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Questions tagged [metrology]

The study of measurements

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1answer
59 views

Definition of a meter and Newtonian law of Gravity

Newtonian law of Gravity: $$F_g = \frac{m_1 m_2}{l^2} G$$ $$G = 6.7 * 10^{-11} \frac{m^3}{kg * s^2}$$ A meter is defined as: the length of the path traveled by light in a vacuum in $1/...
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2answers
199 views

Will the SI units need redefining ever again?

Up until recently, there were obvious problems with the SI definitions of fundamental units, like bits rubbing off the kilogram prototypes (or mercury vapour absorption), and the water used for the ...
0
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1answer
81 views

What is the mass of $N_A$ atoms of carbon-12?

With the recent redefinition of the kilogram, what is the mass of $N_A$ (Avogadro's constant) of carbon-12 atoms? $N_A$ was defined as exactly 6.02214076×$10^{23}$ atoms. Then how close would the ...
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1answer
195 views

Is absolute zero still 0 Kelvin?

Following the recent decision to change the definition of SI units, I understand that Kelvin is no longer defined in terms of the number 1/273.16. Does that mean that absolute zero is no longer ...
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1answer
117 views

What will happen to the International Prototype Kilogram? [closed]

The kilogram has been redefined in a way that does not refer to the International Prototype Kilogram. That kind of makes the International Prototype Kilogram useless. So what will happen to it? Can I ...
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1answer
325 views

A new definition of mass using Planck constant

Tonight in a italian television news channel well known internationally I have heard that almost certainly the definition of mass that we currently know will be obtained by means of the constant ...
7
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1answer
163 views

What will be the uncertainty in $\mu_0$ under the new SI scheme?

As you may be aware, a new SI system is likely to be adopted in November 2018 (see https://www.nist.gov/si-redefinition/kilogram-introduction). Whilst the speed of light remains a fixed quantity and ...
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1answer
243 views

How is the division of physical quantities into base quantities and derived quantities a matter of convention?

Several physics textbooks and even the SI-The International System of Units Brochure(8th-edition) says that, ''The division of quantities into base quantities and derived quantities is a matter of ...
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1answer
66 views

Is quantum physics a reason for the measurement differences of the International Prototype Kilogram

I've recently learned that there is an official Kilogram (IPK) kept in Paris which is used to calibrate, directly or indirectly, all weight measurement tools. I've also learned that each time the IPK ...
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1answer
149 views

What is the value of absolute zero: $-273.15\ \rm °C$ or $-273.16\ \rm °C$?

What is the value of absolute zero? 0K= -273.15 °C Or -273.16 °C It has been confused in different scientific scriptures. The first definition on Dictionary.com, for example.
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2answers
112 views

Is the International System of Units complete?

Are there any known (measurable continuous) physical quantities, which are neither base quantities of SI, nor are derivable from the base quantities? In other words, are there any quantities which ...
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2answers
2k views

What actually is 1 coulomb? Is it number of electrons or the amount of force?

I've just started my highschool, only to land in the beautiful world of electricity and magnetism, I have many queries and dilemmas, so I want some guidance. Now on to the question. I've just read ...
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1answer
75 views

When you see the atomic mass number for an element, does it take into account the atomic mass defect?

Sometimes I read that the official atomic mass number for an element on the periodic table only includes natural isotope ratios, other times I read that atomic mass defect, number of electrons, etc. ...
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3answers
49 views

Since astronomers are adding 'leap seconds' to our years nowadays, does that mean Earth's orbital period is getting longer?

How long was an Earth year several billion years ago? (I'm assuming constant days, even though I know days were much shorter back then).
3
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2answers
946 views

Why can't we define fundamental unit of mass? [duplicate]

In my physics textbook of class $11^{th}$ The kilogram was defined as :- mass of the platinum-iridium standard cylinder kept at Sevre's France But this isn't a proper and scientific definition ...
2
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1answer
85 views

How is the Length of a Meter Physically Measured

I have two parts to this question. First, I understand that the meter is defined as the distance light travels in 1/299,792,458 seconds. But how is this distance actually measured? The second is ...
2
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1answer
208 views

Why was the fraction 1/31,556,925.9747, in the 1956-1968 definition of the second in terms of ephemeris time, chosen?

