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# Chapter 4: The Magic of Drinking with a Straw

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## Alexandra Hohmann

on 17 September 2013

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#### Transcript of Chapter 4: The Magic of Drinking with a Straw

Bernoulli Lift
Bernoulli's principle: for flows of liquid or gas, when the speed of the flow increases, the pressure inside the flow decreases

Airplane wings are shaped so that air moves faster above the wing than below - air pressure is higher below the wing than above, forcing the wing upward
Kristen Cusumano, Alex Hohmann,

Air Pressure
Airplanes and Lift
Airplanes depend on lift to fly - an upward force that counteracts gravity created by the direction and pressure of air flow
About 80% of lift comes from reaction lift, 20% from Bernoulli lift
Summary
Air can compress to accommodate pressure
Pressure = force/area
Pressure has no direction
Despite air pressure, we are able to breath because we have a pressure equilibrium inside our lungs
Hydrostatic pressure is pressure exerted by liquids; at equal depths hydrostatic pressure is equal
Liquids flow so that pressure is equal at equal levels
Airplanes are able to fly fly due to Bernoulli lift and reaction lift
Air can compress

P = F/A

Pressure has no direction

Pressure Equilibrium

Standard atm = 1.03 kg/cm
One paint can is filled to the top, one is not. Both are shot. Which one explodes?

The air molecules can
compress to accommodate
the extra pressure; the
liquid molecules cannot.
Paint Cans
Compressibility
Ice-skating
How do you move on thin ice?

Lay down and slowly move forward. Your weight is distributed.

Pressure and Area are inversely proportional
P = F/A
Reaction Lift
Newton's third law: for every action there is an equal and opposite reaction

Action: Air is pushed downward when it passes beneath an airplane wing that is angled downward

Reaction: Air is pushed upward, causing an upward lift of the wing
Hydrostatic Pressure
http://www.units.muohio.edu/dragonfly/flight/wing.gif
Pressure Has No Direction
Lungs
Pressure Equilibrium
There is pressure on your chest and an equal pressure on your back. Why do your lungs not collapse?

The air pressure in your lungs and outside your lungs is in equilibrium.
Chapter 4: The Magic of Drinking
with a Straw

Hydrostatic pressure is the pressure exerted by a liquid

At equal depths, hydrostatic pressure is equal

The denser the liquid, the higher the hydrostatic pressure exerted by the liquid
Pressure underwater increases with depth

Pressure Under Water
Each plane of liquid has a different pressure than the planes above and below it - pressure increases with depth

When you suck on a straw, the air pressure within the straw decreases, decreasing the surface pressure of the liquid within the straw

The pressure of the surface plane inside the straw is different from the pressure of the surface plane outside the straw - the liquid flows upward so that the pressures of the planes inside the straw align with the pressures of the planes outside the straw

If you are holding your hand out, there is a force of 330 lbs pushing down. Why are you able to hold it up so easily?

Air pressure comes from all sides. There is equal pressure pushing up on your palm.
Palm
The Straw
Evangelista Torricelli

Indicates a change in air pressure

Uses Mercury
Barometer
Underwater, extreme pressure is exerted on your chest

1 meter below the surface, 1,100 kg of pressure are exerted on your chest. Your lungs exert 1,000 kg of outward pressure in return

One meter underwater (approximately 3 feet), is the limit of how far it is possible to snorkel underwater
Snorkeling