Units

3 Key excerpts on "Units"

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  eBook - PDF

  Introduction to Physics

  Mechanics, Hydrodynamics Thermodynamics

  • P. Frauenfelder, P. Huber(Authors)
  • 2013(Publication Date)
  • Pergamon

    (Publisher)

  Hence a definition is the precise formulation of a scientific concept. Quantities which can vary in space or time (most quantities can) must be defined in differential form. To correlate data obtained in the laboratory with the results of applica-tions of the basic principles and empirical laws to real processes, we must consider the theory of errors (see Section 7). 2. T H E THREE STANDARDS OF MEASUREMENT Space, time, and matter (mass) are the independent elements of mechanics. The Units of space, time, and mass are then the independent basic Units of measurement and thus have a fundamental significance. From them, we ι 2 I N T R O D U C T I O N T O P H Y S I C S can derive all the other Units of mechanics by using the basic principles and empirical laws. Physical phenomena can be understood either on a qualitative level, by careful description without the use of mathematics, or on a quantitative level, by means of numerical verification of the relationships in question. To measure a quantity means to compare it with a standard unit of that quantity. The fundamental Units are established by means of definitions; that is, they are completely arbitrary. However, the definitions should be formulated so that the Units are as exact as possible, can be easily reproduced, and lead to simple numerical values for the most commonly occurring measurements. 2-1 The Standard of Length In the classical physics of Newton, the concept of space is understood to be intuitively obvious. It exists independently of time and of any objects embedded in it. A further postulate of classical physics, and one which is broadly supported by experience, is that the geometry of space is Eucli-dean.f In Euclidean geometry, the measurement of space is based on a unit of length. A definition of the standard of length determines the Units of area and volume as well as the unit of length.

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  eBook - ePub

  The Reform of the International System of Units (SI)

  Philosophical, Historical and Sociological Issues

  • Nadine de Courtenay, Olivier Darrigol, Oliver Schlaudt(Authors)
  • 2019(Publication Date)
  • Routledge

    (Publisher)

  The definition of a unit as in equation (4) or equation (5), that does not refer to the relations in the system of quantities, is independent, and dependent is instead a definition, as in equation (6), which does refer to such relations. For example, a definition of length unit based on the relation of length with time and velocity is a dependent definition.

  2.4 Concepts related to the realization of the definition of quantity Units

  The main concepts assumed related to the realization of the definition of quantity Units are:

  1. quantity realizing the definition of a unit;
  2. object realizing the definition of a unit;
  3. calibration;
  4. metrological traceability chain;
  5. calibration uncertainty.

  A measurement process is based on a measurement procedure that specifies a way for comparing the measurand and the unit. This comparison implies the availability of a quantity that realizes the definition of the unit through the realization of a support of that quantity. The object realizing the definition of a unit is called a measurement standard. The comparison with a unit is then generally obtained by realizing a chain of measurement standards, from a primary one to the working standard used in measurement and calibrated against the primary standard through a metrological traceability chain. In the processes of calibration that yields the chain, a calibration uncertainty is produced that generally increases in each step of the chain from the primary standard to the standard involved in the measuring system.

  3 Basic criteria

  The main operative target of the definition of a system of Units is to provide a basis for measurement, for which quantity Units are indeed essential.10

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  eBook - ePub

  Trophic Interactions Within Aquatic Ecosystems

  • Ahmad Hemami(Author)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  Almost everything that we need for living is quantified by measurement, from all those items that we count to what we buy by weight, volume, and so on. Electricity is not an exception, and for various entities that we will discuss, measurement becomes essential. You are already aware of many of the measureable items such as weight, length, and time.

  For measuring anything we need to define a specified quantity of that as the unit of measurement. For instance, one foot is a well-defined unit for length, based on which we can measure and perceive how much, for example, 2, 15, and 1000 ft are. Indeed, without a unit of measure, no measurement can have any meaning.

  Among all the entities that we measure, some can be considered principal, from which other items can be derived. We have already seen examples of this in the measurement of length: If the unit for measuring length is feet (ft), then for area we can use ft2 and for volume ft3 may be employed. In the metric system we have meter (m) for length, and m2 and m3 for area and volume, respectively. Another example is the unit for measuring speed of something moving. If meter (foot) is the unit for measuring length (or distance) and second is the unit for measuring time, then meters per second (m/sec) or feet per second (ft/s) can be used for the unit of speed.

  The most common entities that we use in our daily life and measure are length (distance, height), area, volume, speed, acceleration, force, weight (which has the same unit as force), mass, density, energy, time, and temperature. Other specialized measurements depend on the discipline; for example, in physics, mechanics, chemistry, and electricity, there are other entities that need to be measured, and each one has its unit of measure.

  To measure what we need to measure, a system of measurement is defined, in which the Units are established. There are a few major systems of measurement, some of which are becoming obsolete. Nowadays, the metric system is adopted by most of the industrialized countries. The imperial system (U.S. customary Units) is another system of Units that it is still used in the United States (and some other countries). One of the main objectives of this chapter is to facilitate understanding of the metric system Units and the conversion from the U.S. customary Units to their equivalent metric values for those who are not yet familiar with the metric system.

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