The recent question Why are leap seconds needed so often? pulled up some interesting details about the definition of the second, and I'd like to have some of them confirmed explicitly. I'm ...
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4answers
8k views

In nuclear physics, what length year in seconds is used?

So I'm working on a nuclear physics problem and am looking at radioactive decay. The common unit used for very long decays is years within the literature. Is this the sidereal or tropical year? I want ...
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7answers
7k views

Why are leap seconds needed so often?

In Coordinated Universal Time (UTC), leap seconds are added to account for the slowing down of Earth's rotation. But the slowing down is said to be of the order of milliseconds in a century. Then why ...
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1answer
150 views

Why are the international prototype kilogram and its copies kept under nested bell jars?

I'm watching a YouTube video about the American standard kilogram by the Veritasium channel and noticed something that I'd seen before but never questioned. The standards are kept under two, nested ...
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3answers
144 views

How to define approximate meter using primitive to no tools? [closed]

Imagine you've lost somewhere in the wild due to some catastrophic event, and don't have any measurement tools with you. How do you find approximate meter, millimeter, etc. with materials like sticks ...
2
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2answers
81 views

What experimental bounds do we have on big $G$?

I know that there has been a large amount of controversy surrounding the exact value of the gravitational constant $G$, but I know that there is not a substantial difference in the measured value. So ...
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2answers
835 views

Complete list of fundamental properties [closed]

What are all of the fundamental properties, that is all measurable quantities which are not derived from anything else? Many quantities are derived e.g. area is length squared, velocity is length per ...
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0answers
95 views

Thermometer calibration at temperatures away from the ice point and the triple point of water [closed]

Calibrations of thermometers at 'Ice point' as well as at 'triple point of water' are well known and promoted by standard issuing bureaus. However, I experience difficulty to find a simple and ...
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5answers
73 views

Is there such a thing as a work meter to measure work?

I know there are devices to measure power (e.g., dynamometers) but I can't think of any example of a device that measures work directly. Is there such a thing?
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1answer
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Definitive measurement of the correct “gravity potential difference” and uncertainty of its determination by “conventional measurement”

In this article (doi) describing recent experiments on "Geodesy and metrology with a transportable clock" featuring a "transportable Sr optical lattice clock" there is an interesting remark towards ...
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2answers
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What makes the thorium-229 nuclear transition special?

Thorium-229 has a famous isomer with an excitation energy of only about 7.8 eV. As I gather from the wikipedia page, this transition was discovered essentially by accident from gamma ray spectroscopy....
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3answers
783 views

Definition of Joule?

The Wikipedia definition of Joule is: the energy transferred to (or work done on) an object when a force of one newton acts on that object in the direction of its motion through a distance of ...
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1answer
112 views

Why is the kilogram defined using Earth's gravity? [closed]

Since there are variations of $g$ depending on location on Earth's surface, why not use a reproducible lab experiment using a vertical axis centrifugal balance, and say that one kg is defined by ...
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2answers
354 views

Why must the kilogram standard be based on a kilogram mass object?

Inspired by the accepted answer to a question about the Avogadro Project, why must an object used to define a new standard for the kilogram have a mass of one full kilogram? If a smaller mass were ...
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2answers
5k views

Why are scientists involved in the Avogadro Project using silicon-28 atoms instead of carbon-12?

My question is, why use silicon-28 atoms to calculate the kilogram when you already have carbon-12 atoms defining the constant? Does the Avogadro Project intend to define the constant by replacing ...
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1answer
109 views

Why aren't SI units squared or cubed, e.g. cubic centimetres?

For example when we look at cm^3: the multiplier is then (10^-2)^3 so why don't we write (cm)^3 instead?
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2answers
780 views

Why frequency is a SI-derived unit?

Frequency is a derived SI unit but its unit is 1/s. It uses only time once and no other fundamental quantities. So why is not included in SI base unit?
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1answer
1k views

Is displacement vector fundamental or derived quantity?

