Page 130 - Science Coursebook

9.5 Pressure calculations

Pressure is the quantity that tells us how concentrated a
force is when it presses on an area.
Here is the equation which is used to calculate pressure,
written in words and in symbols:
force
pressure =
area
F
P =
A
where P = pressure, F = force, A = area. 8 N 2 N 2 N
Example: The diagram shows a force of 8 N pressing on
an area of 4 m . 2 N 2 N
2
force 8 N
pressure = = = 2 N/m 2
area 4 m 2
So the pressure is 2 N/m . As the diagram shows, a force =
2
of 2 N is acting on each square metre of the surface.

Question 4 m 2 2 N/m 2
2
1 A force of 6 N acts on an area of 2 m . What force
acts on each square metre? What is the pressure on
the surface?

Units of pressure
The unit of pressure is the pascal (Pa). 1 Pa is the same
as 1 N/m . We can write the answer to the example
2
above as:
pressure = 2 Pa
In the next example, the area is given in square
millimetres (mm ), so the pressure is in newtons per
2
square millimetre (N/mm ).
2
Example: A hammer hits a nail with a force of 200 N.
The point of the nail has an area of 0.5 mm touching a
2
piece of wood. Calculate the pressure on the wood.
force 200
pressure = = = 400 N/mm 2
area 0.5

Question
Hammering a nail.
2 A rectangular block weighs 240 N. The area

of the block in contact with the floor is 20 cm .
2
Calculate the pressure on the floor. (Give your

answer in N/cm .)
2
128 9 Forces in action