Units are standards of measurement used for different types of quantities.
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votes
1answer
121 views
What is volume * mass be called? [closed]
I need a denotation for a rather unusual unit.
$$ \frac{m}{V} $$ is called density. So what is $$ m \cdot V $$ called? Is there a denotation for it? I'd like to get a denotation that could somehow ...
-5
votes
1answer
64 views
How can Young's modulus be dimensionless, but still have units? [closed]
According to this wikipedia entry:
Young's modulus is the ratio of stress, which has units of pressure, to strain, which is dimensionless; therefore, Young's modulus has units of pressure.
From ...
-1
votes
1answer
62 views
Rotation System Dynamics - Unit conversion issues [closed]
I'm having issues with unit conversions when for rotational systems. All of my components (spring, mass, damper) come in Imperial units, so I did all my calculations in Imperial. When I convert to ...
1
vote
4answers
209 views
Is dimensional analysis used outside fluid mechanics and transport phenomena?
Most dimensionless numbers (at least the ones easily found) used for dimensional analysis are about fluid dynamics, or transport phenomena, convection and heat transfer - arguably also sort of fluid ...
4
votes
3answers
112 views
What is the missing proportionality constant in the magnetic levitation formula?
The formula for magnetic levitation is
$$ B \frac{dB}{dz} = \frac{ \rho g }{\chi} $$
but as always, I have a hard time figuring the units in SI. The left hand side is $\mathrm{T^2 /m}$, while $\chi$ ...
0
votes
1answer
58 views
Would a truly physical oscillation still be measured in hertz?
I recently bought a new scroll saw and was commenting to someone about how it was a relatively slow saw... low ... RPMs (thinking like a circular saw). Then it occurred to me that not being a circle, ...
0
votes
1answer
52 views
Formulas for kinetic energy
I was reading ABC of relativity from Bertrand Russell and some formulas about kinetic energy caused me some problems. Here is the extract :
The kinetic energy is, in the usual form ...
5
votes
2answers
170 views
In what order should unit symbols appear?
I am trying to represent the result of a dimensional analysis calculation and I can't find an official document that lists the order that unit symbols should appear.
For example, when I google ...
1
vote
1answer
41 views
What units are needed for the variables when I want the result to be in $erg$ [closed]
$erg$ is an Energy unit, which means that it is equal to $\frac{mass{\cdot}distance^2}{time^2}$. If I want to calculate the kinetic energy of a body in erg units, in what units should the distance and ...
-1
votes
2answers
69 views
Help me check units [closed]
I need to check units in this equation, I know, that there's Pa in the numerator, but what about the denominator?
$p_1$ - pressure in [Pa]
$T_1$, $T_2$ - temperature in [K]
$\rho_w$ - density
$g$ - ...
4
votes
1answer
2k views
How much is 1 electron-volt (eV)?
I am interested in knowing how much is one eV of energy. Everywhere I found are the technical definitions. Can anybody please tell me how much is this much energy. I need something which I can feel. I ...
0
votes
2answers
56 views
Dividing values with units
I'm reading about the subject of heat in a basic physics book. If I am not mistaken the formula to work out how much energy is required to increase the temperature of water is
...
8
votes
5answers
169 views
Physical representation of volume to surface area
I was looking at this XKCD what-if question (the gas mileage part), and started to wonder about the concept of unit cancellation. If we have a shape and try to figure out the ratio between the volume ...
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votes
1answer
56 views
Convert units from cal cm^-1 day^-1. to Wm-2
I am trying to calculate the energy budget of a water body and the last term I'm trying to calculate is the energy advected from the water body ($Q_v$). This equation is given by:
$Q_v = V ( (T_1 - ...
0
votes
1answer
43 views
The units of gain and number of atoms in population inversion in a laser
I am following my university course notes on amplification in laser media, and have come across expressions for the gain of a medium, but the notes are not exactly rigorous... The expression given for ...
3
votes
1answer
160 views
What are units actually?
This question is about the concept of units in physics.
Firstly - do units have a formal mathematical definition? How are they different from pure numbers? Are pure numbers defined to be ratios of ...
1
vote
2answers
128 views
Magnitude refers to number or number with units?
This question is about terminology for physical quantities.
When we talk about magnitude (while talking about scalars and vectors) do we refer to just number or Number along with units?
example: If ...
