Air Pressure

Fluid Forces

Can you feel air pressure?

When you take off or land in an aircraft, your ears may hurt or feel uncomfortable. This is because your eardrums can feel changes in air pressure as the aircraft moves quickly up and down.

But what is air pressure?

What is Air Pressure?

• It is caused by the weight of all the air in the atmosphere pressing down on Earth.

• It is also known as atmospheric pressure.• Pressure = Force / Area• S.I. unit is Pascal (Pa)• A Pascal is 1 N/m²• 1 atmospheric pressure a.t.m. = 101,300 Pa,

101,000 Pa or 101 kPa

Why don’t you feel the atmosphere crushing you?

• The fluids (mostly water) inside your body also exerts pressure. It works against atmospheric pressure.

• There is a weight of 10 N of atmospheric pressure on every cm² of your body.

• Fluid pressure increases as depth increases.• Fluids flow from areas of high to areas of low

pressure.

Pascal’s Principle• A change in pressure at any point

in an enclosed fluid will be transmitted equally to all parts of that fluid.

• Hydraulic devices use liquids to transmit pressure from one point to another.

• Pressure Force = Force Pressure Area Area

Photos I

• A barometer

Photos II

• A barograph

Measuring Air Pressure• Air Pressure is measured in millibars (mb)

on a barometer. • The simplest kind of barometer is a

mercury barometer and pressure is measured in mm (in) of mercury.

• A barograph is a special kind of barometer, which records continuous changes in air pressure.