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. T

    Can variable refrigerant flow outdoor units be combined

    Hey, I noticed that some manufacturers have provided data for pre-combined (2 or 3 modules) to achieve higher loads (limited to about 60 refrigeration tons).I was wondering if a designer could combine modules to achieve a greater even larger(different) loads or am i limited to the combinations...
  2. F

    Age of Universe in Planck normalized units

    Hello, I've a problem. I need to find the age of universe in normalized Planck units. In my case, I'm using Planck mass = 1 and: ##H = 1.18 × 10^{-61} \times t_P^{- 1}## With these values, how can I find the age of the universe? Thanks!
  3. M

    Dimensional Analysis: Solving E = (1/2) mv Equation

    Homework Statement Is the following equation dimensionally correct? Homework Equations E = (1/2) mv where: E = energy m = mass v = speed The Attempt at a Solution 1. I understand that the 1/2 is irrelevant. 2. I broke everything down into length, time, and mass. 3. I got ML^2/T^2 = ML/T 4. My...
  4. E

    Is Voltage Directly or Inversely Proportional to Electricity?

    According Ohm's laws V=IR , which means voltage is directly proportional to electricity, but according to the formula of P=VI= V=P/I, voltage is inversely proportional to electricity, so what's wrong here?
  5. 24forChromium

    Converting units for angular motion

    I think angular velocity can be measured in radians/second. I also heard that if I multiply a value of angular velocity that is in the units of Rad./sec. by a certain value of radius in meters, I will get m/s, but this is the instantaneous tangential velocity. Is any of these right? In...
  6. Mr Davis 97

    Why are we allowed to cancel units?

    As a simple example, when we do chain-link conversion, we are allowed to cancel units in order to obtain the correct answer. However, units are not numbers, so why is this allowed?
  7. J

    What Does W/(g2 cm3) Represent in Power Density Units?

    Homework Statement So I saw a unit "W/(g2 cm3)" And I'm pretty sure it's about power density...and I understand what W/cm3 is. It's watts per cubic centimeter...but what does the g mean? grams? If so, how does that fit in? :/[/B]
  8. Suraj M

    Recombination of 2 Genes Separated by 66 Map Units

    Homework Statement What is the percentage of recombination of two genes separated by 66 map units? 2. Relevant equation -- 3. The Attempt at a Solution Maybe there appears to be no linkage? I don't know, by simple substitution it would be 66%. I don't think that's correct. I mean the book...
  9. A

    Why is μ0 assigned an exact value in SI units?

    Hello all, While I understand the significance of natural units, I am wondering why, in SI units, we are able to assign μ0 an exact value. The speed of light is experimentally determined in m/s, and given the relationship derived from Maxwell's equations, we know that c^2 = 1/√(ε0μ0). Thus by...
  10. C

    Future time measurement units for a tidal locked exoplanet

    Lets assume that a human colony wants to reform time measurement units for more practical. Reasoning: -circadian rhythm a bit exceeds 24h, so perfect day would be around 24h and 10 minutes ( http://www.circadiansleepdisorders.org/info/cycle_length.php ) -readjusting year zero, somewhere...
  11. K

    Mathematica Mathematica: How to use two x-axis with different units?

    Hello, I've collected data with pairs of { resolution, filesize in bit }. Now I would like to use a ListPlot to visualize the data in mathematica. My problem is now showing the x-asis two times. One for the filesize in bit (above) and the other for the filesize in KiB (below). ListPlot[...
  12. M

    Ttorque angular acceleration units check

    Given : A (hypothetical) wheel with a m.o.i. of 1 lb*ft^2 Radius of wheel = 1.9099 inches, therefore circumference of wheel = 1 foot The wheel is at rest, i.e. zero rotation speed. A torque of 1 ft/lb is applied for 1 second Is the result a rotation of 1 r.p.s. (60 r.p.m.) ? thx Mike
  13. C

    Mixing units with functions or derivatives?

    Hi, How do you correctly use units when writing derivatives and functions in math? Example A car goes 17miles per gallon, so a function m with the equation m(g)=17g describes the distance it can go with g gallons. And the derivative dm/dg = 17 miles/gallon. Question: could you write the...
  14. Math Amateur

    MHB Why Does Every Element in U(\mathbb{I}_m) Have an Inverse?

