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Questions tagged [units]

Units are standards of measurement used for different types of quantities.

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4answers
87 views

Converting part of a compound unit

I am working with several different datasets with different units. I want to convert the distances to mm for comparison. The units I'm dealing with are $\mathrm{kg/m^2/s}$, $\mathrm{mm/day}$, and $\...
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1answer
70 views

What does pc(eV) = 300 Br(G cm) mean?

In the January 1983 issue of the American Journal of Physics, Edward Purcell published a one-page "Round-Number Handbook of Physics" (see e.g. http://lemeshko.blogspot.com/2008/06/one-page-round-...
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2answers
469 views

Why is Scan Rate Measured in Hz, not Hz/s?

Spectrometers have a specification called scan rate. As spectrometers measure spectrum, one would think the units of scan rate would be like Hz/s - meaning, the bandwidth scanned per unit time. Like ...
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1answer
4k views

How to convert bars to meters head?

I am trying to convert 1 bar to meters to come up with a conversion factor. Thus, 1 bar = x meters head What is x? From my computations I got something like ...
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2answers
84 views

Keeping track of units

I'm not quite sure how keep track of physical units when differentiaion is involved. I find it important to keep track of units because I want to be able to check the units afterwards. I will discuss ...
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1answer
398 views

How can I convert spectral irradiance to spectral radiance [closed]

I have information on the spectral irradiance incoming from the sun at the top of the atmosphere in units of $\rm mW\ m^{-2}\ nm^{-1} $. The photons hit a hypothetical surface on the earth with 0.3 ...
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1answer
90 views

Is there a rule on which order to write derived SI units?

I noticed from https://en.wikipedia.org/wiki/SI_derived_unit than derived units have to be written in a specific order. For example Newton Metre is written: $$\text{m}^2⋅\text{kg}⋅\text{s}^{−2}$$ ...
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3answers
9k views

What is the unit for work done?

My textbook's equation for work done is: work done = force * distance So this means that the unit should be Nm. However, when I researched on Google, a lot of people were saying that the unit is J....
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2answers
76 views

Vector units with different unit in each component

I'm used to encounter units that come from product or division of units, such that $m/s$ or $kg\cdot m\cdot s^{-2}$. However, I have never seen a unit defined as a sum of units, for instance, $m+J$. I ...
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1answer
57 views

A viscosity unit [closed]

Does the viscosity unit "$\mathrm{ps}$" exist or is it a misprint and refers to poise "$\mathrm{P}$"? Thank you A liquid of $0.014\,\mathrm{ps}$ viscosity and $1.4\,\mathrm{g/cm^3}$ density ...
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1answer
179 views

Converting GPa and TPa to N/m

I have a problem with converting units, in some papers, ultimate tensile strength has been shown with GPa or TPa, but in some papers, it has been presented with N/m. (not newton per square meter) As ...
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1answer
86 views

Why doesn't magnetomotive force have units of force?

Why does it have units of Ampere-turns and not Newtons? Is it a current, or turns mean metres?
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1answer
98 views

Angular Momentum calculation not adding up — Where is my error? [closed]

I'm recreating a calculation from this paper. The authors calculated the angular momentum (L) of a spinning disc with mass = 114.7 grams (0.1147 kg) and radius = 4.75 cm (0.0475 m). The disc spins at ...
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1answer
93 views

How to check if a thing I have is exactly of 1 kg or not?

The definition of a kilogram is as far I know, Kilogram: The mass of a cylinder made of platinum-iridium alloy kept at International Bureau of Weights and Measures is defined as 1 kg. But it is ...
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1answer
58 views

How bright is the night sky in $\frac{\mathrm{W}}{\mathrm{m}^2}$?

I'm looking at the Wikipedia page on sky brightness, which gives the answer in "S10" units. I've looked at the definition of S10 and I'm still completely at a loss as to how to convert that to SI ...
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1answer
188 views

Why does $\frac{1}{4\pi}$ appear in many formulas? [duplicate]

Why do most of the physical equations have $\frac{1}{4\pi}$ as constants? I have seen that many equations have $\frac{1}{4\pi}$ as constants like Coulomb, pendulum problems, etc. Can anyone tell me ...
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1answer
171 views

Reformulate quantum harmonic oscillator Schrödinger equation using dimensionless quantities

I am trying to rewrite a Schrödinger equation using dimensionless quantities but here, the potential is perturbed by $\lambda x^4$: \begin{equation}V(x) = \frac{m\omega^2}{2}x^2 + \lambda x^4\end{...
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1answer
481 views

What unit of measurement is GV?

