science

Metre

Metre 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 Metre rather than just read about it. In short: The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of ⁠1/299792458⁠ of a second, where the second is defined by a hyperfine transition frequency of caesium.

Key takeaways

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

Reference excerpt

The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of ⁠1/299792458⁠ of a second, where the second is defined by a hyperfine transition frequency of caesium. The metre was originally defined in 1791 by the French National Assembly as one ten-millionth of the distance from the equator to the North Pole along a great circle through Paris, setting 10000 km as that quarter of the Earth's polar circumference. In 1799, the metre was redefined in terms of a prototype metre bar. The bar used was changed in 1889, and in 1960 the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. The current definition was adopted in 1983 and modified slightly in 2002 to clarify that the metre is a measure of proper length. From 1983 until 2019, the metre was formally defined as the length of the path travelled by light in vacuum in ⁠1/299792458⁠ of a second. After the 2019 revision of the SI, this definition was rephrased to include the definition of a second in terms of the caesium frequency ΔνCs.

Etymology and spelling The etymological roots of metre can be traced to the Greek verb μετρέω (metreo) ((I) measure, count or compare) and noun μέτρον (metron) (a measure), which were used for physical measurement, for poetic metre and by extension for moderation or avoiding extremism (as in "be measured in your response"). This range of uses is also found in Latin (metior, mensura), French (mètre, mesure), English (meter for measuring instruments, but metre or meter in poetry) and other languages. The Greek word is derived from the Proto-Indo-European root *meh₁- 'to measure'. In English, the use of the word metre (for the French unit mètre) began at least as early as 1797. Metre is the standard spelling of the metric unit for length in all English-speaking nations except the United States and the Philippines, which use meter.

History of definition

SI prefixed forms of metre

SI prefixes can be used to denote decimal multiples and submultiples of the metre, as shown in the table below. Long distances are usually expressed in km, astronomical units (149,597,871 km), light-years (63,000 au; 9.5 trillion km), or parsecs (210,000 au; 31 trillion km), rather than in Mm or larger multiples. "30 cm", "30 m", and "300 m" are more common than "3 dm", "3 dam", and "3 hm", respectively. The terms micron and millimicron have been used instead of micrometre (μm) and nanometre (nm), respectively, but this practice is discouraged.

Equivalents in other units

Within this table, "inch" and "yard" mean "international inch" and "international yard" respectively, though approximate conversions in the left column hold for both international and survey units.

"≈" means "is approximately equal to"; "=" means "is exactly equal to". One metre is exactly equivalent to ⁠5 000/127⁠ inches and to ⁠1 250/1 143⁠ yards. A simple mnemonic to assist with conversion is "three 3s": 1 metre is nearly equivalent to 3 feet 3+3⁄8 inches. This gives an overestimate of 0.125 mm. The ancient Egyptian cubit was about 0.5 m (surviving rods are 523–529 mm). Scottish and English definitions of the ell (2 cubits) were 941 mm (0.941 m) and 1143 mm (1.143 m) respectively. The ancient Parisian toise (fathom) was slightly shorter than 2 m and was standardised at exactly 2 m in the mesures usuelles system, such that 1 m was exactly 1⁄2 toise. The Russian verst was 1.0668 km. The Swedish mil was 10.688 km, but was changed to 10 km when Sweden converted to metric units.

See also ISO 1 – standard reference temperature for length measurements Metric prefix Metrication

Notes

References

Cited bibliography

External links

The dictionary definition of metre at Wiktionary

Illustrations

Worked examples

Example 1 — a first encounter with Metre

Start with the simplest possible case. Write down what Metre 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 Metre 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 Metre 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 Metre

In research: Metre 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 Metre 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: Metre is common in secondary-school and first-year university syllabi. It links to neighbouring topics 1790s introductions, Metre, SI base units, so understanding it makes those chapters shorter.
In everyday life: Look for Metre 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 Metre in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Metre 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 Metre out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Metre in simple terms?

Why does Metre 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 Metre?

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 Metre.