science

Kilometres per hour

Kilometres per hour is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Kilometres per hour rather than just read about it. In short: The kilometre per hour (alternatively spelt kilometer per hour; SI symbol: km/h; abbreviations: kph, kmph, km/hr) is a unit of speed, expressing the number of kilometres travelled in one hour. History Although the metre was formally defined in 1799, the term "kilometres per hour" did not come into immediate use – the myriametre (10,000 metres) and myriametre per hour were preferred to kilometres and kilometres per h…

Key takeaways

Kilometres per hour belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Kilometres per hour to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Kilometres per hour from memory before moving on to harder problems.

Reference excerpt

The kilometre per hour (alternatively spelt kilometer per hour; SI symbol: km/h; abbreviations: kph, kmph, km/hr) is a unit of speed, expressing the number of kilometres travelled in one hour.

History Although the metre was formally defined in 1799, the term "kilometres per hour" did not come into immediate use – the myriametre (10,000 metres) and myriametre per hour were preferred to kilometres and kilometres per hour. In 1802 the term "myriamètres par heure" appeared in French literature. The Dutch on the other hand adopted the kilometre in 1817 but gave it the local name of the mijl (Dutch mile).

Notation history

The SI representations, classified as symbols, are "km/h", "km h−1" and "km·h−1". Several other abbreviations of "kilometres per hour" have been used since the term was introduced and many are still in use today; for example, dictionaries list "kph", "kmph" and "km/hr" as English abbreviations. While these forms remain widely used, the International Bureau of Weights and Measures uses "km/h" in describing the definition and use of the International System of Units. The entries for "kph" and "kmph" in the Oxford Advanced Learner's Dictionary state that "the correct scientific unit is km/h and this is the generally preferred form".

Abbreviations Abbreviations for "kilometres per hour" did not appear in the English language until the late nineteenth century. The kilometre, a unit of length, first appeared in English in 1810, and the compound unit of speed "kilometers per hour" was in use in the US by 1866. "Kilometres per hour" did not begin to be abbreviated in print until many years later, with several different abbreviations existing near-contemporaneously.

With no central authority to dictate the rules for abbreviations, various publishing houses and standards bodies have their own rules that dictate whether to use upper-case letters, lower-case letters, periods and so on, reflecting both changes in fashion and the image of the publishing house concerned, In contrast to the "symbols" designated for use with the SI system, news organisations such as Reuters and The Economist require "kph". In informal Australian usage, km/h is more commonly pronounced "kays" or "kays an hour". In military usage, "klicks" is used, though written as km/h.

Unit symbols In 1879, four years after the signing of the Treaty of the Metre, the International Committee for Weights and Measures (CIPM) proposed a range of symbols for the various metric units then under the auspices of the General Conference on Weights and Measures (CGPM). Among these were the use of the symbol "km" for "kilometre". In 1948, as part of its preparatory work for the SI, the CGPM adopted symbols for many units of measure that did not have universally agreed symbols, one of which was the symbol "h" for "hours". At the same time the CGPM formalised the rules for combining units – quotients could be written in one of three formats resulting in "km/h", "km h−1" and "km·h−1" being valid representations of "kilometres per hour". The SI standards, which were MKS-based rather than CGS-based, were published in 1960 and have since then have been adopted by many authorities around the globe including academic publishers and legal authorities. The SI explicitly states that unit symbols are not abbreviations and are to be written using a very specific set of rules. M. Danloux-Dumesnils provides the following justification for this distinction:

It has already been stated that, according to Maxwell, when we write down the result of a measurement, the numerical value multiplies the unit. Hence the name of the unit can be replaced by a kind of algebraic symbol, which is shorter and easier to use in formulae. This symbol is not merely an abbreviation but a symbol which, like chemical symbols, must be used in a precise and prescribed manner. SI, and hence the use of "km/h" (or "km h−1" or "km·h−1") has now been adopted around the world in many areas related to health and safety and in metrology in addition to the SI unit metres per second ("m/s", "m s−1" or "m·s−1"). SI is also the preferred system of measure in academia and in education.

Non-SI abbreviations in official use km/j or km/jam (Indonesia and Malaysia) km/t or km/tim (Norway, Denmark and Sweden; also use km/h) kmph (Sri Lanka and India) กม./ชม. (Thailand; also uses km/hr) كم/س or كم/ساعة (Arabic-speaking countries, also use km/h) קמ"ש (Israel) км/ч (Russia and Belarus in a Russian-language context) км/г (Belarus in a Belarusian-language context) км/год (Ukraine) km/st (Azerbaijan) km/godz (Poland)

Regulatory use

During the early years of the motor car, each country developed its own system of road signs. In 1968 the Vienna Convention on Road Signs and Signals was drawn up under the auspices of the United Nations Economic and Social Council to harmonise road signs across the world. Many countries have since signed the convention and adopted its proposals. Speed limits signs that are either directly authorised by the convention or have been influenced by the convention are shown below:

In 1972 the EU published a directive (overhauled in 1979 to take British and Irish interests into account) that required member states to abandon CGS-based units in favour of SI. The use of SI implicitly required that member states use "km/h" as the shorthand for "kilometres per hour" on official documents. Another EU directive, published in 1975, regulates the layout of speedometers within the European Union, and requires the text "km/h" in all languages, even where that is not the natural abbreviation for the local version of "kilometres per hour". Examples include:

Dutch: "kilometer per uur" ("hour" is "uur" – does not start with "h"), Portuguese: "quilómetro por hora" ("kilometre" is "quilómetro" – does not start with "k") Irish: "ciliméadar san uair" Greek: "χιλιόμετρα ανά ώρα" (a different script). In 1988 the United States National Highway Traffic Safety Administration promulgated a rule stating that "MPH and/or km/h" were to be used in speedometer displays. On May 15, 2000, this was clarified to read "MPH, or MPH and km/h". However, the Federal Motor Vehicle Safety Standard number 101 ("Controls and Displays") allows "any combination of upper- and lowercase letters" to represent the units.

Conversions 3.6 km/h ≡ 1 m/s, the SI unit of speed, metre per second 1 km/h = 0.27 m/s 1 km/h ≈ 0.62137 mph ≈ 0.91134 ft/s 1 kn ≡ 1.852 km/h (exactly) 1 mph ≡ 1.609344 km/h (* = approximate values)

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Kilometres per hour

Start with the simplest possible case. Write down what Kilometres per hour claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Kilometres per hour before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Kilometres per hour ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Kilometres per hour

In research: Kilometres per hour appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Kilometres per hour in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Kilometres per hour is common in secondary-school and first-year university syllabi. It links to neighbouring topics Non-SI metric units, SI derived units, Units of velocity, so understanding it makes those chapters shorter.
In everyday life: Look for Kilometres per hour outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Kilometres per hour in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Kilometres per hour means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Kilometres per hour out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Kilometres per hour in simple terms?

The kilometre per hour (alternatively spelt kilometer per hour; SI symbol: km/h; abbreviations: kph, kmph, km/hr) is a unit of speed, expressing the number of kilometres travelled in one hour. History Although the metre was formally defined in 1799, the term "kilometres per hour" did not come into…

Why does Kilometres per hour matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Kilometres per hour?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Kilometres per hour.