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. Z

    What Are the Correct Units of Coulomb's Constant?

    Coulomb's constant, which appears in Coulomb's Law, contains ε0. The units of ε0 being F/m Are the units of Coulomb's constant therefore F/m ? Thank you.
  2. M

    Are units considered irreducible

    Definition: Let R be an integral domain. A nonzero, nonunit element r in R is said to be irreducible if whenever r=ab, then a is a unit or b is a unit. My question is are units considered irreducible. This how I understand it, Let v in R be a unit such that v=ab ==> 1=ab(v^-1) ==>...
  3. M

    Calculate reservoir pore volume in field units and initial gas in place?

    Homework Statement Calculate reservoir pore volume in field units and initial gas in place for a formation with total bulk volume of 1E8 m^3. A sample of the reservoir gas mixture has 15 moles of CH4 (Tc = 191K),Pc = 46.1bar), 2 Moles C2H6 (Tc =306K, Pc = 48.8 bar) 3 Moles C3H8 (Tc =...
  4. A

    Solving the Confusing Units Issue in Atomic Energy V(R)

    Okay, so I have an expression for V(R) in atomic units (bohrs, hartrees, etc), where V is an energy. I'm assuming this means that if I plug in a value for R (in bohrs), I get back an energy (in hartrees). But if I take the SECOND derivative of V with respect to R, and I want to convert that to...
  5. M

    Question of units of B, H in Gaussian system

    In "Classical Theory of Fields", by Landau & Lifgarbagez, they give, for example, the force on a charged particle by a magnetic field as: F = \frac{ev}{c} x H where H is the magnetic field intensity. Now, normally written in SI units, this expression would use B and no factor of 1/c...
  6. P

    Permeability of Free Space: Units & Value

    In Hartle's book on General Relativity, page 47 footnote 1, it says: "You might be used to thinking that quantities called \epsilon_0 and \mu_0 are the basic parameters in Maxwell's equations, but \mu_0 \equiv 4\pi \times 10^{-7} is a pure number, and \epsilon_0 = 1/(c^2 \mu_0)." But as far as...
  7. A

    Norms and Units of an Integral Domain

    In an example in my algebra text, (from the section on unique factorization domains) it is describing the ring \mathbb{Z}[\sqrt{-5}], and demonstrating that it is not a UFD. It starts by giving the norm N(a+\sqrt{-5}b)=a^2+5b^2. It remarks that if zw=1, then N(z)N(w)=1, and then it goes on...
  8. T

    So the correct SI units for the quantity A are meters (m).

    I am having a problem finding the correct SI unitsfor the quantity A? In the equation A=√(R/TY) That is A equals the square root of R divided by TY (not to good showing workings on the computer sorry) , the SI units of the quantity R are kg m^3 s^–2, the SI units of the...
  9. B

    Is the Units Group of Z Modulo 34 Cyclic?

    I came across this question. Is the group (ℤ/34/ℤ)x cyclic? We haven't discussed the theorem in class that any units group of Z modulo n is iff n = 1, 2, 4, pk and 2pk (where p is an odd prime). But thanks to Deveno I know about it. So in this case, p =17 works, so the group is cyclic? but is...
  10. D

    Chemistry Relationship between moles of O2 and Ch2 Units in a combustion reaction

    Homework Statement I want to find a relationship between the number of moles of O2 used and the number of CH2 units in an alkane in a complete combustion reaction. Homework Equations The Attempt at a Solution Right now I have come up with a math equation, given X units of CH2...
  11. C

    Converting radiometric units (W/m^2) to photometric units (Lux)

    1. I am doing a lab about solar radiation. We collected data using a lightmeter which measures illuminance in lux. Now to calculate the optical depth of the atmosphere I either need to find an expression for the solar constant in lux or be able to convert illuminance values in lux to W/m². The...
  12. A

    Intuitive understanding of dimension (units)

    I was flipping through a physics text and some of the units seemed pretty 'crazy'. Just wanted to know if they can always be understood intuitively, like you can visualize what's going on e.g. force, mass x acceleration, if I look at it as kg*m/s^2 then it doesn't really make sense to me...
  13. S

