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

learn more… | top users | synonyms

2
votes
2answers
62 views

Is there a difference between Hertz and 'frames per second'?

It's not uncommon that the term 'frames per second' (sometimes abbreviated as fps or FPS) is associated with, or even equated to, the unit Hertz (Hz). I'm not exactly sure how these two concepts ...
0
votes
4answers
88 views

Can someone explain to me how to solve $6.6738410*10^{-11}\ \text{m}^3\text{kg}^{-1}\text{s}^{-2}$, the formula to find the gravitational constant? [closed]

Can someone explain to me how to solve $6.6738410\times 10^{-11}\ \text{m}^3\ \text{kg}^{-1}\ \text{s}^{-2}$, the formula to find the gravitational constant? I'm writing a bunch of reports about the ...
3
votes
4answers
68 views

Measurement in reciprocal metres

I'm trying to name a measurement that is measured in reciprocal length, which is in a draft document for vehicle risk management. It currently says: ...
3
votes
3answers
204 views

Is it possible to change units in order to simplify the value of an exponential?

I have the equation $$F=e^{E_0 i \pi}, $$ where $E_0$ is the time-independent electric field, and $F$ is just some important value I am trying to calculate. Obviously, it would be better if $F=-1$, ...
3
votes
1answer
59 views

$c^4$ in Einstein field equations

I have read many derivations of Einstein field equations (done one myself), but none of them explain why the constant term should have a $c^4$ in the denominator. the $8{\pi}G$ term can be obtained ...
3
votes
1answer
45 views

Problems with units of entropy in statistical thermodynamics

The statistical thermodynamics definition of entropy: $S = kN \ln (W)$ troubles me a lot with the problem of dimenstions. where $S$ is entropy; $k$, the Boltzmann constant; $N$ the number of particles ...
2
votes
0answers
32 views

Units and missing constants in quintessence expressions?

In cosmology, quintessence is an alternative to the cosmological constant. In this approach (described here), we consider a scalar field $\phi$ and its self-interacting potential $V\left(\phi\right)$ ...
2
votes
0answers
42 views

Different factors of $4\pi$ and $\epsilon_0$ in Poisson equation [duplicate]

Some authors claim the Poisson equation is $$\nabla^2 \psi = -\dfrac{\rho}{\epsilon\epsilon_0}$$ (e.g. Wikipedia) whereas other ones (e.g. Andelmann) claim it is $$\nabla^2 \psi = ...
6
votes
3answers
76 views

Units of a discrete Fourier transform

Normally a Fourier transform (FT) of a function of one variable is defined as $$f_k=\int^\infty_{-\infty}f(x)\exp\left(-2\pi i k x\right) dx.$$ This means that $f_k$ gets the units of $f$ times the ...
1
vote
3answers
68 views

How units were defined?

I was wondering how we humans can be sure that one meter is one meter and that one second is one second. Nowadays, except for the Kilogram, all other units are well defined using highly accurate ...
3
votes
2answers
93 views

Energy definition in special relativity

I'm going through the early homework assignments for my special relativity course and I've got myself a little confused about energy. I've got a basic understanding of what the 4-momentum is, having ...
-4
votes
3answers
71 views

Is there a unit system, where all physical laws looks nice?

And by nice looking law I mean with no constants. I mean, what would we need to set, so all laws would without those nasty constants in front of them? (I mean all of them, also $\pi$!) What would it ...
2
votes
1answer
40 views

Differences between 3 apples and 3 kg of apples according to base units

7 base units are defined in physics and other units are derived from these units. When we say "3 kg of apples", we mention its mass and mass is a base unit. But for "3 apples", what is its unit? Is ...
0
votes
1answer
73 views

Units kT = 1… why?

Why is it convenient to choose units kT=1? It came up in this paper: http://jfi.uchicago.edu/~leop/SciencePapers/scaling%20and%20universality%20in%20statistical%20physics.pdf in page 4, equation ...
2
votes
1answer
86 views

Why is charge not taken as a fundamental unit? [duplicate]

According to the definition of electric current, it appears to be a derived quantity. Charge on the other hand seems more fundamental than electric current. Then why is current taken as fundamental ...
0
votes
1answer
48 views

Simple units question (School) [closed]

A tank has 2000 litres of capacity (Ct). It fills at 5 l/s. It is filled with 800 l at time 0. a) How many seconds will it need to fill half the tank. So I did this: X = time. ...
-3
votes
1answer
46 views

