Units of Measurement: Detailed Notes

What Is a Measurement?

Measurement is a quantifiable amount of a physical magnitude. Measurements are formed by a quantity and a unit. Measurements are a key concept related to defining quantities in a uniformed language.

Carl Sagan, the famous astrophysicist, said that human beings are a way that we allow the Cosmos to know itself. Measurement is relevant to the sciences, especially physics, and therefore defines our understanding of the cosmos. It allows us to formulate our theories and test our hypotheses.

What Is a Unit?

Measurements are made to compare and to know an unknown quantity This quantifiable quantity is fixed and is called a unit. The need to have a uniform measurement all over the world has led to the development of units.

As different languages are used to express the same idea in different ways, quantities are expressed in different units. The different magnitudes (length, area, mass, volume, etc) will be quantified using units. The same quantities have a different representation depending on the unit used. Each type of magnitude has its compatible units. Therefore, it is necessary to determine a unit and quantity to give greater precision. 

Important Measurements

How do we measure the great distances between the moon and the earth? How do we measure the masses of huge bodies like the sun? The measurement of length, mass and time can be complex and require indirect methods. 

Measurement of Large Distances

The huge distances between the Sun and the Earth are measured by indirect methods, in this case the parallax method. Parallax measures the displaced distance of an object’s position from the observer’s location.

Mass Measurements

The mass of an object can be found using Newton’s laws. To measure large quantities of matter, indirect methods are again used. In this case, the method uses Newton’s law of gravitation as a basis.

International System of Units

The International System of Units, also called SI, sets a list of standard units for measuring all kinds of physical quantities. The current SI system is the system of units that was defined in 1960 during the 11th General Conference of Weights and Measures. The system was based on the former decimal metric system (CGS). The system has been adopted internationally and is used in scientific practice. The SI encompasses a number of magnitudes/units called basic and defines the so-called derived units. The SI is a system of units that makes it possible to quantify any measurable quantity of interest in research, industry, commerce or society, in fields as varied as health, safety, environmental protection, the acquisition of goods or the invoicing of consumption, etc.

The SI Unit prefixes

In addition to the base units, the SI system also proposes several prefixes for the base units to express different quantities more adequately and accurately. The prefixes serve the function of expressing multiplications or fractions of the units. There are at least 20 accepted prefixes. These prefixes are shown in the following table:

The Anglo-Saxon system 

There is also a system of measurements recognised in some countries, such as the United States and in many English-speaking territories (such as the United Kingdom). This is the English System, where the units are not metric. However, within these systems there are some discrepancies between the US and English systems. This system was born out of the use of measurements using local units over the centuries. Recently, efforts have been made to standardise the units in England, with some of the units tracing their origins back to ancient Rome. However, this system has slowly begun to be replaced by the International System of Units. In the United States, the inertia of the old system and the high cost of migration have prevented change. 

Units and Dimensional Analysis: Unit Conversion

Dimensional Analysis allows us to have a base unit by applying a series of simple calculations. Thanks to this analysis we can convert a quantity expressed in one unit into an equivalent quantity in another unit. These calculations are done by applying a conversion factor that indicates the relationship between the units.

Original quantity x conversion factor = equivalent quantity

This uses the assumption that there is an equality between a unit and its conversion factor with respect to another unit to express the same physical quantity.