Units Definition and 998 Threads

English units are the units of measurement used in England up to 1826 (when they were replaced by Imperial units), which evolved as a combination of the Anglo-Saxon and Roman systems of units. Various standards have applied to English units at different times, in different places, and for different applications.
The two main sets of English units were the Winchester Units, used from 1495 to 1587, as affirmed by King Henry VII, and the Exchequer Standards, in use from 1588 to 1825, as defined by Queen Elizabeth I.The English units were replaced by Imperial Units in 1824 (effective 1 January 1826) by a Weights and Measures Act, which retained many though not all of the unit names and redefined (standardised) many of the definitions.
Use of the term "English units" can be ambiguous, as, in addition to the meaning used in this article, it is sometimes used to refer to United States customary units, which have somewhat different definitions, or to Imperial units, the standard units throughout the British Empire and Commonwealth.

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  1. R

    Units of sound pressure level, example: 120 dB re 20 μPa

    I am trying to do a calculation using a value of -211dB re uPa/V , are these SI units? i.e. if OCV = -211db re uPa/V and I have RL = 20log(Vrms) - OCV Do I need to convert the OCV to standard units before putting it in the equation? Wiki isn't so clear about this:
  2. Ecthe

    Atomic units of frequency and angular frequency

    To calculate the Keldysh parameter, I need to use the optical frequency of a laser in atomic unit. Since for the time: 1 a.u. = 2.42×10-17 s, I would assume that for the frequency: 1 a.u. = 4.13×1016 s-1 which is juste the inverse of the time one. However, I found several sources such as...
  3. Paladin

    I Engineering Design Units & Concepts: Chart & Examples

    Hello, I'm new here. I have a question that I don't think many people can answer. In my Engineering Design class last school year, we use physics on a daily basis. My teacher goes over units and where they come from. He also goes over fundamental measurements like mass and time. He stressed to...
  4. needved

    B Help With Units in General Relativity

    Hi people :) I'm learning some of General Relativity topics but still I am a beginer, uh! i use the Schutz "A first course of general relativity", but i a little confused about the units, the author say it use c = G = 1 all around the book. Just right now i reading chapter 11: Schwarchild...
  5. M

    Units of Inertia and Moment of Inertia

    Hi Unit of 'moment of inertia' is kg.m^2, Which I know. But about Inertia itself, is it a quantity phenomenon or quality phenomenon? What is it's unit?
  6. L

    Question regarding concentration units

    Hi all, I've done some experiments and I'm trying to work out the correct unit of my final concentration. The experiment goes thus far: I added 800 uL of a substance into a litre of my sample (800uL/L). This substance has a density of 1020 kg/m3 and is 60% w/w. I'm trying to work out the amount...
  7. P

    Why Are The Units of Coulombs Law What They Are?

    Why are the units of force used when applying coulombs law (N-M^2)/(C^2)? This is actually a three part question. 1. Why are the units of the permitivity constant (C^2)/(N-M^2)? 2. Why do Q1 and Q2 not contribute to the final units? Each charge is measured in coulombs, but those units don't...
  8. ckirmser

    Stargazing What is the true extent of the semi-major axis for companion stars?

    I'm trying to map out local space using Astrosynthesis and have been looking up stellar data online. When I come to companion stars, rather than use the RA and Dec for the star - only its position at the time of observation - I use the orbital data relative to the primary. One of the...
  9. J

    Why is there a time unit in some inertia units and not in others?

    Why are some inertia units lb-in-s^2 and others lb-in^2. what the difference? The first one is from a gear box spec and the second is from a motor rotor.
  10. E

    Units of constants in transfer functions?

    Hi All Probably a very basic question. What are the units of the constants in transfer functions? It we take a look at the transfer function of a second order system we then have: H(s) = ω02/(s2+2ζω0s+ω02) ω0 is the natural resonance frequency and has a unit of rad/sec. ζ is the damping...
  11. facenian

    I Is There a Mistake in My Understanding of Planck Units and Dimensional Analysis?