    Understanding Specific Heat Units (hg-C)

    Hopefully this is a really simple question to answer. A problem I have uses the following information: The specific heat of air = 1.05 kJ/hg-C. Does anyone know what the hg-C stands for? I know specific heat is usually expressed in terms of J/C, so this is confusing me.
  14. M

    Density of states arbitrary units

    So the problem is following. The density of states in energy space (3D case) represents the number of states per unit volume per energy. This means that the unit is #(number of states)/(cm^3 eV). This result can be seen in many solid-state physics books. I am reading some articles where the...
  15. Y

    Fluid pressure problem and units confusion

    Homework Statement Exercise Find the pressure at a depth of 10 m below the surface of a lake if the pressure at the surface is 1 atm. (Answer: With P0 = 1atm = 101kPa, ρ=103kg/m^{3}, and g= 9.81 N/kg, we have P = P0 + ρgh = 1.97 atm. The pressure at a depth of 10 m is nearly twice that at...
  16. J

    Exploring the Physical Meaning of Planck Units

    I've been looking at the various Planck units, and I'm wondering how they are constructed from the constants involved. Like is there any physical reasoning behind those equations? I've looked all over the place for derivations of the equations or what the actual physical meaning of the units...
  17. S

    Can someone clear up the implications of Planck units once and for all?

    Time and time again I always hear people say that Planck Lengths are the smallest units of meaningful distance. But I don't think this implies nothing can be smaller than this or that spacetime is discrete, correct? It just means talking about anything smaller than this would be like talking...
  18. P

    How should physical units be displayed on chart axes?

    Hello, Just wondered what the correct way is to display physical units on chart axes. Let's say for example thermal resistance of a heatsink extrusion is plotted as a function of length, and so the units for the y-axis are K.W-1 My 'A' Level physics teacher always insisted on (in order...
  19. B

    Exploring the Pros and Cons of Electromagnetic CCB Units

    There are different types of CCB units. Hydraulically powered, pneumatic, electromagnetic and of course mechanical. But, can anyone give me the proper advantages and disadvantages of electromagnetic CCB over other variants. Googled it. But invain. Regards.
  20. J

    Bunsen Coefficient Solubility Calculation Units?

    I've been reading about calculating dissolved gas concentrations in water at varying temperature and salinities using an equation of the form: ln(β)=exp(A1 + A2*(T/100) + A3*ln(T/100) + A4*(T/100) + S*(B1 + B2*(T/100) + B3*(T/100)^2 where β is in units of mL/L, and A# and B# are...
  21. C

    The Meaning of Natural Units: $c=1$

    What is the meaning of a frame of reference where $c=1$ ? Are these the so called natural units?? And which is the purpose of this??
  22. N

    Average Acceleration Units Problem

    1. An electric train initially moving at 25km/h accelerates to 35km/h in 20s. Find its average acceleration in m/s^2. G: v1 = 25km/h v2 = 35km/h Delta t = 20s R: Aav 2. Aav=(v2-v1)/delta t 3. = (35km/h-25km/h)-20s The problem is that the velocities are in km per...
  23. D

    Time constant of a thermal model with multiple masses and units

    Hi, i have modeled a thermal system in simscape: the unit includes a polymer casing inside which there is a heater, heat spreader and a fluid sample in that order. i wish to validate this model by hand calculations such as finding the time constant. However i do not know how to find the time...
  24. H

    Simple Static Pressure problem, with units

    Homework Statement Hi I'm having trouble solving this physics problem if anyone can help it would be greatly appreciated ! Static pressure problem: A bathtub contains water to a depth of 20 inches. Determine the pressure at the bottom of the bathtub in both [psia] and [psig] (a being...
  25. H

    Simple Static Pressure problem, with units

    Homework Statement Hi I'm having trouble solving this physics problem if anyone can help it would be greatly appreciated ! Static pressure problem: A bathtub contains water to a depth of 20 inches. Determine the pressure at the bottom of the bathtub in both [psia] and [psig] (a being...
  26. C

    Resistance in a wire, sanity check about units.

