W. Gilmore
Pressure

Pressure is the amount of force acting on a unit of area

We calculate pressure using the following equation
Force(N)
Pressure =

Area(m2)
The unit of pressure is N/m2 or Pascal (Pa)
This formula is the reason why it is a lot sorer to get stood on by a person
wearing stiletto heels than a person wearing runners if they are the same
weight. The person wearing the stilettos. Her weight/force is concentrated
in one small area. The person wearing the runners. Her weight/force is
spread over a larger area.
E.g. What pressure is exerted on the ground by the box below?
Force = 50N
Area = 5cm x 2cm = 10cm2
Force(N)
(50N)
Pressure =
Area(m2) (10cm2)
Answer = 5N/cm2
W. Gilmore
Pressure and the depth of liquids


Liquids and gases exert pressure in all directions.
If you fill a bag with water and place pin holes at equal positions
around the bag, the water will flow out in equal directions.
BUT
 Pressure increase the further you move down into a liquid.
Why?

The further you swim down, the more liquid on top of you therefore
there is greater pressure above you. The diver below will feel greater
pressure at 20m than at 10m because there is more liquid above him
This is shown below in the laboratory demonstration
Therefore:
 Pressure in a liquid increases with depth

This is why a dam is thicker at the bottom

Deep sea divers wear special protective suits when diving at great
depths
W. Gilmore
Air also exerts pressure

Air is made up of millions of molecules which are pulled to the earth
by gravity

The pressure of the weight of air is called atmospheric pressure
Remember this experiment


Boiled a small amount of water in the can to push all of the air out.
Submerged the can upside down into water.

The can crushed because the atmospheric pressure outside the can
was greater than inside the can

A barometer or an altimeter is the instrument used to measure
atmospheric pressure
W. Gilmore
Atmospheric pressure and the weather




Weather depends on atmospheric pressure
Areas of equal pressure are joined together by lines called isobars
An area of high pressure will not allow water vapour to rise giving
sunny, settled and dry weather
An area of low pressure allows water vapour to rise giving, wet,
unsettled and windy weather