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# 333333333 - PY211 Lecture 20(15 Nov 2016 Fluids[Ch 12 Young...

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PY211 Lecture 20 (15 Nov 2016) Fluids [Ch. 12 Young and Freedman (cont.)]

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Class info Midterm 2: scores posted on Blackboard – Please submit regrade requests by end of the week Problem Set 9: due Friday, Nov. 18 by 5pm. To be posted. Problems from Chapters 12, 13. Ten problems online. Half also submitted on paper Discussion this week: on topics from Chs. 13, 14. Lab: this week in SCI-B9 Lab6: Fluids Help: SCI-121 More help: on Piazza!
Gases vs. Liquids Pressure in liquids and gases are due to a combination of gravitational and thermal contributions: For gases, the gravitational contribution is minimal.

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Pressure in a Static Liquid If point 2 is a vertical distance h below point 1, and the pressure at point 1 is P 1 , the pressure at point 2 is: Note: Point 2 does not have to be directly below point 1. Ex: diving For liquids, however, gravity plays a big role. P 2 = P 1 + ρ gh P 2 = 10 5 Pa + 1000 kg m 3 ( ) 10 m s 2 ( ) h
Pressure in a Static Liquid For a liquid in hydrostatic equilibrium, the pressure depends only on the depth and the surface pressure. A connected liquid in hydrostatic equilibrium rises to the same height in all open regions of the container. Compare the pressures at the bottom of each tube. Is there a pressure difference?

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Pressure in a Static Liquid Consider two connected tubes of different shape. Is the pressure at point 1 greater than at point 2? Pressure is the same at all points on a horizontal line through a connected liquid in hydrostatic equilibrium. The two points are at the same depth, so the pressures are equal. Otherwise, fluid would flow.
Worksheet What is the longest vertical soda straw you could possibly drink from? By sucking on the top and reducing the pressure, the straw is essentially a barometer. Atmospheric pressure pushes the liquid up the straw. The length of the longest straw can be obtained from the formula

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