9/10/10
Post date: Sep 09, 2010 9:26:27 PM
Bell Ringer: A pressure of 500 Pascals is applied all over a roof with an area of 150 square meters. How much total force does the roof experience?
Fluids through openings
when fluids move from a wide to a narrow opening, they have to speed up to get all the fluid through
When fluids move from a narrow to a wide opening, they slow down.
example: putting your thumb over part of a hose's opening to shoot water faster and farther
example: narrow opening at end of a squirt gun to make water shoot faster and farther
example: clear tape over most of sink opening to shoot water fast and get user wet (good April Fool's prank!)
Pressure and movement
Fluids generally move in ways that will equalize pressure differences
Fluids flow from higher pressure to lower pressure (higher pressure side is pushing them harder than the lower pressure side)
Objects in a fluid will move with the fluid according to pressure differences
example: Fire One! game (push on pumps to move plastic piece using air pressure)
example: Compressed air leaking out of a tire into the lower pressure atmosphere
example: Hiss when opening a soda due to higher pressure air (and CO2) inside bottle moving into lower pressure atmosphere.
example: ALL WEATHER is driven by differences in pressure
How planes use pressure
Flight
Newton's 3rd law (to be discussed in the physics part of this class) explains part of how wings generate lift
Wings of a plane push air downward as the plane moves forward
Wings are shaped so that air will have very high pressure below the wing, and lower pressure above the wing.
Air moves from high to low pressure (up), and takes plane up with it.
In the cabin
Planes go very high, where air pressure is low
People have a hard time getting enough air when the pressure is that low
The inside of the cabin is kept at a higher air pressure than the surrounding air, to keep people comfortable
The structure of a plane must be VERY strong to hold all of this extra pressure inside the plane.