Lecture 27: Fluid Mechanics - Pascal's Principle - Hydrostatics - Atmospheric Pressure - Over Pressure in Lungs and Tires

author: Walter H. G. Lewin, Center for Future Civic Media, Massachusetts Institute of Technology, MIT
recorded by: Massachusetts Institute of Technology, MIT
published: Oct. 10, 2008,   recorded: November 1999,   views: 19002
released under terms of: Creative Commons Attribution Non-Commercial Share Alike (CC-BY-NC-SA)

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1. Pressure and Pascal's Principle:

Pressure is a scalar. Pascal's Principle is explained. In hydraulic jacks, a small force is applied to move a large mass a small distance.

2. Gravity and Hydrostatic Pressure:

Because of gravity, pressure increases with depth in a fluid. This is called hydrostatic pressure.

3. Compressibility of Gases vs. Liquids:

Unlike liquids, gases are compressible, so they cushion impacts. Liquids do not act like cushions. This is demonstrated in a very dramatic way by firing a bullet in a sealed can filled with air and one filled to the brim with liquid.

4. Pressure Difference and a Column of Liquid:

The pressure difference between the bottom and top of a vessel of liquid depends only on the height and density of the liquid, not on the area or weight of the column. This is rather non-intuitive.

5. Atmospheric or Barometric Pressure:

The pressure at sea level due to the air above determines the atmospheric or barometric pressure. It can be measured by raising (sucking up) a column of liquid from an open reservoir with a tube sealed at the end where we pump the air out. For every 10 meters depth in water, the hydrostatic pressure increases by about one atmosphere.

6. Submarines and Overpressure:

Cornelis Drebbel is credited with inventing the first submarine operating at a depth up to 5 meters. At this depth, the hydrostatic pressure is about half an atmosphere. A sealed paint can was evacuated to demonstrate the enormous forces acting upon it with an over pressure of about one atmosphere. The can imploded.

7. Overpressure in our Lungs:

The lung capacity, our ability to overcome hydrostatic pressure, is measured with a manometer. This is related to how deep snorkeling works, and why scuba-divers use pressurized air tanks. Professor Lewin demonstrates that by blowing on a manometer, or by sucking on it, we can raise or lower a column of water by about 1 meter (0.1 atmosphere). So why then was he able to suck fluid up a straw of several meters long?

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Reviews and comments:

Comment1 Marc O'Brien, April 6, 2009 at 11:14 p.m.:

You can suck the cranberry juice up so high because you are able to exhale each meter of effort. I would imagine then that how many meters difference you can achieve on the manometer in the earlier experiment has much to do with the limited capacity of your lungs. Internal compression ratio related to internal volume ratio change capability?


Comment2 siddayya, May 12, 2009 at 10:29 a.m.:

thanks very much, please add videos with lecturing of machines working procedure


Comment3 Richard Polkinghorne, August 27, 2009 at 12:44 a.m.:

Thank you so much. I am a retired graduate level M.Sc. educated U.K. person, who just needed to remind myself of Pascals Law. I have just sat entranced watching your lecture. I am amazed that such education is offered free to view in your country. I wish this country had such an enlightened view of education to our 'A level' students. I will view the rest of your series with some enthusiasm.


Comment4 mohammed Hussein Said, October 20, 2009 at 8:53 a.m.:

good and very nice work
i wish to have all lectures of your nice work


Comment5 AMIT KUMAR, November 28, 2010 at 5:15 a.m.:

hi,i love u prof,u r great,and with wat enthusiasm u r teaching thats amazing !!! i wud like to watch all of your videos!!!


Comment6 Para, June 7, 2011 at 12:24 a.m.:

Very nice lecture. I enjoyed every minute of it. Thanks.


Comment7 Vinaya, September 14, 2011 at 7:05 a.m.:

Thank you,
It is a great lecture i have seen,, wonderfully explained.


Comment8 nithin, November 7, 2011 at 4:48 p.m.:

i want HTML about this video


Comment9 ellias, April 21, 2012 at 11:37 p.m.:

it is really a nice lecture.u will be a great model for ethiopian teachers


Comment10 Davor, May 1, 2012 at 7:48 p.m.:

These are indeed great² lectures.

One minor mistake: at 36:40 prof Lewin said force on lung is 100kg at 10m under sea-level. It’s more like 1ton.


Comment11 Claudio Pujol, March 30, 2013 at 10:40 a.m.:

Excelentes las demostraciones


Comment12 Muhammad Arif, March 27, 2015 at 6:16 a.m.:

Really good concept on fluid statics applied for Manometer.


Comment13 Davor form VideoLectures, December 18, 2017 at 10:33 a.m.:

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