1
vote
2answers
44 views
Base quantities and charges
Is there an unit of color charge? I haven't found it, so I suppose that it doesn't exist, if this is right, why? Isn't it supposed that every measurable quantity can be expressed in terms of base ...
0
votes
2answers
138 views
Why metric system uses kilogram as a basic SI unit?
SI system uses all (that I know) measurement basic units as 1 (single) instance: meter, second, ampere, etc, except the KILOgram. It already defined with 1000 multiplier (kilo).
It prevents from ...
1
vote
1answer
100 views
When should angles be expressed in degrees vs. radians?
I am trying to calculate the albedo of a given latitude by following the methods of Brutsaert (1982), I have copied the formula below:
3.6 Shortwave and long-wave radiative fluxes
Albedo ...
5
votes
3answers
180 views
What is a “measure equation” as mentioned by this TeX Users Group guide?
In this TeX Users Group (TUG) document, Typesetting mathematics for science and technology according to ISO 31/XI by Claudio Beccari, the author makes various typesetting recommendations including:
...
0
votes
1answer
64 views
Magnetic Field and the Speed of Light
Is it just a historical choice that both magnetic field and the Lorentz force equation include the speed of light?
I figure that whoever wrote up the equations (in cgs!) could have put both factors ...
3
votes
1answer
60 views
CGS Units for Magnetism
Why does the formula for magnetic field force include the speed of light in the denominator in cgs units? Where does the extra $c$ go in SI units?
0
votes
0answers
28 views
How to convert lb/MMBTU to concentration?
This might be a stupid question but...
I am looking over this document for allowable emissions for PM particles in biomass furnaces.
They are given in lbs/MBTU, pounds per million British Thermal ...
4
votes
5answers
641 views
Does the unit of a quantity change if you take square root of it?
For example, I have a mass, m = 0.1kg and I square root it, giving me m = 0.316 (3s.f.) does the unit still stay as kg, or does it change?
1
vote
1answer
319 views
Converting Notes to Milliseconds
Given a note duration (whole, half, quarter, etc.), a tempo (in beats per minute), and a time signature, how long does that note last in milliseconds?
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votes
2answers
90 views
What Physical Quantity has SI Unit $kg \cdot m$,? [duplicate]
What is the physical quantity that has SI (System International) unit of $kg \cdot m$ (kilogram meters)?
16
votes
6answers
1k views
Why isn't temperature measured in Joules?
If we set the Boltzmann constant to $1$, then entropy would just be $\ln \Omega$, temperature would be measured in $\text{joules}$ ($\,\text{J}\,$), and average kinetic energy would be an integer ...
3
votes
1answer
81 views
Units for physical constants
Someone told me that units for $G$ and $\epsilon_0$ (gravitational constant and Coulomb's constant) are placed there simply to make equations work dimensionally and that there is no real physical ...
2
votes
2answers
48 views
What are the units of these virial coefficients?
I'm reading some papers for calculating the vapor pressure of alkali metals as a function of temperature, and I've come across some familiar-looking virial expansions, but when I tried to work out the ...
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votes
4answers
588 views
Why there is no “Edison” unit in physics? [closed]
In the popular culture the XIX-XX century competition between Thomas Edison and Nikola Tesla is well-known. The example could be the Prestige movie, where there are some "Edison's agents" who sabotage ...
17
votes
8answers
1k views
Is there a symbol for “unitless”?
I'm making a table where columns are labelled with the property and the units it's measured in:
Length (m) |||| Force (N) |||| Safety Factor (unitless) ||| etc...
I'd like not to write "unitless" ...
2
votes
3answers
77 views
Curious relation between the dependance in ℏ of Planck units and units dimensions
Looking at Planck units, there seems to be a curious rule between the dependance in $\hbar$ of a Planck unit and the unit dimensions of the corresponding physical quantity.
Let the dimensions of the ...
1
vote
2answers
91 views
Units and Dimensions - Use of proportionality constant
In units and dimensions we learn about Establishing a Formula :
(example) : to establish a relationship between T (Time Period) , m (Mass) , l (length of the string) and g(acc. due to gravity) - ...
0
votes
0answers
60 views
cgs Gauss' system of units
I had never seen this system until today, and I'm really confused. I've read the wikipedia article about it but I still don't know how to change between this and the international system. For example, ...
