We’re rewarding the question askers & reputations are being recalculated! Read more.

# Questions tagged [units]

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

821 questions
Filter by
Sorted by
Tagged with
10k views

### Can units be plural? [duplicate]

I was in a conversation with my senior engineer where he kept on insisting that we can use plural when we write down any unit. I argued that it is not the 'common' practice or even throughout my whole ...
126 views

### How to convert a quantity in natural units

Suppose I am working with a system of units where $c = G = \hbar = 1$. I can then write e.g. a distance in units of kg by converting with a factor of $$\frac{c^2}{G}$$ Now if I have an energy in ...
259 views

### Why is $k$ taken as 1 in the derivation of $F=kma$? [duplicate]

In the derivation of F=ma, when we reach the point F=kma, we take k=1. Why can't we take 'k' as some other value?
87 views

### What allows us to treat physical units in algebra?

I have been thinking about this problem: $$Speed = \frac{Distance}{Time}$$ Following this, is makes sense that the units of speed is m/s. However, I do not follow why we are able to divide units to ...
42 views

### Scaling atomic units with energy

In Hartree atomic units, $e = m_e = k_e = \hbar = 1$, and \begin{align} 1&\mbox{ a.u. of energy is }E=\frac{m_ee^4}{(4\pi\varepsilon_0 \hbar)^2}=27.21\mbox{ eV.} \\ 1&\mbox{ a.u. of time is }\...
265 views

I need get intensity of an LED light source, but I cannot convert lux to $W/M^2$ How does one convert illumination to irradiance? And what is the irradiance formula?
265 views

### Are ‘fundamental measures’ a thing?

The question I want to ask is: What measures are needed to describe the physical world and what are the fundamental ones of those, in the proper sense of the word fundamental? But that might be too ...
40 views

### Definition of a calorie - confusion

Wikipedia defines a calorie as the (approximate) amount of energy needed to raise the temperature of 1 gram of water by 1 degree Celsius at a pressure of 1 atmosphere. Is it the case then that, ...
85 views

76 views

### Is there a simple way to represent this concept? [closed]

If something is represented in meters per second, it means the object is changing by x amount of meters every y amount of seconds. But when you have a unit of something similar to, "Meters · Seconds," ...
6k views

### How to convert the electron volt into Velocity unit?

About this question,I found this from the Wikipedia:nuclear fission produces neutrons with a mean energy of 2 MeV (200 TJ/kg, i.e. 20,000 km/s), which qualifies as "fast". For my question, I only knew ...
235 views

145 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 $g$ ...
277 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 ...
58 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: $$F_{dyne}=\dfrac{p^2}{r_{cm}^2}=\dfrac{1^2}{1^2}=1 dyne$$ ...
294 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 ...
3k views

### 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 ...
70 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 ...
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 ?
111 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,...
1k 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 ...
116 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?