Convert an angle

Can someone show me a formula in order to convert an angle from [V] to degree or rad? (i use aPotentiometer measuring steering angle) Edit: Well let me exlpain it. I photo of my experiment is shown ...
0
votes
2answers
34 views

Which charge value to use and Why [closed]

$1\mu C=10^{-6}C$ vs $e=1.60\times10^{-19}C$. I am having trouble understanding when to use for $q$, $10^{-6}C$ or $1.60\times10^{-19}C$. I had a problem where $a=Eq/m$. I don't know which value of a ...
0
votes
2answers
52 views

Calculating Pressure in CGS units

calculate pressure in CGS units using following data : $$ \mbox{Specific gravity of mercury},\gamma_{Hg}=13.6\\ \mbox{Density of water}, \rho=10^3{\rm kg/m^3}\\ \mbox{Gravity}, g=9.8{\rm m/s^2}\\ ...
2
votes
3answers
93 views

convert units for spectral irradiance

How can I convert $$ W m^{-2} sr^{-1} nmm^{-1} $$ to $$ W m^{-2} nm^{-1} $$ I have the following matlab code to illustrate the spectral energy distribution of solar radiation: ...
1
vote
1answer
42 views

Power loss due to eddy currents

I am curious about estimating power losses due to eddy currents. Looking on Wikipedia I find an expression for power dissipation under limited circumstances, $$ P = \frac{\pi^2 B^2 d^2 f^2}{6k\rho D} ...
0
votes
1answer
65 views

The position of a particle at any time $t$ is given by $S = V0/a [1-e^{-at}]$. What are the dimensions of $a$ and $V_0$?

To find the dimensions of and V0, I must know the dimension of S and e. So I want to know it.
1
vote
3answers
104 views

Boltzmann constant--why units aren't joules per degree PER MOLECULE

I didn't think the information I looked at did a very good job of explaining the Boltzmann Constant, kB, but once I did this next simple calculation, it seemed clear that what I didn't understand were ...
3
votes
1answer
106 views

How to interpret $t^2$? [closed]

I can't think of the meaning of squaring the Time (multiplying it by itself). It makes sense in Mathematics. But how can you figure it out in nature (or physics). As an example, the formula ...
5
votes
3answers
68 views

Is there a conveniently small unit of volume for talking about molecules?

When discussing energy transformations on the molecular scale, we usually use electronvolts as the energy unit. This is handy because chemical bond energies are a few electronvolts in magnitude. I ...
11
votes
4answers
287 views

Why isn't it $E \approx 27.642 \times mc^2$?

Sorry for the strange question, but why is it that many of the most important physical equations don't have ugly numbers (i.e., "arbitrary" irrational factors) to line up both sides? Why can so many ...
1
vote
2answers
104 views

Which is the most fundamental constant between the Planck constant $h$ and the reduced Planck constant $\hbar$?

This question is related to Planck units (also called natural units, absolute units or God's units). I'm wondering which constant is the most fundamental and should be normalized to 1. I would like as ...
0
votes
1answer
132 views

How to determine Young's modulus of a wire by Searle's method?

The question is: A student performs an experiment to determine the Young's modulus of a wire, exactly 2m long, by Searle's method. In a particular reading, the student measures the extension in ...
0
votes
2answers
144 views

Taking force, mass and length as base units, find the dimensional formula of velocity [closed]

My doubt is that how can force be considered as a base quantity. Is that possible? How can I represent the dimension of velocity using it?
4
votes
2answers
1k views

What is the symbol Å?

I saw this symbol like: $$\lambda=3000\overset{\circ}{\text{A}}$$ and I don't know what this means. Is it a frequency? (since $\lambda$ is usually used for frequency)
0
votes
2answers
45 views

Lattice pattern

I'm trying to find and outline a non-primitive conventional unit mesh, I'd also like to find any mirrors planes and rotional symmetry axes. I first thought it would be like this: , but I was told ...
2
votes
1answer
99 views

How to pronounce $\textrm{eV}\!/c^2$

It seems that $\textrm{eV}\!/c^2$ (and its multiples) is commonly used as the unit of mass in particle physics. For example, David Griffiths uses it quite naturally in Introduction to Elementary ...
5
votes
3answers
348 views

Why didn't we replace our SI units with a better system? [closed]

Intro It seems to me that the SI units we use today are nothing but the result of a historical 'coincidence'. I recently began researching about natural (absolute) systems of units, which are ...
0
votes
2answers
46 views

