Basics


    Pressure

    Pressure is defined as force per area .

    The area is measured in m 2 , the force in N.The unit of pressure is N / m 2 , which is also called Pa (Pascal).

    In symbols:

    P = F / A

    Example

    What is the pressure of your feet on the floor?

    Area: width times length

    8 cm * 25 cm = 0.08 m * 0.25 m

    = 0.02 m 2

    Area: both feet= 0.04 m 2

    Force= Weight

    = mass * gravity

    = 70 kg * 9.8 m/s 2

    = 686 kg m/s 2 = 686 N

    Pressure :

    P = F / A = 686 N / 0.04 m 2 = 17,150 N / m 2

    Air: Is it light ? Or is it heavy?

    Air Pressure :

    Atmosphere 16 km = 10 miles high

    Total mass 5000 billion tons !!

    P air = 100,000 N / m 2

    = 1000 * 100 N / m 2

    = 1000 mb
    Here a new unit is introduced, 1 mb (millibar) corresponds to 100 N / m 2 .

    From this we learn that the air pressure is

    P air = 1000 mb


    This corresponds to weight of

    10 mwater ( at sea level)
    Measure air pressure with barometer.

    Air pressure at sea level

    760 Torr 760 mm Hg

    29.9 inches Hg

    1015 mb at sea level

    is not associated with a direction

    Watch a summary on pressure here,
    and on atmospheric pressure here.

    Ch. Elster
    Aug 26 14:27:03 EDT 2020
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