision. B) During the collision C) AUer the collision D) All of the aboe E) Only A and C aboe As long as there are no external forces acfng on the system Mechanics Lecture 11, Slide 7

8 It s a ector equation r dp dt p 1 tot = r F Net, External p init p 2 p 2 p 1 p final θ Mechanics Lecture 11, Slide 8

9 CheckPoint Suppose you are on a cart inifally at rest that rides on a fricfonless track. If you throw a ball off the cart towards the leu, will the cart be put into mofon? LeU Right A) Yes, and it moes to the right. B) Yes, and it moes to the leu. ConseraFon of momentum means the cart must moe to the right since the ball moes to the leu. C) No, it remains in place. Mechanics Lecture 11, Slide 9

10 CheckPoint Suppose you are on a cart which is inifally at rest that rides on a fricfonless track. You throw a ball at a erfcal surface that is firmly ajached to the cart. If the ball bounces straight back as shown in the picture, will the cart be put into mofon auer the ball bounces back from the surface? LeU Right A) Yes, and it moes to the right. B) Yes, and it moes to the leu. C) No, it remains in place. Mechanics Lecture 11, Slide 10

11 CheckPoint LeU Right A) Yes, and it moes to the right. B) Yes, and it moes to the leu. C) No, it remains in place. B) The ball ends up moing the right direction so the cart must moe to the left to consere momentum. C) At first the cart will moe right but then when the ball hits the wall the cart will moe back to the left. Mechanics Lecture 11, Slide 11

12 Your Question Say we took the same cart from the first checkpoint problem and put walls on either side of the man. If the man then threw a ball at one of the walls so that it would just bounce between the two walls, would the cart be set in motion and if so, in what direction? LeU Right A) The cart would moe back & forth as the ball bounced. B) The cart would not moe C) The cart would keep moing in the direcfon opposite the throw Mechanics Lecture 11, Slide 12

13 Clicker Question Suppose you are on a cart that is moing at a constant speed toward the leu on a fricfonless track. If you throw a massie ball straight up (relafe to the cart), how will the speed of the cart change? LeU Right A) Increase B) Decrease C) Will not change As long as there are no external forces acfng on the system, P total is consered Mechanics Lecture 11, Slide 13

14 CheckPoint Two balls of equal mass are thrown horizontally with the same inifal elocity. They hit idenfcal stafonary boxes resfng on a fricfonless horizontal surface. The ball hilng box 1 bounces back, while the ball hilng box 2 gets stuck. Which box ends up moing faster? A) Box 1 B) Box 2 C) same 1 2 Lets think about a slightly different ersion of this Mechanics Lecture 11, Slide 14

15 Clicker Question Two equal- mass balls swing down and hit idenfcal bricks while traeling at idenfcal speeds. Ball A bounces back, but ball B just stops when it hits the brick. Which ball is more likely to knock the brick oer? A B A) A B) B C) Equally likely. Mechanics Lecture 11, Slide 15

16 demo A B ΔP A ΔP B ΔP A > ΔP B The change in the momentum of the ball is bigger in A So the block gets a bigger kick in A Mechanics Lecture 11, Slide 16

17 CheckPoint Two balls of equal mass are thrown horizontally with the same inifal elocity. They hit idenfcal stafonary boxes resfng on a fricfonless horizontal surface. The ball hilng box 1 bounces back, while the ball hilng box 2 gets stuck. Which box ends up moing faster? A) Box 1 B) Box 2 C) same 1 2 Mechanics Lecture 11, Slide 17

18 Ballistic Pendulum m M H A projecfle of mass m moing horizontally with speed strikes a stafonary mass M suspended by strings of length L. Subsequently, m + M rise to a height of H. Gien H, what is the inifal speed of the projecfle? Mechanics Lecture 11, Slide 18

19 Breaking it down into steps before during auer m M H splat Which quanffes are consered before the collision? A) momentum (horizontal) B) mechanical energy C) both momentum and mechanical energy Mechanics Lecture 11, Slide 19

20 Breaking it down into steps before during auer m M H splat Which quanffes are consered during the collision? A) Momentum (horizontal) B) mechanical energy C) both momentum and mechanical energy m = ( m + M ) V Mechanics Lecture 11, Slide 20

21 Breaking it down into steps before during auer m M H splat Which quanffes are consered auer the collision A) momentum (horizontal) B) mechanical energy C) both momentum and mechanical energy 2 (1/ 2)( + ) = ( + ) M m V M m gh V 2 = 2gH Mechanics Lecture 11, Slide 21

22 Ballistic Pendulum m M H m = ( m + M ) V V 2 = 2gH V = m ( m+ M) m+ M = 2gH m = 2gH 1+ M m Mechanics Lecture 11, Slide 22

23 Ballistic Pendulum Demo L L L L m M H M + m d In the demo we measure forward displacement d, not H: L-H H d L L 2 H = = d 2 L + ( L H) L 2 d 2 2 = 1+ M m d g L (mph) = 3d(cm) for d << L Physics 211 Lecture 11, Slide 23