    I am reading Joseph J. Rotman's book, Advanced Modern Algebra and am currently focused on Chapter 1: Groups I. I need some help with the proof of Proposition 1.52. Proposition 1.52 reads as follows: I have several related questions that need clarification ...Question 1 In the above text...
  15. G

    Indivisible units of time and the quantum eraser experiment

    If we take an indivisible unit of time (a Planck time perhaps), could the possible retro-causality hinted at in the Quantum Eraser Experiment be a process where the retro-causality goes back along the same plank unit rather than back in time per se Metaphor - I don't travel to a different train...
  16. Deepak K Kapur

    Derived Units: Exploring Why We Multiply & Divide

    Aa...ha... I can log in now... So, without wasting much time (one never knows how much time one has...) My next question... I saw the list of derived units on wikipedia. All of them had the basic units either multiplied or divided. Why aren't basic units added or substracted to get derived...
  17. H

    Why is (not) the resulting unit of this equation rad/s ?

    Hi guys, My first time here. Looks like a nice forum with friendly members. I have a question. I'm kinda confused by this (and I know it's a shame since I'm on my third year University, almost making my degree). What I remember from my kinematics/dynamics classes this (or similar) equation...
  18. Amy1127

    Is it a bad idea to take 19 units in 16 weeks, or delay O-Chem to next term?

    In order to transfer on time from community college and prepare for the MCAT I have to really scrunch my classes. I was planning on taking Calc 3, Liner Algebra, O Chem 1, and Physics 1 (mechanics) at a whopping total of 19 units in 16 weeks. Is this doable? I would not be working and all of my...
  19. P

    Units for pressure in a cylinder

    I am calculating pressure on a spherical cylinder of 10 mm diameter by wind tunnel. when measuring values in manometer filled with water which density is 1000 kg/m^3 so by hydro-static formula the pressure difference is [P2-P1=ρg(h2-h1)] so by this formula the ρ unit is in kg/m3 and gravity(g)...
  20. Avatrin

    Units of Wave function (Schrodinger equation)

    Hi I thought I knew the answer to this question until I encountered the following question: What is the unit of R(r)? We are of course talking about the radial part of the solution to Schrodinger's equation in spherical coordinates (i.e. \psi(r,\theta,\phi) = R(r)\Theta(\theta)\Phi(\phi)). I...
  21. R

    What is the Effective Mass Equation for Excited Electrons in Units of Angstroms?

    Homework Statement Homework Equations None. The Attempt at a Solution I understand the question, for a phonon it can excite an electron from the valance band to the conduction band between any values of k. If it were a photon it could only excite the electron at specific k values. Anyway so...
  22. S

    Gaussian units vs Lorentz-Heaviside units

    The ##e^2## of Gaussian units becomes ##\frac{e^2}{4 \pi}## in Lorentz-Heaviside units. Is this the only difference between Gaussian units and Lorentz-Heaviside units?
  23. HaLAA

    Show the group of units in Z_10 is a cyclic group of order 4

    Homework Statement Show that the group of units in Z_10 is a cyclic group of order 4 Homework EquationsThe Attempt at a Solution group of units in Z_10 = {1,3,7,9} 1 generates Z_4 3^0=1, 3^1=3, 3^2=9, 3^3= 7, 3^4= 1, this shows <3> isomorphic with Z_4 7^0=1 7^1= 7, 7^2= 9 7^3=3 7^4=1, this...
  24. L

    Calculating Force Constant for a Harmonic Oscillator: Units and Conversions

    Homework Statement A molecule is a harmonic oscillator, and has a fundamental frequency of 1000 cm−1 and a reduced mass of 10.0 amu. From this, determine the magnitude of the force constant, and express it in units of aJ ˚A−2 (atto joules per angstrom). Homework Equations ω=√(k/mr) The...
  25. shanepitts

    Units Mismatch in Analytical Mechanics Textbook

    In my Analytical Mechanics textbook it states that the terminal velocity of a spherical object, whilst taking the linear and quadratic terms into consideration, is vt=[(mg/c2)+(c1/2c2)2]1/2-c1/(2c2) The units don't seem to match here. Is this normal? This is not a homework inquiry, it for...
  26. S

    What are the units of Lz and L^2?

