Appendix B International System of Units
475
B.2.3 Mass
The kilogram is the basic metric unit of mass, equal to the mass of the international prototype kept in the form of a platinum-iridium cylinder at the
International Bureau of Weights and Measures at Shres. A kilogram is very
nearly equal to the mass of 1,000 cubic centimetres of water. The mass per
unit volume is called density and is discussed in detail in Sect. 1.2.l.
B.2.4 Temperature
For studies of marine systems, absolute temperatures are usually not of interest
and for the purpose of this book, temperature is expressed in °C (Celsius),
rather than in oK (Kelvin), where 273.15°K = O°C. The change of 1°C is the
same as a change of 1 0 K. Apart from degrees Celsius, degrees Fahrenheit (OF)
are sometimes used. The conversion between these units is as follows:
(B.3)
(B.4)
B.3 Derived Units
Many other quantities useful in fluid mechanics have derived units, which are
defined in terms of the base units. We list now some of them which are used
in this book.
B.3.1 Frequency
Frequency is the number of oscillations of a system during one second and it is
expressed in hertz (Hz), where 1 hertz = 1 cycle/1 second. In this text, angular
frequency is also used. The units of angular frequency are 1 radian/l second.
As 27r radians = 1 full cycle = 360°, therefore:
360°
180°
180°
1 radian = - - = - - = - - ~ 57.3°.
27r
7r
3.1416
(B.5)
B.3.2 Velocity
Velocity defines the rate at which the position changes with time and it is
expressed in metres per second (m/ s). Traditionally, in sailing and shipping,
and sometimes in oceanography, the velocity of wind, ocean currents and the
speed of the vessel is expressed in knots, where:
1 nautical mile
1852 m
1 knot =
= - - = 0.51 m/s ~ 0.5 m/s.
1 hour
3600 s
(B.6)
475
B.2.3 Mass
The kilogram is the basic metric unit of mass, equal to the mass of the international prototype kept in the form of a platinum-iridium cylinder at the
International Bureau of Weights and Measures at Shres. A kilogram is very
nearly equal to the mass of 1,000 cubic centimetres of water. The mass per
unit volume is called density and is discussed in detail in Sect. 1.2.l.
B.2.4 Temperature
For studies of marine systems, absolute temperatures are usually not of interest
and for the purpose of this book, temperature is expressed in °C (Celsius),
rather than in oK (Kelvin), where 273.15°K = O°C. The change of 1°C is the
same as a change of 1 0 K. Apart from degrees Celsius, degrees Fahrenheit (OF)
are sometimes used. The conversion between these units is as follows:
(B.3)
(B.4)
B.3 Derived Units
Many other quantities useful in fluid mechanics have derived units, which are
defined in terms of the base units. We list now some of them which are used
in this book.
B.3.1 Frequency
Frequency is the number of oscillations of a system during one second and it is
expressed in hertz (Hz), where 1 hertz = 1 cycle/1 second. In this text, angular
frequency is also used. The units of angular frequency are 1 radian/l second.
As 27r radians = 1 full cycle = 360°, therefore:
360°
180°
180°
1 radian = - - = - - = - - ~ 57.3°.
27r
7r
3.1416
(B.5)
B.3.2 Velocity
Velocity defines the rate at which the position changes with time and it is
expressed in metres per second (m/ s). Traditionally, in sailing and shipping,
and sometimes in oceanography, the velocity of wind, ocean currents and the
speed of the vessel is expressed in knots, where:
1 nautical mile
1852 m
1 knot =
= - - = 0.51 m/s ~ 0.5 m/s.
1 hour
3600 s
(B.6)