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