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 metal block measures 3cm x 5cm x 6cm
It has a  mass of 6kg.

The Earth’s gravity is 10N/kg so the weight of the block is 6kg x 10 = 60N

What pressure does the brick excert on the table when stood on its end? First calculate the downward force
6kg x 10 = 60N

Now work out the area touching the table:

The area of the narrow end = 3cm x 5cm

= 15cm2

Now work out the pressure:

pressure = force ÷ Area

ˆ pressure = 60÷ 15

ˆ pressure = 4 N/cm2

Example 2
What will happen to the pressure if the block is stood on its largest side? Downward force   (6kg x 10) = 60N

Area of side  = 5cm x 6cm

= 30cm2

Pressure = force / area

⇒ pressure = 60 / 30

pressure = 2 N/cm2

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

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

More area = less pressure
Less area = more pressure

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?