Your Comments. Nutcracker

Transcription

1 Nutcracker Your Comments I don't understand the pressure differences in different sized pipes. To me, I would think that a pipe with a larger diameter has the smaller pressure and a skinnier pipe has a larger pressure. Can you explain why its the opposite? "Liquids are compressible, otherwise shock and sound waves would not exist!... Thank you for this awesome class! I hope this comment gets put in the presentation!!!! I like fish. (a.k.a. my brain is fried so I'm just writing a random comment.) I am sad that this class is ending. but i am also happy that it is over. because physics has made me feel dead inside. yet so alive at the same time. Sorry about the small amount of time spent on prelecture and checkpoint. My bed looks sooo comfy. I'm still very new to the grading standards in the university. Is a C- still a passing grade for this class? Also, I feel as if I need more practice for the final exams in addition to the practice final that is on the webpage. How else would you suggest that I study for this final? Since we had to make so many assumptions in this unit (and throughout this entire course), when will we ever apply what we learned to real life situations? Oh and also, will there be office hours next week for the people who are taking the conflict exam? This is the end. Hold your breath and count to ten. Feel the earth move and then, hear my heart burst...again. Mechanics Lecture 25, Slide 1

2 Physics 211 Lecture 26 Today s Concepts: A) Moving Fluids B) Bernoulli s Equation Mechanics Lecture 26, Slide 2

3 Clicker Question This picture is A) Real B) Fake Mechanics Lecture 26, Slide 3

4 Vacuum gun demo: Atmospheric pressure = big force Mechanics Lecture 26, Slide 4

5 Clicker Question Through which hole will the water come out fastest? A DP = rgdy B C Mechanics Lecture 26, Slide 5

6 Continuity Equation Flow Rate (volume/time) is the same everywhere a.k.a. Pipe doesn t leak v A = v A 1 1 Mechanics Lecture 26, Slide 6

7 Continuity Equation We used the formula A1v1=A2v2, to figure out the speed of the water in the top pipe. It seems we ignored the fact that it was going straight up. Shouldn't gravity have affected it, making it go slower? No height will change pressure but not speed v A = v A 1 1 I'm not sure if I'm missing something obvious, but what causes the fluid to go up in the pipe? Difference in pressure (force) Mechanics Lecture 26, Slide 7

8 CheckPoint Water flows through a pipe that has a constriction in the middle as shown. How does the speed of the water in the constriction compare to the speed of the water in the rest of the pipe? A) It is bigger B) It is the same C) It is smaller The same amount of water flows through every part of the pipe during an amount of time, so it will flow faster in the constriction Mechanics Lecture 26, Slide 8

9 DK = W TOT Mechanics Lecture 26, Slide 9

10 Remember Work Kinetic Energy Eqn W W = DK P A d P A d = mv mv Vol Vol 1 1 P P = rv rv 1 1 Mechanics Lecture 26, Slide 10

11 Bernoulli s Equation a.k.a. Energy Conservation (When height doesn t change) Mechanics Lecture 26, Slide 11

12 CheckPoint Water flows through a pipe that has a constriction in the middle as shown. How does the pressure of the water in the constriction compare to the pressure of the water in the rest of the pipe? A) It is bigger B) It is the same C) It is smaller Mechanics Lecture 26, Slide 12

13 Clicker Question Two empty pop cans are placed about ¼ apart on a frictionless surface. If you blow air between the cans, what happens? A) The cans move toward each other. B) The cans move apart. C) The cans don t move at all. Mechanics Lecture 26, Slide 13

14 Bernoulli s Equation when height changes Mechanics Lecture 26, Slide 14

15 CheckPoint Water flows from left to right along a pipe as shown. The right end of the pipe is twice as high as and also has four times the area of the left end. Which of the following statements best relates the pressures at the ends of the pipe? A) P L = 2P R B) P L = P R wtf bent pipe? C) P L = ½ P R D) The relative size of P L and P R depends on the speed of the flow. h R P R h L 0 P L 1 1 P rv rgh = P rv rgh L L L R R R Bernoulli s Equation Mechanics Lecture 26, Slide 15

16 Clicker Question Suppose the water isn't moving. The right end of the pipe is twice as high as and also has four times the area of the left end. Which of the following statements best relates the pressures at the ends of the pipe? A) P L = 2P R B) P L = P R C) P L = ½ P R D) P L P R = rg (h R h L ) h R P R h L 0 P L 1 1 P rv rgh = P rv rgh L L L R R R Bernoulli s Equation Mechanics Lecture 26, Slide 16

17 Clicker Question Water flows from left to right along a pipe as shown. The right end of the pipe is twice as high as and also has four times the area of the left end. Which of the following statements best relates the pressures at the ends of the pipe? A) P L = 2P R B) P L = P R C) P L = ½ P R D) P L P R =rg (h R h L ) 1 / 2 r (v R 2 v L2 ) h R P R h L 0 P L 1 1 P rv rgh = P rv rgh L L L R R R Bernoulli s Equation Mechanics Lecture 26, Slide 17

18 Clicker Question We just saw that P L P R can be written in the following way: 1 P P = rg h h v v 2 ( ) r( ) L R R L L R Is there any reason this has to mean P L = 2P R or P L = P R or P L = ½ P R A) Yes B) No h R P R h L 0 P L 1 1 P rv rgh = P rv rgh L L L R R R Bernoulli s Equation Mechanics Lecture 26, Slide 18

19 CheckPoint Water flows from left to right along a pipe as shown. The right end of the pipe is twice as high as and also has four times the area of the left end. Which of the following statements best relates the pressures at the ends of the pipe? A) P L = 2P R B) P L = P R C) P L = ½ P R D) The relative size of P L and P R depends on the speed of the flow. h R P R h L 0 P L 1 P P = rg h h v v 2 ( ) r( ) L R R L L R Mechanics Lecture 26, Slide 19

20 Flow rate = A * v (units of m 3 /s) FlowRate = R v Mechanics Lecture 26, Slide 20

21 FR = R v = R v v 1 1 R = v 2 1 R2 Mechanics Lecture 26, Slide 21

22 Volume = Flow rate x time time = Volume / Flow Rate Mechanics Lecture 26, Slide 22

23 1 1 P rv rgh = P rv rgh P = P r v v 2 ( ) same Mechanics Lecture 26, Slide 23

24 Mechanics Lecture 26, Slide 24

25 Please explain the roof problem from the homework. Bernoulli s Equation Assume the roof is flat. Mechanics Lecture 26, Slide 25