    Since ##c=1## we have ##[l]=[t]##, ##\hbar=1## and from Einstein law ##E=\hbar\omega\rightarrow [E]=[t^{-1}]##, ##G=1\rightarrow [E]=[m]^2[t^{-1}]## then we must have ##[m]=1## but since ##E=mc^2\rightarrow [E]=[t]^2## Can someone please point out my mistake
  12. S

    What Are SG Units? Oil & Gas Industry Explained

    Hi all, I am currently trying to understand a paper which presents a model for sand production and wellbore collapse in the oil&gas industry. The paper refers to stresses and pressure in "s.g." instead of Pa or other pressure/stress units... values tend to range from 0.4 s.g. for cohesive rock...
  13. D

    How to Convert Capacitance in cm to Farads for a Transmission Line Experiment?

    Homework Statement Basically I have to convert from Gaussian Units in a Transmission Line Experiment. I don't get what the point of expressing Capacitance in cm is and I find this very confusing. Homework Equations The equation I've been given is C'=Q'/V=w/4πh, (1) where the prime denotes "per...
  14. Si1entShad0w

    B Understanding the Equations and Units

    I have been struggling through my physics class this summer (it doesn't help that 5 months worth of material is crammed into 2 months in an online classroom). This site has helped me out a lot, but I am still stumbling through understanding when to use certain equations. When I started reading...
  15. P

    Can I Convert Volts to Electron Volts in Photoelectric Effect Calculations?

    Homework Statement HI, I have this photoelectric effect problem, it is not so difficult but when i got to the units and red about it , i got confused. The Problem : Photo-electrons are completely hold up by electric potential with potential of 3V. The work function is given. I have to...
  16. pixel

    I Can c be set equal to 1 in certain systems of units?

    Going off on a tangent here, and I know this is done a lot, but I have never understood the advantage. Just to save some typing? Showing c explicitly in the equation seems to be more instructive, especially to someone new to the subject.
  17. S

    Calculating Stopping Potential in Photoelectric Effect Experiment

    Homework Statement In a photoelectric experiment, a stopping potential of 2.70 eV is measured when ultraviolet light of wavelength 380 nm is incident on the metal. If blue light of wavelength 440 nm is used, what is the new stopping potential in eV? Homework EquationsThe Attempt at a...
  18. O

    I How long can you be exposed to gamma radiation for

    So my question is, how long can you be exposed to a gamma radiation for? Specifically, I've been working with a sealed cobalt-60 source for a final presentation. Now my professor noted to me that these sources we're using aren't radioactive enough to cause us any harm. However, I've been...
  19. Amine_prince

    I Calculating Energy Released from Mass Defect in Units of Kg and MeV/C2

    So 1u = 931.5Mev/c2 = 1.67 10_27 kg . the thing is when i calculate E=mC2 with both units the calculations don't match . i can't understand why .
  20. kq6up

    I Spectra Units: cm^(-1)? | KQ6UP

    I am seeing units for atomic/molecular spectra listed as ##cm^{-1}##. Is this based on the wave number ##k## for ##\Psi=Asin(kx-\phi)##? Thanks, KQ6UP
  21. R

    B What Does the Unit m-3/2 Mean in the Hall Petch Equation?

    Hi. Stress is measured on Pascals. It's force divided by area. Area of course is m x m. So when stress is given it's in units of MN / m2 or MN m-2. Okay. I'm looking at something called the Hall Petch equation. There a constant in it labelled k. In an example, k is given as 0.45 MN m-3/2...
  22. Flyx

    Understanding Pain Threshold: Measuring Pain and its Units

    What is pain threshold? Can pain be measured? If so, how? What are the units? Thx :-p
  23. J

    How can we use a base e system to count and represent numbers?

    I once heard a mathematician (Adrian Banner of Princeton I believe), say the most natural way to count would be in units of ##e^1##, I've been thinking about this recently and can't think of how this would work, and how it would be more natural. Does anyone have any ideas on where to start?
  24. T

    Difference between density and unit weight with same units

    In my lectures and notes my professor keeps writing out density as lbs/ft3 and unit weight as lbs/ft3. For a material such as concrete it could have a density of 167 lbs/ft3 but a unit weight of 103 lbs/ft3. I am confused as to how these two values are different even though they have the exact...
  25. RJLiberator

    Units / Zero divisors in Comm Ring

    Homework Statement Let F(ℝ) = {ƒ:ℝ->ℝ} define (f+g)(x) = f(x)+g(x) (f*g)(x) = f(x)*g(x) F(ℝ) is a commutative ring. ƒ_0(x) = 0 and ƒ_1(x) = 1 a) Describe all units and zero divisors b) Find a function f such that ƒ≠ƒ_0, ƒ≠ƒ_1, and ƒ^2 = ƒ Homework Equations A unit is an element r ∈ R, which...
  26. L

