Pressure in a liquid   Atmospheric pressure  Measuring air pressure

Pressure is defined as force per unit area.

The pressure exerted by a force tells us concentrated the force is .

Pressure is calculated as : PRESSURE = FORCE ÷ AREA

The UNITS of pressure we use are newtons per square centimetre (N/cm² )

(The normal SI unit of pressure is a newton per square metre (N/m²), also called the pascal)

If the area that a force is acting on INCREASES then the pressure will DECREASE.

Example 1
A wooden brick measures 3cm x 4cm x 6cm
It has a  mass of 3.6kg and so  exerts a downward force of 36N on the table.

What pressure does the brick exert on the table when stood on its end 

Answer: First work out the area touching the table:

The area of the shaded end = 3cm x 4cm

= 12cm²

Now work out the pressure:

pressure = force ÷ Area

ˆ pressure = 36 ÷ 12

ˆ pressure = 3 N/cm²

Example 2
What will happen to the pressure if the brick is stood on its side

Answer: Area of side B = 3cm x 6cm

= 18cm²

Pressure = force / area

ˆ pressure = 36 / 18

ˆ pressure = 2 N/cm²

Notice how the pressure gets less when the brick is stood on a larger area.

If the area increases then the force is more spread out making the pressure LESS.

eg Explain how snow shoes work:

A boy wearing snow shoes is less likely to sink into the snow because his weight is more spread out making the pressure on the snow LESS.

If the area decreases the force is more concentrated making the pressure MORE.

eg Why is teh wire of a cheese cutter so thin?

With a thin wire the downward force is concentrated on to a very small area. This increases the pressure of the wire on the cheese.

Pressure in a liquid

The pressure increases with depth. The can shown has three holes in it. More water squirts from hole C because it is lower down than the other holes. Any two points same depth will be at the same pressure. The pressure at D (above) equals the pressure at C. Water pressure acts equally in all directions. Water squirts out of the hole at right angles to the hole, even though gravity is pulling the water downwards

A dam wall is thicker near the base because the water pressure is greater lower down in the water.

A water tank is placed high up in order to increase the water pressure near the tapThe height of water being supported is sometimes called the head of water

Atmospheric Pressure

Caused by the weight of air pushing down (see also KINETIC THEORY)

The air presses with a force of about 10N on every square centimetre. (10 N/cm²)

Air pressure gets LESS as you rise up in the air (which is why a balloon expands as it rises).

Air pressure normally acts on all surfaces of an object which is why we don't notice it.

Atmospheric pressure is responsible for helping us breath, filling a syringe, making a vacuum cleaner suck.

If all of the air is removed from a container the pressure of the air pressing on only one surface can cause the can to collapse.

Air pressure crushing a tin can

Measuring air pressure

Air pressure can be measured using a bourdon gauge

Atmospheric pressure is measured with a barometer

There are two kinds of barometer: mercury barometer and aneroid barometer

The pressure of the atmosphere (as measured by a mercury barometer) = 760mm (76cm)

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