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.