Questions tagged [units]

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

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5answers
171 views

Do the units of $G/c^4$ make intuitive sense?

In classical Newtonian mechanics, gravity is explained by: $$F=G \frac {Mm} {r^2}$$ Where $F$ is the force due to gravitation, $M$ and $m$ are the masses of the two bodies in question, $r$ is the ...
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6answers
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Why is it “bad taste” to have a dimensional quantity in the argument of a logarithm or exponential function?

I've been told it is never seen in physics, and "bad taste" to have it in cases of being the argument of a logarithmic function or the function raised to $e$. I can't seem to understand why, although ...
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1answer
251 views

Units and the momentum eigenstate in position basis

Consider $\psi(x)=\langle x|\psi\rangle$. Such a wavefunction must have dimensions of inverse square root length to satisfy the normalization condition. Why then does $\langle x | p \rangle = \frac{1}{...
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3answers
139 views

Why the time period cannot depend upon the angular velocity and angular acceleration of the pendulum?

To derive the time period $T$ of a pendulum using dimensional analysis it is assumed that it depends upon the mass $m$ of the bob, the length of the string $\ell$, the acceleration due to gravity $g$ ...
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1answer
245 views

What determines whether a unit is a base unit? [duplicate]

I've had base units explained to me as units that cannot be broken down into a combination of other units. The Ampere is considered a base unit, however it can be written as coulombs per second. Is ...
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1answer
55 views

Why am I getting two different results in emu and SI unit?

I am computing force between two magnetic poles each of one unit pole (in emu) and situated one centimeter apart. In electromagnetic units: $$F_{dyne}=\dfrac{p^2}{r_{cm}^2}=\dfrac{1^2}{1^2}=1 dyne$$ ...
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1answer
254 views

Converting Hubble constant to GR Units

I'm working my way through Thomas Moore's General Relativity Workbook and having a problem with Exercise 24.4.1 (p. 288), which asks you to verify Hubble constant values (with errors) for a variety of ...
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7answers
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Why are work and energy considered different in physics when the units are the same?

There is a question that explains work and energy on stack exchange but I did not see this aspect of my problem. Please just point me to my error and to the correct answer that I missed. What I am ...
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0answers
69 views

Units of a quantum electromagnetic field

The recalling $\mathbf p \rightarrow \mathbf p -q\mathbf A$, the vector potential has units $$ \langle \mathbf{A} \rangle \leftrightarrow\Big[\frac{\text{momentum} }{\text{charge} }\Big]$$ If we take ...
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1answer
45 views

$k_e$ (constant of Coulomb force) is sometimes seen in calculations for atomic radii and sometimes not

One of our formula for radius of atom was $\frac{n^2h^2}{4\pi^2me^2}$. And another one was, $\frac{n^2h^2}{4\pi^2me^2K}$. For hydrogen, Why $k_e$ is sometimes included and sometimes excluded ?
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2answers
109 views

In a force model, $F = ma$, how to understand the units?

For instance, in an aerodynamic force model, the force terms can be lift terms, drag terms, both of which have translational velocities as factors in their models; but, velocities are in units of, say,...
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4answers
962 views

Units in the power equation

I have a question with the formula power = energy / time. I know it's a simple formula, but when substituting into this formula, Does the energy have to be in joules or watts (no kJ or something ...
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1answer
113 views

Finding a substitution of $\mu_0$ to convert from SI to c.g.s in EM laws [closed]

In c.g.s, we set $$k=\frac 1{4\pi\epsilon_0}=1$$ which gives $$\epsilon_0=\frac 1{4\pi}$$ This conversion works, for example, in Gauss' law: in SI $$\vec{\nabla}\cdot\vec{E}=\frac\rho{\epsilon_0}$...
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Why does a calculation to count objects covering a certain area seem to give nonsensical units?

Suppose you want to estimate the number of atoms in a rectangular sheet of graphene. You might estimate the sheet to have $10^{7}$ atoms along one edge and $2*10^{7}$ atoms along the other edge. ...
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2answers
982 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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1answer
98 views

Dimensions/Units and Matrix Inverses

I was going through a homework problem which is essentially a math review in a kinematics frame. This group of problems start as follows, Given $a_x$, constant acceleration, and initial conditions, ...
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2answers
1k views

Why is unit of pressure (psi) used to determine things like bite force of an animal?