I am reading this paper; at page 12, it saids: Lockwood et al. (1986) found that the recovery time was independent of rigidity in the range ~ 2 to ~ 5 GV what is GV? I thought that rigidity was ...
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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 ...
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1answer
84 views

Rewriting Schroedinger's equation with $\hbar c$?

I am solving a special case of the radial component of Schroedinger's equation numerically. The equation looks like this: $$ u''(r) = \frac{2\mu}{\hbar^2}(V(r) - E)u(r). $$ $V(r)$ is a potential and $...
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1answer
170 views

Laser beam diameters: What are the units of 1/e^2?

I own an argon ion laser head with a beam diameter of '0.70' (using $1/e^2$ width). What is the unit on the 0.70? And can it be converted to something more intuitive, such as meters?
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0answers
68 views

How do you properly derive atomic units from SI units?

I'm learning atomic units and I'm having trouble figuring out how to derive the atomic units version of energy, time, electric field, magnetic field, etc. For example the Wikipedia page has a list of ...
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1answer
277 views

What is the largest practical unit of mass? [closed]

I have seen in internet and books that the largest practical unit for mass is Chandrasekhar limit which is 1.4 times the mass of sun. It gives the minimum mass for a star to become a neutron star. But ...
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2answers
314 views

Multiplication of units

I can understand the operation of dividing two units for example: 1 m/s it means that the object covers the distance of 1m in one second but really I can not understand the operation of multiplication ...
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1answer
80 views

Is the seconds cancel out in equation? [closed]

Do the seconds cancel out in this equation leaving seconds instead of seconds squared?
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1answer
193 views

Simple 2D Curl example with units?

I'm studying Calculus, not physics, but was curious how units work when we consider curl over a 2D velocity field. Given $\mathbf{F} = M(x,y)\mathbf{i} + N(x,y)\mathbf{j}$ the curl is defined to be $(\...
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0answers
122 views

How is thrust per unit power measured?

Typically, thrust efficiency is measured in specific impulse, as thrust per fuel consumption rate: $$I_\text{sp} = \frac{F_\text{thrust}}{\dot{m}_\text{fuel}} \quad \text{or} \quad I_\text{sp} = \...
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1answer
62 views

Converting units, ternary phase diagram [closed]

I have a task with a ternary phase diagram with units given in mol/mol, but in the description I am also given a feed-to-solvent ratio at 2.5 kg/kg. The molecular masses (and in the equation you see ...
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1answer
61 views

Understanding the statcoulomb conceptually

I've just learned about the statcoulomb, which is basically a way to express charge when we didn't have units of charge (I think), with the definition $$1 \mathrm{statC} = 1 \mathrm{dyne}^{1/2} \...
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6answers
639 views

Give an interpretation of what $c=\frac{1}{\sqrt{\varepsilon_0\mu_0}}$ actually means

From my later school days on, the formula that impresses me the most is $$c=\frac{1}{\sqrt{\varepsilon_0\mu_0}}$$ I know how this can be derived, both from Maxwell Equations and more intuitively. I ...
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3answers
3k views

What is a “decade” as a unit of measure (ex. a decade of the EM spectrum)?

Reading through papers and online sources about radio galaxies, I kept stumbling across a term--a "decade" of the electromagnetic spectrum. Radio galaxy emission encompasses "11 decades of the EM ...
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5answers
136 views

The value of $c$ in $E=mc^2$

In Einstein’s formula $E = m c^2$, $c$ is the speed of light. However the speed of light is not a unique number. It can be expressed in kilometres per second or miles per second for example, which ...
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2answers
831 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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3answers
85 views

Is energy always measured in units of mass × length$^2$/time$^2$ in physics?

Is it always $ M\,L^2/T^2$? Is special relativity different from general relativity regarding the units of energy?
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1answer
880 views

What unit of measurement is a BeV?

