Appendix B International System of Units
477
kilograms. Therefore, when the weight is expressed using units of kilograms
(or pounds), the value must first be converted from kilograms (pounds) to
newton before beginning any calculations, i.e.:
1 kilogram(weight) = 9.806N,
(B.I0)
and
1 pound(weight) = 4.448 N.
(B.11)
However, in everyday life, the distinction between mass and weight does not
matter. If an Englishman buys 2.2 pounds of apples at the supermarket and
a Polish woman buys 1 kilogram of apples at the farmer's orchard, they both
get the same amount of produce.
B.3.6 Pressure
The pressure at any point is the force per unit area at a point. In the SI
system, pressure is expressed in N/m 2 , and a pressure of 1 N/m 2 is called a
pascal (Pa), i.e. 1 Pa = 1 N/m 2 . This is a very low pressure. Therefore,
for practical applications, the multiplications of pascal are used, for example,
1 hPa = 10 2 Pa, 1 kPa = 10 3 Pa, 1 MPa = 10 6 Pa. In particular, atmospheric
pressure is usually expressed in hPa, and normal atmospheric pressure at sea
level, called the normal atmosphere, is equal to about 1000 hPa = 10 5 Pa =
1 bar (the exact value is 1.01325 x 10 5 Pa at 15°C). This pressure corresponds
to the pressure of 1 atmosphere (1 atm) in the old metric system, where:
kg (weight) of force
1 atm = 1
2
'
cm
(B.12)
in which 1 kg (weight) is the force needed to induce an acceleration of g =
9.806 m/s 2 for a mass of 1 kg. Thus:
kg (weight)
9.80665 N
5 N
5p
b
1
= 1 atm =
: : : : : : 10 - = 10 a = 1 ar.
cm 2
1O- 4 m 2
m 2
(B.13)
The magnitude of pressure determines it completely.
B.3.7 Shear Stress and Shear Force
Similarly to pressure, shear stress is force per unit area. However, now this
force acts parallel to a surface. For example, the force due to water flow over
the sea bottom induces shear stress at the bottom. As the shear stress force is
divided by the area over which it acts, it has the same units as pressure, i. e.
N/m 2 .