    Homework Statement A x cm long wire has a diameter of y mm and a resistivity of 4.0x10-7 ohm-cm. Calculate its resistance. Homework Equations R = p*L/A The Attempt at a Solution First, I convert the given information into SI units: Length = x*10^-2 m Diameter = y*10^-3 m...
  27. J

    Why is Counting Down Considered Less Work in Recursive Functions?

    In class, we're learning about recursive functions. One example given was figuring out 1 + 2 + 3+ 4 + 5 + 6 + 7 + 8 + 9 + 10. My professor stated to address this, we start with 10 in the function and decrement for the next recursive call, so 10 + f(9). He said going the other way would be...
  28. X

    SI Units Lengths: Defined by Light Speed

    After the discovery of the speed of light, it may have made a bit of sense to redefine the meter as \frac{1}{300,000,000} the distance light travels in a second. I understand that larger units like the light year may have been distorted a bit, and that conversions would be required for such...
  29. O

    How Do You Calculate the Orbital Period of a Satellite?

    Homework Statement An Earth satellite is observed to have a height of perigee of 100 n mi and a height of apogee of 600 n mi. Find the period of the orbit. Homework Equations (1) Period = (2*pi/sqrt(mu)) * A^(2/3) (2) rp+ra = 2A Where: Mu is the Standard Gravitational Parameter...
  30. S

    Pressure depth problem in English units instead of metric

    Homework Statement Consider a submarine cruising 32 ft below the free surface of seawater whose density is 64 lbm / (ft^3). What is the increase in the pressure in psi exerted on the submarine when it dives to a depth of 172 ft below the free surface? Assume that the acceleration due to...
  31. G

    Interval notation and specifying units (v vs. t graph)

    Homework Statement #5 and #6 of the attached pdf on page 5 (graph on page 4 relates to said problems) Note: The professor did not cover how to do a problem like this, and the book does not have a similar problem, nor any other handouts. This definitely was a unique question. I'm unsure if I...
  32. A

    Why Does Faraday's Law Appear Differently in Various Units?

    This is maddening and i cannot find a concise explanation anywhere despite the simplicity of this question. I keep coming across faradays law expressed as \text{curl}\left(\mathbf{E}\right) = -\frac{1}{C^2} \frac{\partial\mathbf{B}}{\partial t} Im only used to working in SI units so all i...
  33. N

    [abstract algebra] Isomorphic group of units

    Homework Statement Given that gcd(n,m)=1, prove that \mathbb Z_{nm}^\times = \mathbb Z_n^\times \oplus \mathbb Z_m^\times. Homework Equations / The Attempt at a Solution I can prove both groups have the same amount of elements (using Euler's totient function), but I can't figure out...
  34. F

    Units Question: Avagadro's Constant

    Generally, I see Avogadro's constant being given with units mol-1. Now to me that doesn't seem very... descriptive. But as they say there's more than one way to skin the cat, so I was wondering if these other ways of thinking of it are correct: The conversion factor from grams to amu. The...
  35. M

    How can I convert the units of B to match the Kennicutt Schimdt law?

    Homework Statement I need to infer the observed star formation efficiency of the Kennicutt Schimdt law starting from a volumetric SF law. The KS law is a relationship between gas and stellar surface density that we can approximate as: \frac{d\Sigma_*}{dt}=A\Sigma_{gas}^{1.5} Where A...
  36. BWV

    Hbar c=1 and EV: Understanding Inverse Energy Units

    if using hbar=c=1 and ev for mass/energy what are appropriate distance units? Wikipedia talks about inverse energy without really explaining them
  37. 1

    Determine units of variables in formula

    Homework Statement During a short interval of time, the velocity v in m/s of an automobile is given by v=at^2 + bt^3 where the time is t in seconds. The units of a and b are... Homework Equations The Attempt at a Solution Alright, here's the thing. A friend gave me some old...
  38. E

