Units & Measurement

UNITS & MEASUREMENTS:

UNITS (dimension standard):                      

A quantitative description of physical things.

To get something accurately, one has to measure the physical quantity and has to represent that measurement by a number. Such a measurement is made by comparing the quantity with a standard; this standard is called a unit. The result of each measurement should be expressed in terms of units.

For example, mass is a physical quantity that refers to the quantity of matter contained in an object. The unit kilogram (kg) is one of the standard units used to measure mass.

Note: Standard unit is a universally (worldwide) accepted quantifiable language that helps everyone to understand the association of the object with the measurement. The objective of using it is to provide and maintain a common language of measurement. Standard units are made up of 7 base units which are used for defining 22 derived units.

One meter is the distance traveled by light in vacuum during the time interval of 1/299 792 458 of a second

The International System of Units (SI Units)

Seven physical quantities have been selected as base quantities in the 14th General Conference on Weights and Measurements, held in France in 1971. These quantities form the basis of the International System of Units, abbreviated as SI (from its French name Systeme International) and popularly known as the metric system.

Standards of Length, Time, and Mass

Length (L): In 1983, the precision of the meter was redefined as the distance traveled by a light wave in vacuum in a specified time interval.

Time (T): Standard of time derived from a device known as atomic clock. Cesium is most common element that is typically used in the construction of atomic clocks because it allows us to attain high accuracy. The SI system fixed the unit of time as Second.

One Second is the time taken for the cesium atom Cs to perform 9 192 631 770 oscillations to emit radiation of a specific wavelength.

Quantities & its Units:

Physical quantity (mass, volume or density) is the result of a measurement.

Physical quantity = Numerical value x Unit

Examples are:

Mass of Water: 25 liter

Mass of Iron: 2 kg

Density of Copper: 8.96 gm/cm3

Units are treated like algebraic quantities, and may be multiplied, divided, and cancelled. Thus, the expression (physical quantity/unit) is simply the numerical value of the measurement in the specified units, and hence is a dimensionless quantity.

Mass, Volume & Density:

Mass (M): a measure of the quantity of matter in an object. The term “mass” and “weight” are often used interchangeable, in actual scientific meaning, weight is the force that exerts by gravity on an object. The value of weight can differ from planet to planet.

One Kilogram: The SI unit of mass, 1 kilogram, is defined as the mass of a platinum-iridium alloy cylinder kept at the International Bureau of Weights and Measures in France.

A second standard Mass

Atomic masses can be compared with each other more precisely than the kilogram.  By international agreement, the carbon-12 atom, 12C, has been assigned a mass of 12 atomic mass units of carbon-12.

MKS System, CGS System & FPS system:

The MKS (meter, kilogram, second) system of units is the fundamental core of the SI system of measurement. There are two other major systems of units besides the SI units.

-        CGS system

-        FPS system

The CGS (cent-meter, gram, second) system of units is a variant of the metric system. These are derived from fundamental base units. FPS system of units uses the foot, pound, and second as its base units.

Although, MKS system is more popular than the CGS system, each has its own application. For example: In measuring the volume of liquids and solids the liter and the cubic meters are always used whereas their counterparts in the CGS system, the cubic-centimeter is less often used. In contrast to this, grams and milligrams are usually used in medicine than kilogram. The advantages of MKS system over CGS system is that we cannot measure long distance or heavy weight using CGS system.

Volume can be defined as the three dimensional space enclosed by a boundary or occupied by an object. The volume of an object is measured in cubic units such as cubic centimeters, cubic inch, cubic foot, cubic meter, etc.

Density:

Density is the weight of a substance for a specific volume. It is a measure of mass (m) per volume. It is determined by dividing mass with its volume (v). (d = m/v). For example, the density of water is 1 gram per cubic centimeter, the density of earth is 5.51 grams per cubic centimeter.