# Questions tagged [units]

A unit is a quantity chosen as a standard in terms of which other quantities may be represented. Examples include Joules, Moles, Seconds, etc.

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### Why is there no special unit there for velocity/speed?

Heat, time, and electrical current have separate units, namely, calorie, second and ampere, respectively. However, there is no special unit for velocity. We still use $\frac{\rm m}{\rm s}$. Why? ...
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### Weights & Measures - barleycorn GRAIN

The medieval and non-metric Imperial data is that 7000 grains make a Pound weight avoirdupois; 5760 grains for Troy & Apothecaries. Inconveniently, the only scientific data I can find of a fair ...
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### What makes the right angle special enough to be distinguished in the French metric system?

When introducting the metric system, the French tried to decimalise the degrees used for angles. They defined the right angle to contain 100 gradians. Why was the right angle chosen? A somewhat ...
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### Motivation behind original definition of metre? [duplicate]

The first original definition of the metre, presented by the French Academy of Sciences in 1795 is: The length of the metre is one ten-millionth of the Earth quadrant, the distance from the North ...
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1 vote
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### Were units of area/volume always in terms of squares/cubes?

Throughout our known history of geometry were the units representing areas and volumes always in terms of squares and cubes respectively? Take ancient Egyptian formulas as an example, the fact that ...
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### In which units did Sir Isaac Newton define force at that time as SI system didn't exist then?

Sir Isaac Newton led the foundation of his famous laws of motion during the 17th Century but at that time SI system hadn't existed. So in which units did he define force? Did he define it in some ...
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### Why historically the hour was divided into 60 minutes and when it had started? [duplicate]

Why and when was the hour divided into exactly 60 minutes (and not for example 70 or 80)?
1 vote
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### The unit Ångström

The website http://ucum.org/ucum.html tells us whether every unit is metric or not. Metric units can be multiplied by a power of 10 and can be combined with a prefix. One ångström is defined as 0.1 ...
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### Names of the electromagnetic units in SI

The unit of electric charge is the coulomb, named after Charles Augustin de Coulomb. This makes sense because Coulomb's law talks about the force between two charges. Likewise, it also makes sense to ...
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### Were the kilogram, the grave, etc. meant to be units of weight, units of mass, or was it ambiguous at the time?

It is sometimes stated that the early metric units of 'weight' really were meant to be exactly that: units of weight (i.e. force), not mass. However, is that really so? In fact, did the people ...
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### Why is one meter as long as it is?

The metre is defined as the length of the path travelled by light in a vacuum in 1/299 792 458 of a second Why is this so? Who decided that 1/299,792,458 of a ...
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### Why volt instead of volta?

Why is the volt not identical to the full name Volta, unlike the other electrical units ohm, ampere, coulomb, tesla, weber and henry? Is there a historical explanation, was the volt introduced at a ...
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### When did non-SI double prefixes go out of use?

In old physics and engineering publications from the 1950s or so, it's common to see non-SI "double prefixes", such as a "millimicrosecond pulse", or a "10 micromicrofarad" capacitor. These units are ...
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### Why are revolutions per minute (RPM) still used instead of hertz (Hz)?

When did people start using Revolutions per Minute (RPM) to measure motors, engines, other devices and where did the term originate? Why do we continue to use it instead of an SI unit like Hz? From ...
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1 vote
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### Where did the notation for dimensions of a quantity come from?

The current SI Brochure defines the dimension of a certain physical quantity as something separate from its units, and treated in detail in its §2.3.3: This is, by now, pretty bread-and-butter ...
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### When and how did usage of the term Centigrade give way to Celsius? Are/were they in fact numerically identical?

Discussion below the question Does the US National Weather Service use Celsius or Fahrenheit? and correction of my original use of "Centigrade" to the modern Celsius lead me here. For ...
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### Why does the US keep survey miles and feet?

The United States adopted the international mile and foot in 1959, but for some reason they decided to keep alongside the older units as survey mile and survey foot. These units are almost equal to ...
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### Did the French ever use the decimal time in science?

Have you ever seen a scientific paper/document that uses the decimal time introduced during the French Revolution? I'm not looking for any paper that uses decimal time like in astronomy, but instead ...
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1 vote
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### Was the value of the mole invented or discovered in chemistry?

For example, $\pi$ is not an invention, it is a discovery which was natural, that is ratio of the circumference of a circle to its diameter. But when we define a meter it is not a natural value it is ...
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### How was the historical definition of the meter used in practice?

The meter was initially defined as $10^{-7}$ times the distance of the north pole to the equator. How exactly was this definition used to fabricate the actual meter sticks from which the standard ...
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### What if we measure a physical constant to higher accuracy? [closed]

So, the SI system of units has the basic philosophy of defining all 7 of the base units in terms of universal constants such as the plank constant, the speed of light, etc. These are all measured, ...
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### Metre definition using a seconds pendulum

I have always heard that the first and most prominent definition of the metre was to use the length of the seconds pendulum - pendulum with the period of exactly 2 seconds. However, in the end it was ...
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### Why is an inch (in the English Imperial system of measure) as long as it is?

