Basics


Pressure

Pressure is defined as force per area.

The area is measured in m2, the force in N.
The unit of pressure is N / m2, 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 m2

Area: both feet= 0.04 m2

Force= Weight

= mass * gravity

= 70 kg * 9.8 m/s2

= 686 kg m/s2 = 686 N

Pressure :

P = F / A = 686 N / 0.04 m2 = 17,150 N / m2

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 / m2

= 1000 * 100 N / m2

= 1000 mb

Here a new unit is introduced, 1 mb (millibar) corresponds to 100 N / m2.

From this we learn that the air pressure is

P air = 1000 mb


This corresponds to weight of

10 m water (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