    Finding units for power via Dimensional Analysis? Makes no sense

    Homework Statement "Power is the rate at which work is performed. Power is signified by W for watts. Power is also the amount of energy transmitted in a unit of time (s). Energy is measured in joules. Joules have the units of Newton∙meters. A Newton is a unit of force. It...
  39. A

    Question about units for angular velocity, time constant

    Here is a link to page in a book which contains an example problem: http://imgur.com/OPrlw.jpg" In the book, they work out the natural frequency of a hydraulic cylinder and come out with an answer in rad/sec. This number is then inverted to get a time constant, and the resultant unit is...
  40. J

    Units of Coulomb, trouble understanding the question

    Homework Statement Two charges, q1 = 4.9 e C and q2 = -3.6 e C are located 105nm apart, what is the size of the force between the two charges?Homework Equations F=kqq/r^2 The Attempt at a Solution I did it according to the formula and I got the answer wrong, so I checked it using an online...
  41. G

    Math Challenges: Find Pairs, Units Digit, Perfect Squares, Last Digit

    1. Find the number of pairs (m, n) such that 2m - 2n = 63 in which m and n are nonnegative integers. 2. What is the units digit of 625 - 324 ? 3. How many perfect squares divide the number 4!*5!*6! ? 4. What is the last digit of the sum 1! + 2! + 3! + ... + 2010! + 2011! ? Thanks!
  42. G

    Representation of units with abbreviations

    Homework Statement I just now came to the realization that I don't know the standard for representing units. When do I include "." after the abbreviation and when do I not? Thanks. For example mi. or mi m or m. yr. or yr What's the rule? I'm sure there's got to be some general...
  43. E

    Frink - the ultimate physics calculator and units converter

    It only seems to have been mentioned a couple of times in passing, but Alan Eliasen's Frink calculator and programming language (https://frinklang.org/) makes unit conversions simple (converting to SI internally then back to any desired target units. User-defined units may be added, too.) Very...
  44. Y

    Natural Frequency Question SI Units

    i have a basic question about the natural frequency of a system. for a mass (M), spring (k constant) undamped system the natural frequerncy is: w_n=sqrt(k/M) the units of w_n according to a lot of resources i found on the internet & textbooks are [rad/sec], my question is why? if i use...
  45. E

    Inability to calculate kinetic energy if I use SI units

    Homework Statement If a 2000 lb car with a velocity of 88 ft/s has by my calculations a kinetic energy of 240 kJ. But for a 909kg car with a velocity of 26.73 m/s I calculate 324.7 kJ. Same velocity, same mass, different energy calculation. Homework Equations English units: Kinetic...
  46. A

    How to convert length units to inverse cm

    how to convert length units to inverse cm.For example arngstroms,nanometers to cm^-1.Thanks
  47. M

    Question about gas leak engineering units

    I have a somewhat mundane question that I hope somebody can help me with. I am working with several standards for performing leak testing on hermetically sealed electronics packages using helium bombing. The question I have is about the engineering units used in the leak test specifications...
  48. C

    Fukushima Fukushima: Unit 2 Discharge - Why Differs from Units 1 & 3?

    Fukushima - Unit 2, what happened different to Units 1 and 3? There are recent discussions about this topic going on, so I think it deserves its own thread. According to attachement IV-2, Unit 2 is responsible for more than 90% of the overall emissions. So there's one big question: What was...
  49. G

    Calcium current to amount of substance units (mol)

    Hi there... I have a differential equation with to terms: \frac{d[Ca2+]}{dt}=f\Big(\frac{3I_{Ca}}{z 4 \pi r^3 F}-k_{Ca}[Ca2+]\Big) I think that the first term converts the Ca2+ (density of?) current to mol units. But, where does it comes from? z is the valence; in this case of the...
  50. B

    What are the SI units for measuring magnetic fields?

    Hi, I was curious as to what webers and teslas measure. As I read up on them I was able to speculate that teslas measured a magnetic fields dimensions and webers measured a magnetic fields strength; obviously though, this is a mere speculation: so please correct me if I am mistaken. Thank you
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