    Homework Statement From quantum numbers: n=2, l=1, m=-1 find the total energy, Lz and L^2? Homework Equations E = -Er/n^2 (I think, although it refers to the total energy we have the principle quantum numbers so I'm good to go. Lz = mh(bar) L^2= l(L+1)(hbar)^2 The Attempt at a Solution...
  27. Avatrin

    Removing units from Schrodingers equation

    Homework Statement I am working on a problem regarding an electron in a one dimensional finite square well. I start off with \psi(x,0) which is the symmetric solution to the time-independent Schrodinger equation for the well. Then I can use Euler's method to find \psi(x,t) for the other values...
  28. SanEng02

    Problem with Pressure Equation units

    I was doing a problem that required me to initially find the amount of pressure that was acting on a bubble, I decided that I had to use the equation P=P(STP) + ρgd. But looking at the equation, the units for the ρgd part of the problem doesn't make sense about how that gives me a unit of...
  29. M

    Solving Unit Conversion Problems: Changing from ft to gal/min

    I think this problem is straightforward , but I'm not used to these units. (Normally I use SI units) From a physics book (U.S version ) I have this equation: ##h=10 + 4.43*Q^2## where the unit of ##h=[ft]##, and ##Q=[ft^3/s]## now the book rewrites the above equation, such that...
  30. R

    Understanding Kinematic Viscosity Units

    I don't understand units associated with kinematic viscosity. Let's recap dynamic viscosity (η): η = Shear stress (τ) / rate of shear strain (m/s)/m) η = F/A / (velocity (m/s) / gap distance (m)) η = Force( N) / Area (m2) / (velocity (m/s) / gap distance (m)) Stress (F/A) is pressure...
  31. Lobos

    Universal? or Multiple Units of Measure?

    Firstly, I'd be happy to be corrected if I'm not using the right wording or just plain incorrect with my approach. Is it practical to suggest that units of one measurement system could possibly represent the measurement of things not only at the macro level, but represent particles with and...
  32. C

    Units of Fourier expanded field

    If I write the basic scalar field as $$\phi(x)=\int\frac{d^3k}{(2\pi)^3}\frac1{\sqrt{2E}}\left(ae^{-ik\cdot x}+a^\dagger e^{ik\cdot x}\right),$$ this would seem to imply that the creation and annihilation operators carry mass dimension -3/2. That's the only way I can get the total field...
  33. B

    Energy units in the potential well problem

    Homework Statement (Please look at the attached file too) In one dimension time independent potential well, I want to know what is a suitable unit for energy (electron volts or joule) Homework Equations The Attempt at a Solution In the attached picture, I've tried to analyze each...
  34. J

    Atmospheric Temperature and Air Molecule Density Calculation

    Homework Statement At an altitude of 50 km, the average atmospheric temperature is 0C. what is the average number of air molecules per cm^3 of air at this atmosphere? Given formula: P(sub h) = P(sub 0) x e^(-Mgh/RT) = -(Mgh x 10^5)/2.303RT Assuming P = 1 atm at sea level and h in km...
  35. Drakkith

    Which Object is the Most Dense Based on Given Measurements?

    Homework Statement Of the objects below, which is the most dense? A. an object with a volume of 13 dm3 and a mass of 1.29 × 103 g B. an object with a volume of 0.00212 m3 and a mass of 4.22 × 104 mg C. an object with a volume of 3.91 × 10-24 nm3 and a mass of 7.93 × 10-1 ng D. an object with...
  36. S

    How are units of measure defined in Quantum Mechanics?

    In classical physics, the definition of a unit of measure involves a deterministic process (e.g. "the amount of force necessary to give a kilogram mass the acceleration of 1 meter per second square"). How are such units defined in Quantum Mechanics? ( Do we replace references to quantities...
  37. S

    What really are units? Why can we ignore them, like in class?

