Length
Length is a basic scalar quantity that measures a linear dimension, i.e. the amount of extension in one principal direction rather than in all three directions of space.
Length is quantified by comparing the object to be measured with an appropriate reference instrument.
To better describe the concept of length and the quantities that derive from it, let us consider some examples.
If you are placing desks in a classroom, you may want to know the length and width of the desks (rudimentary amounts), or if you are working at a desk, you may want to know the surface area that supports the desk (i.e. are you interested in the derived quantity, the area). The physical quantity at the basis of the measurement of these properties is precisely the length.
Meter, multiples and submultiples
The SI unit of length is the metre. Over the centuries, the meter has had many definitions. In the 18th century it was defined as one ten-millionth of the distance between the earth's poles and the equator along the meridian that runs through Paris. In the 19th century, the Sevres meter (Paris) was defined, consisting of a platinum-iridium rod, the temperature of which was maintained at 0 ° C, the length of which corresponded to the previously given definition, and all samples from other countries. . The XI International Conference on Weights and Measures, held in Paris in 1960, defined the meter as the length equal to 1650763.73 wavelengths of the red-orange line of krypton 86 in vacuum. The most recent definition dates back to 1983 given in the XVII International Conference of Weights and Measures. One meter is defined as the distance light travels in a vacuum in 1/299792485 of a second. The metric system is officially used in physics to measure length. Naturally, when studying physical phenomena involving length, it is necessary to decide whether to use multiples or divisors of the meter to facilitate the management of the studied quantity. For example, if you're estimating the distance between two cities, you should measure it in kilometers (1km = 1000m), not meters. Alternatively, the length of a pencil should be expressed in centimeters.