    How to change density of states to eV

    I know that if I multiply e^3/2 (e= 1.602x10^-19) to the unit below I can change it to eV Can somebody help me I don't know why
  27. S

    Finding the Dimension and Lifetime of G_F in Natural Units

    Homework Statement In natural units, the inverse lifetime of the muon is given by ##\tau^{-1}=\frac{G_{F}^{2}m^{5}}{192 \pi^{3}}##, where ##m## is the muon mass, ##106\ \text{MeV}##. What is the dimension of ##G_{F}## in natural units? Put in the factors of ##\hbar## and ##c## so that the...
  28. A

    Calculating Equations in SI Units: Boltzmann Constant & More

    Excuse me, If I want to calculate one equation in SI units. Which value of Boltzmann constant I should deal with. Also, the electron temperature it can be used in Kev or it should be in ev. Also, the nm must be converted to meter or it can be used in nanometer.
  29. B

    What is the difference between amu and u in atomic mass units?

    Homework Statement What is the difference between amu and u? In my book they seem to use u but on my online homework it uses u and it seems to be the same thing. Homework Equations chemistry has too many special units The Attempt at a Solution atomic mass unit 1/12 mass of an atom of carbon...
  30. Raptor112

    Bose Equilibrium Distribution and Atomic Units

    Homework Statement For my project I need to compute the average the number of photons given by the expression: ##\bar{n}= \frac{e^{-\bar{h}\omega/\kappa T}}{1-e^{-\bar{h} \omega / \kappa T}}## where ##\kappa## is the Boltzmann constant and ##\omega## is the oscillator frequency. For the...
  31. A

    What is the conversion factor for hydraulic conductivity units?

    Hi I am new to the forum, I am using darcy's equation to model a tumor - tissue medium. In the literature I found that K - Hydraulic conductivity is given by cm^2/(mmHg.s) But in comsol it's given by (m/s) Is there another term involved in this?
  32. T

    Help with graph units and interpretation

    Hello, I have been doing research with a certain biological substance and its capacitance and conductivity. The following graphs are from a computer connected to a BDS (broadband dielectric spectrometer): I understand the fundamental units, but Re(C'), Re(C''), Re(σ'), lm(σ'') do not make...
  33. C

    Examples of units with non-integer exponents

    Hi everyone, Would anyone be able to give me a few examples of places where units with non-integer exponents come up? I know of a couple right now: noise voltage density (?) in electronics has units \text{V}/\text{Hz}^\frac{1}{2} and the statcoulomb has equivalent units...
  34. C

    Calculating Kinetic Energy in English Units

    Homework Statement An object has a mass of 10 lbm. b. Find the change in kinetic energy ft·lbf when its velocity decreases from 50 mph to 15 mph. Homework Equations KE=1/2mv^2 Ke=(ft*lbf) 1MPH=1.4667 ft/s The Attempt at a Solution When calculating this, does it need to be in slugs, lbm, or...
  35. 1729

    Simplifying SI Units: What Quantity Does JNW⁻¹kg⁻¹ Denote?

    Homework Statement What physical quantity does ##\mathrm{J\cdot N\cdot W^{-1}\cdot kg^{-1}}## denote? Homework Equations You can simplify using mechanics equations or ones from electrodynamics. Be creative, I guess. The Attempt at a Solution ##\mathrm{J\cdot N\cdot W^{-1}\cdot kg^{-1}}##...
  36. avikarto

    LH vs Gaussian units in Biot-Savart Law

    I have a question regarding the relationship between the Biot-Savart formula in Gaussian and Lorentz-Heaviside units. In Gaussian, we have a ##\frac{1}{c}## outside the integral, but in LH units we have a ##\frac{1}{4\pi}\frac{1}{c}##. This does not make sense, considering the transformation...
  37. jdawg

    Calculating Shear Stress in a Fluid Flow with Varying Velocity Profile

    Homework Statement The velocity profile between two plates separated by a distance, h=2 {m}, has a flow distribution given by: The dynamic viscosity of the fluid is given as 1.5E-1 {Pa s}. The upper wall is moving at velocity u0 of 7 {m/s}. The pressure gradient dp/dx is -0.015 {Pa/m}. Find...
  38. Stephanus