-1
votes
2answers
83 views
Which units has the relation $E=mc^2$?
Fast everyone knows the relation $E = mc^2$ but I still ask me, which units the relation has.
$c$ is the velocity of light. It means, a constant, isn't it? We therefore just have the relation $E = ...
1
vote
1answer
67 views
Difference between nautical and terrestrial miles
Does someone know the historical reason behind the difference in physical units between nautical and terrestrial miles?
1
vote
2answers
110 views
How to check units?
I've got: $Q=\frac{Er^2}{k}$
how to check the units?
I start with $\left[\frac{\text V}{\text m} \, \text m^2\right]$, tried replacing $[ \text V ]$ with $\left[ \frac{\text J}{\text C} \right]$, but ...
0
votes
3answers
253 views
Temperature in CGS (Gaussian) units
I've been struggling with conversion from Gaussian to SI units for sometime, trying to figure out how derived units in CGS (current, charge etc) relate to the SI units.
But I couldn't find any ...
2
votes
1answer
226 views
What really is Planck's constant and what are its origins?
In the physics texts I have read and other info online, they says Planck's constant is the quantum of action or that it is a constant of the ratio of the energy of a particle to its frequency. Im ...
2
votes
1answer
118 views
If we are using [eV] as a unit for energy, what, then, should the unit for mass and distance be?
This seems obvious but it is confusing since I know that in MKS (i.e. SI) system, we use Joule with Meter with KG.
But if we're using electron volt (1.6e-19 J) for energy, should we change the mass ...
0
votes
1answer
71 views
Buoyancy fluxes in a stratififed fluid and units
I am calculating the buoyancy flux (B) for a stratified fluid as follows:
where g = 9.81 m/s; alpha = 1.6 x10^{-5} + 9.6 x10^{-6} x (20 degC); S = 100 Wm^{-2}, p0 = 1000 kg/m3, and Cpw is the ...
4
votes
3answers
530 views
Why is 1 AU the distance between the Sun and the Earth?
Why 1 AU is defined as the distance between the Sun and the Earth? (approximately if you like to be precise)
An astronomical unit (abbreviated as AU, au, a.u., or ua) is a unit of length ...
0
votes
1answer
71 views
What is the significance of Planck force?
I have been curious to find what could be the significance of Planck force? It is calculated by the formula $c^4/G$,
where $c$ is the speed of light; $G$ is the gravitational constant. Thus (the speed ...
1
vote
1answer
89 views
Clarification on factors of $c$ in the Lorentz' force
I was told that Lorentz' force is given by
$${\bf F}= q{\bf v} \times {\bf B}.$$
But I have read that it is given by $${\bf F}= \frac{q}{c}{\bf v} \times {\bf B}.$$
Why have these two relations ...
1
vote
3answers
196 views
Factors of $c$ in the Hamiltonian for a charged particle in electromagnetic field
I've been looking for the Hamiltonian of a charged particle in an electromagnetic field, and I've found two slightly different expressions, which are as follows:
$$H=\frac{1}{2m}(\vec{p}-q \vec{A})^2 ...
2
votes
3answers
120 views
Correct expression for D'Alembert operator in $c=1$ units
In QFT texts with $c=1$ units (most of them), D'Alembert operator is written as:
$$\Box ={\partial^2 \over \partial t^2} - \nabla^2$$
For pedagogical purposes, however, some texts don't set $c=1$, ...
4
votes
7answers
369 views
What exactly are we doing when we set $c=1$?
I understand the idea of swapping from unit systems, say from ms$^{-1}$ to kms$^{-1}$, but why can we just delete the units altogether?
My question is: what exactly are we doing when we say that ...
1
vote
2answers
177 views
Showing that position times momentum and energy times time have the same dimensions
I've been asked to show that both the position-momentum uncertainty principle and the energy-time uncertainty principle have the same units.
I've never see a question of this type, so am I allowed to ...
4
votes
2answers
338 views
What are the units of the quantities in the Einstein field equation?
The Einstein field equations (EFE) may be written in the form:
$$R_{\mu\nu}-\frac {1}{2}g_{\mu\nu}R+g_{\mu\nu}\Lambda=\frac {8\pi G}{c^4}T_{\mu\nu}$$
where the units of the gravitational constant $G$ ...