Significant error conversion

So here is my question: Say we have measured something to be 15,67 mm and the significant error is $\pm 0,01$mm. then we convert the measurement to meter to be 0,01567m would the significant error ...
1
vote
2answers
70 views

Why is the Coulomb taken as the unit positve test charge, though $1 C$ charge is a high value? [closed]

Why is the Coulomb taken as the unit positve charge, though $1 C$ charge is a high value?
1
vote
1answer
45 views

Unit Conversion [closed]

I need to convert this expression $ 81752 \left( \frac{BTU}{in^2 h ºR} \right )$ to $ \left( \frac{Kg}{s^3 ºC} \right )$ Is my draft correct? $ 81752 \left( \frac{BTU}{in^2 h ºR} \right ) ...
7
votes
2answers
341 views

Why is the absolute zero a rational number in Celcius?

From the question "Why is the absolute zero -273.15?" I understood that 1°C is the 100th part of the difference of melting and boiling temperature of water (this is my high school physics, maybe there ...
2
votes
3answers
67 views

Is dimensional analysis always sufficient to establish equivalence of quantities?

In dealing with the Biot-Savart law, it was argued that $$ q\frac{d\vec{s}}{dt}\equiv Id\vec{s} $$ using the fact that the units are equal. Does this kind of argument always work? It seems too ...
-3
votes
2answers
48 views

Other units of resistance [closed]

Resistance is $\frac{V}{I}$, and we get the unit $\Omega$. Another unit is $\frac{W}{A^2}$. How do you derive that unit?
2
votes
3answers
99 views

What is a proportionality constant? (Planck's constant)

I understand that Planck's constant is essentially the ratio between the energy of a photon and its frequency. There are 2 things that im trying to verify: isn't the number that Planck's constant ...
3
votes
2answers
193 views

Exponential or logarithm of a dimensionful quantity?

I have a unit measure, say, seconds, $s$. Furthermore let's say I have a dimensionful quantity $r$ that is measure in seconds, $s$. What is the unit measure of $e^r$? ($1/r$ is in $Hz$.) My question ...
1
vote
2answers
78 views

double of temperature

I'm not a physicist, but I was having a discussion with a friend and I could not convince her about the main point. I asked her: "If I am in a place which temperature is 10ºC and she is in a place ...
2
votes
1answer
39 views

Does the unit of Inertia include radians? [duplicate]

The unit for angular acceleration $\alpha$ is: $$\mathrm{rad/s^2}$$ The unit for torque is $\mathrm{Nm}$: $$\mathrm{kg\ m^2/s^2}$$ And their relationship with Inertia is: $$I = \tau/\alpha$$ So ...
0
votes
1answer
68 views

Confusion about units of angular momentum

According to multiple sources the SI units for angular momentum are kg * m$^2$ / sec I am confused about the derivation for this. Here is what I have done: $$L = I \cdot \omega \\ = m \cdot r^2 ...
2
votes
2answers
116 views

Why do we set $x^0 = ct$ instead of $x^0 = t$?

When we deal with Special Relativity and we start considering spacetime instead of space and time each at once, we usually see books saying that we consider a space with four coordinate $x^\alpha$ ...
3
votes
2answers
68 views

Maxwell's Equations: Induction

What is the reason for some writing Faraday's Induction Law as $$ \nabla \times E= -\frac{1}{c}\frac{\partial B}{\partial t} $$ versus $$ \nabla \times E= -\frac{\partial B}{\partial t} ?$$
0
votes
0answers
47 views

Why are cgs units the norm in astrophysics?

Other physics communities, e.g. the particle physics one, have their own set of units, custom-tailored to their own needs. Now, the astrophysics community is somewhat similar, in that a lot of ...
1
vote
1answer
79 views

What volume of helium at standard ambient pressure and temperature is required to lift one kilogram of mass? [closed]

I used the Ideal Gas Law PV = nRT where P is the pressure of the gas P = 1.033 kgf/cm squared V is the unknown volume of the gas n is the amount of substance of ...
1
vote
2answers
91 views

Units in time dilation calculation

I am currently working through Brian Greene's "World Science U" course on special relativity, and I have a question regarding one of the calculations performed for an exercise on time dilation (MODULE ...
2
votes
1answer
58 views

Is there a standard reference where entropy is set equal to zero in property tables?

For practical considerations, it seems that entropy is only meaningful as a difference between states, like $\Delta s$ going from state A to state B. For an ideal gas, for instance, standard formulas ...