Page 128 - Science Coursebook

9.4 Pressure

A carpenter uses nails to join two pieces of wood sharp blunt

together. The hammer hits the nail on its flat head. The
nail’s sharp point pushes into the wood.
Why does a nail have a sharp point? The force of the small larger
hammer is concentrated on a small area so that the nail area area
will be able to penetrate the wood.
If a nail is blunt, the force of the hammer is spread over
a bigger area and the nail will not go into the wood.

The idea of pressure
If a force is concentrated on a small area, we say that the
force is creating a high pressure. If the force is spread If it is sharp, a nail can be hammered further
over a bigger area, the pressure will be less. with the same force.
If you lie on a bed, the force of your weight is spread
over a large area. You will make a shallow dip in the
mattress.

However, if you stand on the bed, your weight will be
concentrated on a much smaller area. You will make a
much deeper dip in the bed.

Question Standing makes a deeper dip in a bed.
1 Who will create a greater pressure, a heavy
person standing on a bed or a lighter person
lying on the bed? Explain your answer.

High pressure, low pressure
Sometimes we want high pressure, sometimes we want
low pressure. Here are some examples.
Sharks have sharp teeth so that, when they bite, the
force of their jaws will create enough pressure to crush
their prey.
A knife is better at cutting when its blade is sharp. Then A shark.
the force pushing on it will be concentrated on a small
area to create a high pressure.

Camels have wide feet so that their weight is spread over
a large area. Then the pressure will be low and they will
not sink into the sand.

A snowboard is wide and does not sink into the
snow. The Inuit people wear wide snowshoes for the
same reason.
An Inuit holding his snowshoes.

126 9 Forces in action