My question is about the length of the inch which is a subunit of the Imperial foot. Is there any connection whatsoever between the Imperial system for units of measure and the dimensions of the ...
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1 vote
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### On Units of Arc

Why aren't turns used more often in math? It seems much more fundamental of a unit compared to degrees and radians. The value of a turn is definite, and other units can be derived from it. The problem ...
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### Why not proton volt instead of $eV$?

Why is eV used instead of proton V even if they would give the same value? I have this view: from electrochemistry's Faraday's law, scientists first calculated the charge of an electron. Then, they ...
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### Old square bracket notation for units

As discussed in this answer https://physics.stackexchange.com/a/77691/667 there are several common conventions for the notation $[q]$ of a physical quantity $q$. However, I often see people to put ...
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### Did English ever use a third (1/60 of a second) for measuring time?

An hour has been divided in sixty minutes since medieval times. During the 16th and 17th century, clocks measuring a second subdivision emerged. Today we still use the sexagesimal system for ...
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### Why is the SI prefix k- lower case?

In the SI unit prefixes, there's a general pattern of using uppercase prefixes for multipliers larger than 1 and lower case for prefixes that are smaller than one. However, this is not a universal ...
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### "Cycles" to "Hertz": Why the shift circa 1970?

Before circa 1970, it was more common to say "kilocycles" than "kilohertz" (and analogously for "mega-," "giga-," etc.). What contributed to "Hertz" becoming more common than "cycles"? cf. the Google ...
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### When was the electronvolt first introduced?

When was the unit "electronvolt" (eV) first introduced, and by whom? The definition of the eV requires the knowledge of the electron charge $e$, so it must have been introduced after 1909, the date ...
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### What happened to the meter bars?

Throughout the centuries the meter has been redefined a number of times. At one point it was defined in relation to the distance from the north pole to the equator. Seeing how that is a bit of an ...
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### when did polynomial coefficient matching start for solving equations?

Coefficient matching feels rather natural when solving equations and checking dimensions, however in footnote 2 to "Two alternative derivations of Bridgman's theorem" (Berberan-Santos M N, Pogliani L, ...
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### Is the name of German company Siemens related to the unit of conductance?

We have heard about the name of the famous German company Siemens.My friend said that this is because they were the first to introduce conductance bridges and hence named after the unit of conductance ...
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### History of cubit - Galilei, Kopernik, Newton

Can cubit from Galileo Galilei books " Dialogue Concerning the Two Chief World Systems", "Two New Sciences", Newtons "Philosophiae Naturalis Principia Mathematica" or Mikołaj Kopernik books convert to ...
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### Former definitions of MKS units

The meter and second, at least, have been redefined several times since they became common in science. Sometimes when referencing historical documents, it's useful to know how the units of the time ...
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### Why does the start of the calendar year not correspond to a natural event?

Why is Jan. 1, the start of a new year, several days after the Winter Solstice, instead of coinciding with a solstice or equinox or other natural annual event? Note: The question does not ask why ...
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### When are units that are named after persons given their names?

There are many scientific units named after people, of which most were given their names a relatively long time ago (Watt, Newton, Celsius, etc). I wonder if the scientists who "discovered" these ...
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### Why is there no named unit for momentum but there is one for energy?

Momentum and energy play very similar roles in mechanics, each being changed by the application of force over a interval. For energy the interval is in space and for momentum it is in time. Both have ...
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### On the work of Sadi Carnot

This PDF file contains an English translation of Sadi Carnot's Refelections on the motive power of fire. It also contains commentary by Lord Kelvin. And some chapters about the work and life of Carnot,...
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### How did each base quantity get its name? [closed]

I've tried searching everywhere, but I can't seem to find anything related to how the quantities got named! Base quantities: Mass Distance Time Temperature Electric current Quantity of a substance ...
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### Why is kg the standard unit for mass and not g in SI?

Why is $\mathrm{kg}$ the standard unit for mass and not $\mathrm{g}$? I know that there is the kilogramme des Archives which is a kilogram and not a gram. But originally on April 7, 1795 the gram was ...
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### Why does JPL at NASA give two different values in astronomical units for planetary distances?

Why is it that JPL (Jet Propulsion Lab) at NASA gives astronomical unit values for the planets for two different time periods, namely 3000 B.C. to 3000 A.D., and 1850 to 2050?
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### What is the importance of SI units in Physics?

Every quantity has some SI units like distance, time, speed etc. Why do we prefer SI units for these quantities?
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### Who invented the mole?

Who invented, or first used, the concept of the mole? I did my own research and the closest I came was Avogadro’s constant, which was made not by Avogadro but rather by Jean Baptiste Perrin. However, ...
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### Two competing definitions at the birth of the unit "meter"

It might be famous that $\pi^2$ is a good approximation to the gravitational acceleration in the unit "meter per second squared". My explanation for this is the seconds pendulum, which was proposed ...
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