# Tagged Questions

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

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### Why did scientists need to invent light years?

Why did scientists need to invent light years? What's so important about having a light year? I have been learning that a light year is $9.461 \times 10^{15} \, \mathrm{m}$. My question is, why are ...
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### Convert cubic feet per sec to feet per sec

Is there a way to convert cubic feet per sec to feet per sec. Or in general volumetric flow to velocity? I want to know the time taken by water to travel from point A to point B. I have the distance ...
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### Units inside a logarithm

I have troubles understanding a seemingly simple integral in a physical context. Take a look at $\int_{V_1}^{V_2} \frac{\mathrm{d}V}{V}$ which appears in isothermal expansions (V being the volume of a ...
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### How to indicate that a unit is dimensionless [duplicate]

For my dissertation I am preparing a list of symbols used in the text, which basically is a table that consists of the symbol, a short explanation and the dimension it has as indicated below: ...
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### I read, some time ago about a dimensionless constant in physics [closed]

and, my terminology is probably off, but, I think I can explain with an example. Take a Newton, which can be described as a KG*Meter/Second^2 - which frankly, written that way, looks confusing to me, ...
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### Complex dimensional analysis

Does complex numbers have physical dimensions? Is it sensible to talk about the dimensional analysis of $Z$ where $Z$ is the impedance of a mechanical oscillatory system? Or is it the $|Z|$ which has ...
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### Units of rutherford singly differentiated cross section for secondary electron production

I have an equation that I have found in several papers which I am currently using for a project, including Waligorski (1986) and this book page 32. 4.2 BUTTS AND KATZ MODEL Butts and Katz ...
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### How to find gravitational acceleration given $k$ in a power function

I have a problem that has been bugging me for a week, and I can't figure it out. We know the power function: $$y=kx^n$$ I also have a set of data: I took the natural log of both of these data ...
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### Neutrino decoupling and units

I have a problem that states: In the early Universe, neutrinos can be created and destroyed by the process: $$\nu \bar{\nu} \leftrightarrow e^{+}e^{-}$$ The thermally ...
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### Proper way to write the units to indicate that they include an offset?

Past a certain point of complexity, I get rather confused with physical units, so I am asking a physicist for help. I have a code that represents temperature, with a resolution of [0.5 °C], whose ...
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### What unit would the answer be in?

So I have used this formula: $$W= PV_1 \times \ln\frac{V_2}{V_1}$$ and I have converted my values to cubic meters and pascals. So Work Done, $W$, what would be the unit for that answer? I already ...
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### Ultimate Planck Frequency [closed]

Is there an upper limit to the Planck Frequency? Does it simply depend upon the inverse of the Planck Time unit?
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### Are there Planck units for weak or strong “charge”, similar to the electromagnetic Planck charge $\sqrt{4~\pi~\epsilon_0~\hbar~c}~$?

Are there Planck units for "charge" of weak or strong interaction, similar to the Planck unit of electromagnetic charge: $\sqrt{4~\pi~\epsilon_0~\hbar~c}$ ? Are there perhaps direct substitutes, ...
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### Is it possible to prove that units can be manipulated algebraically?

With expressions such as $$4\ \mathrm{\frac{m}{s}} \times 2\ \mathrm{kg} = 8\ \mathrm{\frac{m}{s}} \times 1\ \mathrm{kg}$$ We can justify that a $2\ \mathrm{kg}$ mass moving at $4\ \mathrm{m/s}$ has ...
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### What are the proposed realizations in the New SI for the kilogram, ampere, kelvin and mole?

The metrology world is currently in the middle of overhauling the definitions of the SI units to reflect the recent technological advances that enable us to get much more precise values for the ...
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### In the expresion $4\sin (\theta) \frac{d \theta}{d t}=\frac{dy}{dt}$ Which is the correct unit of $\theta$ ¿ radian or degrees? [closed]

If I change units of an angle for radians to degrees in the next expresion $$4\sin (\theta) \frac{d \theta}{d t}=\frac{dy}{dt}$$ the value of $$\frac{dy}{dt}$$ changes. For example at a rate of ...
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### What really generates time evolution?

A fundamental principle of quantum mechanics, as far as I can tell, states that the Hamiltonian generates time evolution. A common result about generators are the following: let $\mathrm T$ be the ...
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### Do the probability density and the probability current density have a unit

I could not find what is the probability density and the probability current density of one-dimensional Schrödinger equation units?
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I have a table of data containing irradiance of light at different wavelengths. This is how it looks like for 300.5 nm: Wavelength, nm: 300.5 Wavelength, $\mu$m: 0.3005 W/m$^2$/$\mu$m: 403 W/...
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### Why do universal constants have the values they do?

This is meant to be a generic question of the type that we get repeatedly on this site, in different versions: The origin of the value of speed of light The gravitational constant G theoretically? ...
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### Reconciling Units in Classical System Analogies: Why Does Torque Have Units of Energy?

In classical physics we often cast an analogy between translational and rotational systems Force < > Torque Energy < > Rotational Energy Momentum < > Angular Momentum and considering SI ...
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### Operations on physical quantities [closed]

I know what quantities like meter and second are, they are a certain quantity of one-dimensional space and a certain duration of time respectively. And I know what a measurement of a quantity using a ...
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### What does the $c$ in $eV/c^2$ stand for?

I have been wondering(also searching) for what does the $c$ in eV/$c^2$ stand for? (For example, mass of the electron is $0.511 \, \text{MeV}/c^2$.) I have read that this unit has been derived from ...
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### Should I always include units at every step?

I've seen some controversy when solving physical equations on whether to put units all the time after I insert a numerical value to a variable with dimensions or to put the final unit at the last ...
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### Special relativity and missing factors of $c$

I am doing problems in a textbook called 'Introduction to Classical Mechanics' by David Morin. In one of the questions it says the following: In the lab frame, two particles move with speed $v$ ...
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### Do doomsday arguments influence doomsday hypotheses?

The doomsday argument supposes that in the absence of any other knowledge, if we know the age of something now, we may assume that we are seeing it in the middle of its lifetime and then calculate our ...
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### Units of acceleration & Newton's 2nd Law

I tried to use Newton's second law, $F=ma$, to calculate the acceleration of an object. \begin{align}\frac{F}{m}&=\frac{ma}{m} \\ a&=\frac{F}{m}=\frac{30\,\rm N}{1.2\,\rm kg}=25\rm\frac{N}{kg}...
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### Heavyside Lorentz units and Gaussian units

What is the relation between Heavyside Lorentz units and Gaussian units? From this Wikipedia article, I noticed there are $4\pi$ difference between them, but my TA wants more. Is there other ...
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### Acceleration format and interstellar units

Calculating with m/s$^2$ is very helpful when dealing with acceleration on the human range, as accelerating from rest at 4 m/s$^2$ for 3 seconds will give a velocity of 12 m/s. g is also a very ...
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### How to recover units?

Theorists frequently use convenient units like $\hbar=1$ or $m=2$ or whatever is useful to simplify the notation in the problem. And after all the calculations are done the units are recovered based ...
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### Units of Hubble Time and Hubble Constant?

When we have Hubble constant and it's inverse Hubble Time (1/H) what units are they measured in? I know Hubble constant is in "km/s per Mpc" but is there any other units which are popular used with it,...