    All my life the approach has been as follows: In math class I learn the rules and almost always deal with purely numerical problems. In physics class I apply the things learned in math class but this time our quantities have units. Now, once the equation is well put and has dimensional...
  38. H

    Units of plancks law in terms of wavelength

    The dimensional term in Planck's law (after getting rid of steradians) is [itex]\frac{h c^2}{\lambda^5}[\itex]Which seems to yield units of J*m/s. However, Wikipedia suggests that the units are J/(m3*s). If wikipedia is right, how do you get that answer?
  39. physicsisgreat

    Units in quantum mechanical problem about 4-He

    Homework Statement Units of the problem are chosen to be such as KB = 1 and ħ = 1 so that energies are expressed in Kelvin and lengths in Å. The professor says the resulting value of ħ2 / 2 m for a 4-He atom is 6.0599278: how is that possibile? Homework Equations Natural units websites give...
  40. K

    Variables This should be easy -- Matched units on LHS & RHS?

    Variable Units x Meters (m) v Meters per second (m/s) t seconds (s) a Meters per second squared (m/s^2) These variables appear in the following equations, along with a few numbers that have no units. In which of the equations are the units on the left side of the equals sign consistent with...
  41. A

    Finding Units Modular Arithmetic

    Homework Statement I am required to find the units of ℤ8. Homework Equations I have that ##\bar{a}## = [a]n = { a + kn, k ∈ ℤ } ##u## ∈ ℤn is a unit if ##u## divides ##\bar{1}##. The Attempt at a Solution I'm not sure how to go about this. My lecturer wrote out the multiplication table...
  42. S

    Making sense of the units of entropy: J/K

    Hi everyone, I have a conceptual question about entropy. I understand perfectly why mathematically the units of entropy are energy per temperature (SI: J / K). However, I would like to better understand the significance of these units. For example, the SI units for speed/velocity are m / s...
  43. R

    What Do Lbf, Lb, and Lbm Really Mean in American Units?

    Ok i am too confused with these american units. I have a couple of questions about them Q1) Is lbf a force. Is lbm a mass. If yh does it mean 1 lbm = 32.174 lbf. Q2) Is lb equal to lbm or lbf. Q3) Is lbs equal to lb or lbm or lbf. Q4) Let's say 1 lbm = 32.174 lbf. and from many sources i...
  44. SalfordPhysics

    Comp Sci Fortran - equation of motion, astronomical units

    Homework Statement Euler method : Plot the trajectory of a body moving under the influence of the suns gravity from initial conditions x=1, y=0, vx=0, vy=1. My trouble is figuring out my function. Homework Equations d2r / dt2 = -r/r3 The Attempt at a Solution What I have been doing...
  45. marcus

    ΛEPRL quantiz'n of cosmological horizon area in Planck units

    The ΛEPRL spin foam model presented 25 November at ILQGS by Haggard and Riello achieves an interesting quantization of the cosmological constant. Basically this is done on slide #10 around minute 15 of the audio. http://relativity.phys.lsu.edu/ilqgs/haggardriello112514.pdf...
  46. J

    Units of acceleration [f/s^2] vs [g]

    Homework Statement Force, mass, acceleration Homework Equations F=ma The Attempt at a Solution Hi I am having a hard time understanding the units of acceleration. Normally I am used to distance/time^2 being the standard unit [ m/s^2 , ft/s^2 ] Now I am seeing this gravity unit [g] being...
  47. acehansen

    How Do You Calculate IV Drip Rates for Medications?

    Homework Statement Your patient weighs 70 kg the prescription states 0.5 mg/kg/hr. You have a bag of 250 mg in 50 mL. At what rate (mL/hr) will you set the pump? Homework Equations I don't think I have to convert the units of this equation, just cancel them out... but I'm not entirely sure...
  48. E

    How Much Force is Needed to Move a Wire with Soapy Solution?

    1. Calculate the force needed to move the wire in the figure (Figure 1) if it holds a soapy solution (surface tension is 0.025 N/m) and the wire is 16.0 cm long. Express your answer to two significant figures and include the appropriate units. 2. γ(surface tension) = F (force) / l (length)...
  49. C

    How do we measure the units in physics?

    Hi, When i Look at SI derived base units like Joule, Newton, pascal and base units like the Ampere which is defined as "The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre...
  50. C

    Angular momentum Units a very basic question

    I have a very basic questions about units for angular momentum. The measure is in kg m^2/s Angular velocity is in radians/s and therefore radians do not appear in the units. Here is my question, can we leave this in degees/s? Sure its not used but is it wrong? If we are dealing with...
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