We know that we have 7 fundamental quantities (all scalars) and length is one of them. I classify velocity as a derived quantity. What about a position displacement vector? How do I classify ...
2
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1answer
255 views

What's the point of normalizing air pressure in the weather report?

If I were to check the weather report on a typical day where I live I would probably see that the air pressure is something like 101kPa. I understand that this has been normalized to sea-level. In ...
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2answers
1k views

Proper reference for the International System of Units

I would like to cite a relevant document that guides usage of SI units. It seems like currently there are two nearly identical literature sources: 8th edition of SI Brochure issued by BIPM, 2006; ...
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2answers
813 views

What is the name and symbol of the derived unit for one thousand metres? [closed]

I'm still getting confused of what a derived unit is. I used the internet to find an answer but I'm still getting confused.
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3answers
8k views

Is the fact that 100 kPa equals about 1 atmosphere accidental?

Typical atmosphere near sea level, in ambient conditions is around 100,000 pascals. But the pascal, as the unit, is not defined through Earth atmospheric pressure. It's defined as one newton per ...
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1answer
183 views

Speed of light in vacuum - is it really a constant and what is the accurate value? [duplicate]

Let's suppose that the speed of light would not be a constant but a function of something. As is quite clear now, universe is expanding exponentially. If the speed of light would actually be a ...
0
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1answer
720 views

Why is a Coulomb not a fundamental unit? [duplicate]

Why is a coulomb not a fundamental unit but an ampere is considering that a coulomb is more ' fundamental '?
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2answers
4k views

Is 12 amu for Carbon-12 exact or rounded?

I'm having a difficult time understanding how Carbon-12 has an atomic mass of 12. Each proton and neutron has an atomic mass that is just slightly above 1 amu, so wouldn't Carbon-12 also be above 12? ...
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1answer
145 views

Means to improve precision and accuracy

I have sensor measuring a distance to a rotating disk. Later on I want to use these values to produce some results like the range of the values. What options do I have to improve accuracy and ...
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3answers
2k views

How can we define the kilogram in terms of Planck's constant?

Scientists are going to redefine the kilogram by using the value of Planck's constant. What is the physics behind this would-be definition of a kilogram? How is Planck's constant related to kilogram? ...
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2answers
379 views

Redefining the kilogram using Planck's constant instead of the density of water among other examples

The kilogram is in the process of being redefined in terms of Planck's constant so as to eliminate its dependence on a physical artefact. Since the length and temperature units are already precisely ...
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2answers
214 views

Why are the electric and magnetic constants where they are?

$ε_0$, the electric permittivity and $μ_0$, the magnetic magnetic permeability were introduced in Coulomb's Constant and Ampere's Constant in order to make units and magnitudes match, in Coulomb's ...
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2answers
600 views

Watt (Kibble) balance and the kilogram - how does the dependence on $g$ get eliminated?

The standard ${kg}$ is now in the process of being redefined by the watt balance (rather than the lump of metal in Paris) A watt balance is very simple, you measure the force needed to support a mass ...
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3answers
202 views

Base unit definition and realization under relativistic conditions

Question Section 1.5 on page 107 of the full SI brochure, Le Système international d’unités, 8ᵉ édition, comments on physically realizing unit definitions when accounting for relativistic effects (I ...
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1answer
147 views

How much variability was there in the O=16 atomic mass standard, and how does it compare to the shift to the current carbon standard?

This is a follow-up question to this excellent answer by David Hammen on the reasons and history of the choice of carbon as the element used to define the atomic mass unit. As related in that answer, ...
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3answers
20k views

Why was carbon-12 chosen for the atomic mass unit?

The atomic mass unit is defined as 1/12th the mass of a carbon-12 atom. Was there any physical reason for such a definition? Were they trying to include electrons in the atomic mass unit? Why not ...
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4answers
901 views

Why are electrical units (specifically, electrical current) considered a base unit?

Note: this is NOT a question why current is the base unit as opposed to charge—that’s because measuring $1 \ \mathrm{ A }$ through a wire is easier to measure in a lab than is $1 \ \mathrm{ C }$ in ...