I am studying this paper by Gleeson and Axford from 1968. They often measure the energy of a particle in BeV: does it mean GeV, like a Billion of eV?
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1answer
54 views

How can distance and temperature be commensurable?

I am sorry to intrude. The discipline I know something about is metaphysics rather than physics itself. And on Stack Exchange I spend my time on English Language and Usage. I am struggling ...
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0answers
23 views

Mixed unit conversion

I have a silly question. I am not used to different unit systems and I am stuck with trivial problem. I have expression for energy of harmonic oscillator: $$ \Delta E = \frac{1}{2}\Omega^2\Delta Q^2. ...
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2answers
611 views

Units on the electrostatic force equation

So here we have the electrostatic equation: Fe = Ke * q1 * q2 / r^2 The units of the constant Ke are N*m^2/C^2 And this is because we want it to cancel out with the product of q1 and q2 which unit ...
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1answer
151 views

The Gauss's law for gravitational field and the unit system

Here $g$ is the gravitational field, $G$ is the gravitational constant, and $M$ is the total mass in the volume $V$. I wonder if this formula holds for any unit system. That is, does the coefficient $...
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2answers
198 views

Definition of stress-energy tensor

The image from the wiki article on the stress energy tensor gives $T_{00}$ as $1/c^2$ times the energy density. I believe this is incorrect and that the $1/c^2$ factor should be dropped. Am I ...
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3answers
140 views

Dimensionless consistency and quantities

I am a chemical engineering student learning about dimensionless quantities. This is a practice question that I am trying. The Van der Waals equation of state can be used to predict the behaviour ...
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2answers
123 views

How does the gradient affect units in physics?

I intuitively understand the gradient in a mathematical sense, especially the fact that it points in the direction of maximum increase and easily tells us the function's sensitivity to change in each ...
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2answers
150 views

Minimal substitution, four-potential and units

When we make the minimal substitution \begin{equation*} p^\mu\rightarrow p^\mu+\frac{e}{c}A^\mu \end{equation*} the four-potential $A^\mu$ must be proportional to $1/e$ in order to ensure the whole ...
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2answers
179 views

Is there a SI unit for space-time?

Space and time are routinely combined into space-time nowadays, which implies that the SI meter and second should be combined into a single SI unit such as [meter-second]. So far, I haven't come ...
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4answers
238 views

Why do physicists multiply the time component of a space-time vector by $c$ to define the conventional four-vector?

When I think of a space-time vector, I think of it as a geometrical object in Minkowski space with the physical properties of magnitude and orientation wrt other space-time vectors. I can use four ...
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0answers
274 views

Are Heaviside-Lorentz unit and Gaussian unit natural units?

I am studying electromagnetism and have encountered the Heaviside-Lorentz unit and Gaussian unit. In these unit systems, the constant $c$ keeps occurring. For example, the Maxwell equations are of ...
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3answers
190 views

Are there any physical quantities whose units are defined using n-th roots, logs, sins, etc. of SI units?

All the SI derived units I know are defined as products of SI units raised to an integer power (e.g. the coulomb is measured in $\text{s}\cdot\text{A}$, the pascal in $\text{kg}\cdot\text{m}^{-1}\cdot\...
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2answers
60 views

Why $c$ needs to be added in numerator and denominator while solving a de Broglie equation word problem?

15.3 Calculate the de Broglie wavleength of a 0.05 eV ("theremal") neutron. Making a nonrelativistic calculation, $$ \lambda=\frac{h}{p}=\frac{h}{\sqrt{2m_0K}}=\color{red}{\frac{hc}{\sqrt{2\left(...
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1answer
77 views

Dimensionality Dilemma: Dimensional Analysis Invalidates my Mathematical Model

I am trying to derive an equation that describes the rotational motion of an "auto-unravelling system": systems comprised of a material (string, chain, cloth etc.) wound around a cylinder and left to ...
2
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1answer
302 views

What's the matter with Planck mass $M_P$ in Einstein-Hilbert action?

The Einstein-Hilbert (EH) action is often written as $$S_{EH}=\frac{c^4}{16\pi G}\int d^4x \sqrt{-g}R\tag{1}$$ and often as $$S_{EH}=\int d^4x \sqrt{-g}M_P^2 R\tag{2}.$$ Comparing (1) and (2), one ...