Page 131 - Science Coursebook

9.5 Pressure calculations

Calculating force
We can rearrange the equation for pressure like this:

force = pressure × area
F = P × A
Example: A footbridge has been designed to withstand a
2
pressure of 2000 Pa. The area of the bridge is 15 m . What is
the greatest force the bridge can withstand (resist)?
force = pressure × area = 2000 × 15 = 30 000 N
That is the weight of about 50 people. Take care! The bridge
might break if they all stand close together – that would
create a greater pressure.
A footbridge.

Question
3 When the wind blows, it creates pressure on any surface it blows against. A
large window has an area of 3.5 m . If the wind causes a pressure of 2000 Pa
2
on the window, what force acts on it?

Activity 9.5

Squashing foam

You can squash a piece of plastic foam by applying

pressure to it. Place a stiff piece of wood on top of
the foam. Place weights on top of the wood.
There are three things to measure:

• the area of the wood pressing down on the foam 5 kg
• the force pressing down on the foam
• the thickness of the foam as it is squashed.
Calculate the pressure acting on the foam.

Use different weights to change the pressure. Draw
a graph to show h ow the thickness of the foam
depends on the pressure.

Summary
force
• pressure =
area
• force = pressure × area

9 Forces in action 129