526
APPENDIX A. METRIC (SI) SYSTEM
used in coastal engineering applications. For other SI standards, refer to
the cited references.
A.2 SI Units and Symbols
The standard SI units and symbols are given in Table A.l. This table
includes the base units, the supplementary units, and the commonly used
derived units. The SI symbols on the table are given in their preferred
form, i.e., upper or lower case. Some of the derived units in Table A.l can
be expressed in terms of other units as well as in terms of base units. It is
best to use, and to become familiar with, the unit symbols as given in the
table. For example, pressure (or stress) could be expressed as newtons per
square meter (N/m2), but it preferable to use pascals (Pa).
A.3 Units Accepted for Limited Use
Some units from different systems are accepted for limited use in the SI
system. These are shown in Table A.2. In certain applications these non-SI
units have distinct advantages, such as degrees for plane angles, but their
usage should be kept to a minimum with the preferred SI unit being used
when possible. For example, liter is restricted to measurement of liquids and
gases, and no prefix other than milli- should be used with liter. Likewise,
the term hectare (square hectometer) is limited to measurement of land or
water areas.
A.4 Mass, Force, and Weight
The main source of confusion in using SI units is the relationship between
kilograms and newtons. The kilogram is a unit of mass, and the newton is
a unit of force. Mass is a property of a body and remains constant for that
body. When a body having a certain mass is accelerated, then it exerts a
force which is given by Newton’s Second Law as:
F — m a
where
F - force (newtons)
m - mass (kilograms)
a - acceleration ( meters per second2)
APPENDIX A. METRIC (SI) SYSTEM
used in coastal engineering applications. For other SI standards, refer to
the cited references.
A.2 SI Units and Symbols
The standard SI units and symbols are given in Table A.l. This table
includes the base units, the supplementary units, and the commonly used
derived units. The SI symbols on the table are given in their preferred
form, i.e., upper or lower case. Some of the derived units in Table A.l can
be expressed in terms of other units as well as in terms of base units. It is
best to use, and to become familiar with, the unit symbols as given in the
table. For example, pressure (or stress) could be expressed as newtons per
square meter (N/m2), but it preferable to use pascals (Pa).
A.3 Units Accepted for Limited Use
Some units from different systems are accepted for limited use in the SI
system. These are shown in Table A.2. In certain applications these non-SI
units have distinct advantages, such as degrees for plane angles, but their
usage should be kept to a minimum with the preferred SI unit being used
when possible. For example, liter is restricted to measurement of liquids and
gases, and no prefix other than milli- should be used with liter. Likewise,
the term hectare (square hectometer) is limited to measurement of land or
water areas.
A.4 Mass, Force, and Weight
The main source of confusion in using SI units is the relationship between
kilograms and newtons. The kilogram is a unit of mass, and the newton is
a unit of force. Mass is a property of a body and remains constant for that
body. When a body having a certain mass is accelerated, then it exerts a
force which is given by Newton’s Second Law as:
F — m a
where
F - force (newtons)
m - mass (kilograms)
a - acceleration ( meters per second2)