Physics Lesson 1.2.1 - Length

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Welcome to our Physics lesson on Length, this is the first lesson of our suite of physics lessons covering the topic of Length, Mass and Time, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.


By definition, length is the measurement or extent of something from end to end; the greater of two or the greatest of three dimensions of an object. Thus, for example, in a rectangle, length represents the extent from end to end of the longest side. However, in Physics length is used to express the numerical value of a measurement taken by using a ruler, metre stick, distance sensor etc. We say "length of wire", "length of rope" "length of my finger" etc., which means the term "length" is not necessarily related to geometrical shapes. It has a wider use in physics.

As we have discussed in the article "Units. Systems of Units. Fundamental and Derived SI Units", the unit of length in the SI system is "metre", (abbrev. m). Metre is an arbitrary established unit of length. According to the most recent definition, a meter is defined as the distance traveled by light in vacuum during a time interval of 1/299, 792, 458 of a second. The original metre represents the distance between two engraved lines on a platinum-iridium bar now kept at Serves near Paris at the International Bureau of weights and measures.

Units of length change 10 by 10. We use one of the prefixes discussed in the abovementioned article to express multiples and submultiples of metre. The most used of them are:

kilometre (km) = 1000 m = 103 m
hectometre (hm) = 100m = 102 m
decametre (dam) = 10m = 101 m
decimetre (dm) = 0.1m = 10-1 m
centimetre (cm) = 0.01m = 10-2 m
millimetre (mm) = 0.001m = 10-3 m

Other popular units of length are: micrometre μm = 10 -6m, nanometre nm = 10-9m, Angstrom Å = 10-10m, Astronomic unit au (the distance from the Earth to the Sun) = 1.5 × 1011m, and Light year ly = 9.461 × 1015m.

More Length, Mass and Time Lessons and Learning Resources

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  7. Continuing learning units and measurements - read our next physics tutorial: Significant Figures and their Importance

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