Whether in tv documentaries or journals, whenever they talk about an animal's bite force, it's measured in PSI anytime imperial units are used (ex: National Geographic, NIH Journal). Many even seem to ...
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2answers
687 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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1answer
154 views

Electromagnetic Dirac equation in CGS units

The Hamiltonian of the Dirac equation in with an electromagnetic field in SI units is $$H=\gamma^0\left[ mc^2+c\gamma^k\left( p_k-\frac{q}{c}A_k \right) \right]+qA^0$$ (from https://en.wikipedia.org/...
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3answers
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Velocity squared in natural units

I encountered a mental block in my understanding of natural units. According to natural units, $c = 1$, which would imply that if we wanted to convert velocity from SI units ($m/s$) to natural units (...
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1answer
1k views

Are the Amp and coulomb equivalent?

1 coulomb is 1 ampere per second and 1 ampere is 1 coulomb per second therefore they are always equivalent. They would only be different if you changed the time but the units themselves are based only ...
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1answer
450 views

Why are ft-lbs used to describe muzzle energy? And how do you work with it?

Whether it's kicks, punches, or bullets, people use ft-lbs or joules to describe kinetic energy. But how is this useful? Why use torque to describe linear motion such as a bullet? And how does one ...
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1answer
230 views

What are the units for the C parameter in the formula for surface excess concentration?

I came across this with a lecturer while discussing the Gibbs-Adsorption Isotherm. The lecturer insisted that the unit for the concentration parameter in the the formula surface excess concentration ...
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1answer
181 views

How do the radian have a unit? [duplicate]

The radian is defined as the ratio of the circumference and the radius. Both are measured in meters. So there should not be a unit for that. But we use 'rad' as the unit of the radian value. The ...
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0answers
69 views

TINKER molecular modeling software package- Boltzmann constant units [closed]

I use the TINKER molecular modeling software package (version 7.1.2) for computational physical chemistry. There is an extra file (extra1.f) included for the user to edit and incorporate in their own ...
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1answer
123 views

Confusion over the standard units used in special relativity and particle physics? [duplicate]

I am new to relativity and particle physics and I am struggling to understand the conventions for the dimensions that are used in particle physics. I have read this and as I understand it: $$\sqrt{s}=\...
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1answer
492 views

Does the diffusion coefficient depend on units of concentration?

I'm sure this is an elementary question, but I was struggling to explain the following concept to a (math) student recently and it exposed my own deficiencies in discussing units. In Fick's law(s), ...
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4answers
9k views

What is the unit (dimension) of the 3-dimensional position space wavefunction $\Psi$ of an electron?

I googled for the above question, and I got the answer to be $$[\Psi]~=~L^{-\frac{3}{2}}.$$ Can anyone give an easy explanation for this?
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4answers
287 views

What units is the moment of inertia for a point mass?

What are the units used in the integral equation for the point mass moment of inertia. Specifically what are the units of the double integral over a region R of $r^2· \rho· dA$, where $r$ stands for ...
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2answers
135 views

Why is multiplication much more ubiquitous than addition in physics? [closed]

This may sound shamefully naive, but I have no good explanation. My only guess would be that multiplication is much less "demanding" to be included in formulae, due to the fact that addition demands ...
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1answer
242 views

What are the actual electromagnetic units in the foot-pound-second (FPS) system?

So I've been reading this Wikipedia article, and I came across the cited note that someone has derived the electromagnetic units for the FPS system. However, I can't find that publication so does ...
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3answers
98 views

How to interpret units such as $m^{-2}$? [closed]

I'm doing a project of mangroves and found this chart http://www.habitat.noaa.gov/coastalbluecarbon.html It lists how much carbon mangroves and a few other ecosystems absorb but the chart uses m^-2/yr^...
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1answer
620 views

How does one convert spectral radiance from $\mathrm{(W\: cm^{-2} \: sr^{-1} )/ nm}$ to $\mathrm{ (W \:cm^{-2} \: sr^{-1} )/ cm^{-1}}$? [closed]

How does one convert spectral radiance from $\mathrm{(W\: cm^{-2} \: sr^{-1} )/ nm}$ to $\mathrm{ (W \:cm^{-2} \: sr^{-1} )/ cm^{-1}}$? The first one is the radiance represented by wavelength and ...
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2answers
77 views

Semantics: Unit/second versus unit-second [closed]

I've been trying to make sense of an unusual bit of mathematical and notational semantics in my electrical engineering studies: Velocity is typically given in units of metres per second ( $ v = \frac{...
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4answers
2k views

How many Newtons are in a kg exactly?

