Page 132 - Science Coursebook

9.6 Pressure in gases and liquids

To inflate (blow up) a toy balloon, you must push

air into it. When the balloon has been blown up,
the pressure of the air inside presses outwards. This
tells us that air and other gases can cause pressure.
Liquids can also cause pressure. The photograph

shows a fireman using a high pressure hose to put

out a fire.
Diving deep

Divers have to be careful if they want to dive to a
great depth in the sea.

The weight of water above presses down on them. A fireman at work in Mumbai, India.

The greater the depth they go, the more water
there is above them pressing down, and so the
pressure is greater. 0 100 000

10 200 000
diver
Question 20 300 000
A+I 1 The photograph shows an experiment on depth / m 30 400 000 pressure / Pa
water pressure. Water squirts out faster from
the hole at the bottom of the bottle than at the 40 500 000
top. Use the idea of pressure to explain why.
50 600 000
Pressure increases with depth in water because
the weight of water pressing on the diver
increases with depth.

Holes in the bottle allow water to escape.

The pressure of the atmosphere
If you climb a high mountain, the air gets thinner (less dense) as you go higher. Its
density decreases. Because the air is less dense at the top of a mountain, its pressure is
less. We call the pressure of the air atmospheric pressure.

Atmospheric pressure is greatest at sea level. This is because, at sea level, we have the
weight of the whole atmosphere above us, pressing down. It is as though we live at the
bottom of an ‘ocean’ of air.

Question

2 Atmospheric pressure is about 100 000 Pa. Calculate the force of
the atmosphere on a person whose surface area is 2.0 m .
2

130 9 Forces in action