    Exploring the Confusing Units in Physics: Electron Volt & Volt

    Dear PF Forum, I'm interested in the future. How mankind will benefit from fusion power. But there are some terms that I don't understand. Electron Volt. https://en.wikipedia.org/wiki/Electronvolt Volt https://en.wikipedia.org/wiki/Volt Perhaps this yes/no question will confirm my confusion...
  39. D

    Natural units and dimensional analysis

    As far as I understand, a natural system of units is one in which the numerical values of ##c## and ##\hbar## are unity. However, they still have dimensions, indeed ##[c]=LT^{-1}## and ##[\hbar]=ML^{2}T^{-1}##. How is it the case then, that certain quantities, such as the action ##S##, can be...
  40. Seismic101

    Units of P1 and P2 in Coulomb's Law

    - What are the units of P1 and P2 in Coulomb's law for magnetic poles? (picture attached). - What is the accurate name of 'P' : magnetic pole strength? Magnetic pole intensity? PS: Sorry if my questions are too basic for this forum- I am not a Physicist.
  41. joneall

    Units if conversion between covariant/contravariant tensors

    I am still at the stage of trying to assimilate contravariant and covariant tensors, so my question probably has a simpler answer than I realize. A covariant tensor is like a gradient, as its units increase when the coordinate units do. A contravariant tensor's components decrease when the...
  42. Samboy

    What are the best elective units for a Physics undergraduate degree?

    Hey everyone, I'm new here but have been following a lot of the threads recently. I just wanted your opinions on what elective units I should study whilst doing a undergraduate degree in Physics. I've been told Computer Science units will be handy as well as more Maths but is there anything...
  43. L

    E=mc^2 Conversion to eV=u: What Happens to the Distance Units?

    Hi, I'm new to the site and not sure if I'm posting in the right place as this is not exactly a homework problem, but just a problem in general. I'm looking at E=mc2 and how the units can be broken down into eV=u (I think?). I don't know what happens to the distance units, though, if we keep...
  44. R

    What units will this equation have?

    Let u be density of something (heat, smell, etc.). Assume the something only diffuses; there’s no convection or ballistic transport. Let’s work in one spatial dimension (x). Then u satisfies the diffusion equation, ∂u/∂t = D ((∂^2)u/(dx^2))
  45. T

    Why divide by 1.059 to convert from slugs*ft/(s^2) to lbf?

    Homework Statement Hello guys, I just have a very quick question about English units. On my fluids homework, I got the answer 9714.806 slugs*ft/(s^2) which seems to be right according to the solutions manual. The homework problem is asking for the force , and the solution manual is dividing...
  46. DiracPool

    Why do power supply units "blow"

    I just recently finished a new computer build for $280, or so I thought. I did it reluctantly. I was hoping to hold on to my old Core2Duo I built back in 2007 until 2022 when they stopped supporting Windows 8. In fact, I probably would have pushed it even until they stopped supporting Windows...
  47. ProjectTruth

    Help Me Comprehend This.... (change of units)

    Hello, First off, I apologize if this is in the wrong section. I didn't think this would constitute as a homework problem,as I'm not really asking to solve a problem,but more, I'm asking you all to help me comprehend and mentally vizualize this. So I had a story problem that went along the...
  48. Calpalned

    What are the units for a in Kepler's 3rd law?

    Homework Statement Kepler's 3rd law can be written as ##p^2=a^3## If p, the period, is given in years, what are the units for a? Homework Equations n/a The Attempt at a Solution The answer is AU. Is there a proof for this or is this merely a definition? Thank you.
  49. C

    Unit Conversion in Bending Stress Calculations

    Homework Statement This is the problem I am working through. I am up to (b). I am struggling with the units. https://www.flickr.com/photos/93763273@N05/8815418518/lightbox/ My question is, why is the final answer in N/m^2? and why does the workbooks list stress in the units of N/m^-2 the...
  50. S

    Convert absorption coefficient units between Hitran, Lifbase

    Hi, I have some modelling code that uses data from Hitran 2012 (https://www.cfa.harvard.edu/hitran/Download/HITRAN2012.pdf). I want to use some data from Lifbase but unfortunately the absorption coefficient units do not match. Lifbase units are given in m3 J-1 s-2 whereas Hitran units are...
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