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.

Example: 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.

Example: Why is the wire of a cheese cutter so thin?