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2.3: Untitled Page 15

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    18148
  • Chapter 2

    necessarily used in the creation of alternate units, and this leads to complications which in turn lead to errors.

    Table 2‐2. Alternate Units of Length

    1 kilo meter (km) = 103 meter (m)

    1 deci meter (dm) = 10‐1 m

    1 centi meter (cm) = 10‐2 m

    1 milli meter (mm) = 10‐3 m

    1 micro meter (μm) = 10‐6 m

    1 nano meter (nm) = 10‐9 m

    2.1.2 Systems of units

    If we focus our attention on the fundamental standards and ignore the electric charge, we can think of the SI (Système International) system as dealing with length, mass and time in terms of meters, kilograms and seconds. At one time this was known as the MKS‐system to distinguish it from the CGS‐system in which the fundamental units were expressed as centimeters, grams and seconds. Another well‐known system of units is referred to as the British (or English) system in which the fundamental units are expressed in terms of feet, pounds‐mass, and seconds. Even though there was general agreement in 1960 that the SI system was preferred, and is now required in most scientific and technological applications, one must be prepared to work with the CGS and the British system, in addition to other systems of units that are associated with specific technologies.

    2.2 Derived Units

    In addition to using some alternative units for length, time, mass and electric charge, we make use of many derived units in the SI system and a few are listed in Table 2‐3. Some derived units are sufficiently notorious so that they are named after famous scientists and represented by specific symbols. For example, the unit of kinematic viscosity is the stokes (St), named after the British mathematician Sir George G. Stokes (Rouse and Ince, 1957), while the equally important molecular and thermal diffusivities are known only by their generic names and represented by a variety of symbols. The key point to remember concerning units is that the basic units represented in Table 2‐1 are sufficient to describe all physical phenomena, while the alternate units illustrated Table 2‐2

    and the derived units listed in Table 2‐3 are used as a matter of convenience.

    Units

    17

    Table 2‐3. Derived SI Units

    Physical Quantity

    Unit ( Symbol)

    Definition

    force

    newton (N)

    kg m/s2

    energy

    joule (J)

    kg m2/s2

    power

    watt (W)

    J/s

    electrical potential

    volt (V)

    J/(A s)

    electric resistance

    ohm ()

    V/A

    frequency

    hertz (Hz)

    cycle/second

    pressure

    pascal (Pa)

    N/m2

    kinematic

    stokes (St)

    cm2/s

    viscosity

    thermal

    square meter/second

    m2/s

    diffusivity

    molecular

    square meter/second

    m2/s

    diffusivity

    While the existence of derived units is simply a matter of convenience, this convenience can lead to confusion. As an example, we consider the case of Newton’s second law which can be stated as

    time rate of change

    force acting

      of linear momentum

    (2‐5)

    on a body

     of the body

    

    In terms of mathematical symbols, we express this axiom as

    d

    f

    mv

    (2‐6)

    dt

    Here we adopt a nomenclature in which a lower case, boldface Roman font is used to represent vectors such as the force and the velocity. Force and velocity are quantities that have both magnitude and direction and we need a special notation to remind us of these characteristics.

    Let us now think about the use of Eq. 2‐6 to calculate the force required to accelerate a mass of 7 kg at a rate of 13 m/s2. From Eq. 2‐6 we determine the magnitude of this force to be

    2

    2

    f

     (7 kg)(13m/s )  91 kg m/s

    (2‐7)

    18