Fluids: Pressure

Page 3

This text is meant to accompany class discussions. It is not everything there is to know about fluids, hydrostatics, and fluid dynamics. It is meant as a prep for class.

Click for the questions that go with this reading

Brownian Motion

It is typical to think of molecules as small stationary objects. In reality they are moving in random directions all the time. Below is a 36 second video showing this motion.

The video can be found at https://youtu.be/cDcprgWiQEY (YouTube,. 'Brownian Motion - Nanoparticles In Water'. N. p., 2015. Web. 29 Aug. 2015. )

Pressure at Depth

If you were to hold up a cube a metal, (50 N,), a wood cube, (5 N) and a brick, (10 N.) How would you estimate the total amount of weight you are holding up?

Hand holding up weight cubes

How would you find the weight on the bottom of a graduated cylinder filled with 3 fluids of different densities?

What if you had to find the PRESSURE on the bottom of the cylinder filled with 3 different fluids?

This YouTube video can be found at https://youtu.be/sEVMbcYu8rM

"Gauge" Pressure

What is the gauge pressure inside of an open 2 liter bottle?

The Barometer

Pressure in a fluid is due to the number of collisions molecules have with a surface. The greater the number of collisions or the force exerted by the molecules, the greater the force exerted on a surface.

Because of Brownian motion, the pressure is exerted in all directions at the same depth. The scuba diver and the location in the cave, at the same depth below the surface as the diver, experiences the same pressure. Also because of Brownian motion, the pressure at this depth is exerted EQUALLY in all directions. (When talking about any body at depth, don't over think the situation. Ignore the height of the body unless the problem specifically addresses it.)

Pressure

Pressures you need to memorize to solve problems

An atmosphere is abbreviated "atm"

1 atm = 1.0 x 10⁵ ^N/_m²

1 atm = 1.0 x 10⁵ Pa (standard SI units for all equations)

1 atm = 760 mm mercury

Example Problem

Question
Solution
Video Solution

Diving Bell image A diving bell is 2560 m below the surface of the ocean and feels a pressure of 25,942,000 Pa. If the round window on the diving bell has a radius of 15.0 cm, then how much force is pressing on the window?

This solution is showing all the pieces you need for credit: What you are attempting to do, the equation with only variables, the equation with numbers and units, the answer with units and it is all logically organized.

Pressure Solution

[ YouTube Video Solution Goes Here ]

(Image credit:"Graduated cylinders-diagrams" by Wersję rastrową wykonał użytkownik polskiego projektu wikipedii: Polimerek, Zwektoryzował: Krzysztof Zajączkowski - Wersja rastrowa: http://pl.wikipedia.org/wiki/Plik:Menzurki.png. Licensed under GFDL via Commons. )

by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)

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