I understand that it is supposed to equal the gravity coefficient, which is defined as exactly 9.80665. But when i search google i saw one result saying 9.80665002864, and at first i thought it may ...
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2answers
546 views

Gravity acceleration - why is it inverse time squared?

I've looked up and understand the reason behind the number of the acceleration of gravity $(9.8)$. But whenever people describe it they say it's $9.8 \text{m/s}^2$. I don't understand why it's ...
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1answer
65 views

Units of a vision [closed]

I have a basic question about physical units. Say I have the following value: a= $\frac{m1}{m2·g}$ m1,m2 being masses and g being gravitational acceleration. What would the units of this value be? ...
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1answer
857 views

What's the conversion between apparent magnitude and lux?

I know the apparent magnitude of the sun, but I'd like the units in lux. Please also direct me to a reliable source if possible.
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0answers
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Metre Definition from speed of light [duplicate]

Why is 1 metre defined as the distance travelled by light in 1/299792458 seconds rather than 1/300000000 seconds?
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4answers
216 views

Representing dimensions in Dirac delta function results

The Dirac delta function can be defined as $$\delta(x)=\frac{1}{2\pi}\int_{-\infty}^\infty e^{itx}dt$$ From this we see that the dirac function has units of $x^{-1}$. How do we represent the units ...
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1answer
385 views

How to understand the units of Dispersion Measure?

In astrophysics and pulsar astronomy, a typical quantity is the dispersion measure or DM, which is the integral of the electron number density along the line of sight, $$ DM = \int_0^d n \, dl$$ up ...
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1answer
416 views

Can Amps be used as a unit of measure for the amplitude of an EM wave?

As far as I know the definition for the amplitude of a wave is the distance between the line $y=0$ and the peak of the wave which in most cases is going to be a unit of length. I also understand that ...
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1answer
68 views

Why does this paper report potential energy in kHz, and what does it mean?

I am interested in studying theoretically the double well system shown in the figure below, which shows an experimental demonstration of an atomtronic battery. Caption: Figure 1. (a) ...
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4answers
662 views

How to express an energy in natural units

If I have some angular frequency e.g. $\omega [\text{rad } \text{s}^{-1}]$, I can easily express this as an energy as $E = \hbar \omega [\text{ J}]$. Now suppose I am working in Natural (Planck) ...
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1answer
301 views

Dimensions of charge in natural units

The natural units $$G = c = \hbar = k_{B} = 1$$ set fundamental constants of gravity, relativity, quantum physics, and statistical physics to simple numbers. However, surprisingly, the natural ...
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2answers
44 views

Optimization problem and measurements

This is more of math problem, but my doubt is about the measurement units of the final answer so I figured I'd post it here. Problem: A lighthouse is located on a small island 3 km away from a ...
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1answer
40 views

d'Alembertian inconsistency

I'm reading The Feynman Lectures on Physics, Volume II, Chapter 25. I'm confused as to why the d'Alembertian is defined in there as follows (Page 25-7, Eq 25.20): $$ \Box^2 = \frac{\partial^2}{\...
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1answer
212 views

Dimensional analysis problem

I am asking this question here because i think this is fundamentally linked to physics as it revolves around around dimensional analysis and physical quantities. Background: Amount of substance is a ...
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1answer
74 views

Imperial to Metric lenght [closed]

I'm from Metric based country and have an unusual for me imperial notation. Google didn't clear that up for me so I'm asking it here: I have a wall with measures: 10'7-1/4'' What's 7 ? is it tenth of ...
0
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1answer
660 views

Calculating the unified atomic mass unit [duplicate]

The masses of electron, proton and neutron (in SI units) are (approx.): \begin{equation} m_e=\text{electron mass}=9.109\times 10^{-31}\ \text{kg} \end{equation} \begin{equation} m_p=\text{proton mass}...