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. G

    Dimensional Analysis and units of time

    time = x(min) distance = 1(y) y = unknown unitsI think the answer should be 1(y)/ Min. This is not correct becase 1(y) is unknown. Any help? I have the answer but am confused
  2. P

    Trivial question about units (Hans Bethe Small Hole Diffraction)

    http://www.physics.miami.edu/~curtright/Diffraction/Bethe1944.pdf Hans Bethe's paper Jackson doesn't take any issue with Hans Bethe's paper as far as I can tell when he references it. That leads me to believe the two sets of dipoles are equivalent and there happens to be an "old school"...
  3. John Greger

    I Units of trigonometric functions?

    What are the units of the trigonometric functions sinus, cosinus etc? If I take say Sin(0.5), what would the units of the output be?
  4. K

    I'm not sure which units I should convert this to

    Homework Statement:: Hello, I have a simple question which goes like this: Medical ultrasound waves travel at about 1392.8 m/s in soft tissue. Higher frequencies provide clearer images but don't penetrate to deeper organs. Find the wavelengths (mm) of 1.7-MHz ultrasound used in fetal imaging...
  5. B

    Help with Thermodynamics process equations and units

    For part 1. I need to find the temperature, I rearranged the equation so that: T = pV/nR (28000000)(0.45) / (28.96)(8.314) This gives me a value of 52331.4 I am unsure where the units I have used in the equation are correct. I have used pa for pressure, or should I be using kpa? this would...
  6. T

    B Reduced mass to atomic mass units conversion help please

    Hello I am trying to understand how to write the reduced mass into atomic mass units but i am confused how it was done. The equation is $$m_1m_2/(m_1+m_2)$$ For two similar masses in my particular case i have: $$m^2 / 2m = 1/2 * m$$ Then to convert to atomic mass units, the book says it...
  7. I

    Converting Units to Find Density of Unit Cell

    I don't understand the ratio [ 1 g / 6.02*10^23 amu] in the following conversion factor: [228.2 amu / 4.75*10^(-23) cm3] * [ 1 g / 6.02*10^23 amu] = 7.98 g/cm3 (This calculation is in a problem where you're finding the density of a unit cell in g/cm3 - though it has nothing to do with the...
  8. SamRoss

    B Rigidity: Number of Newtons Required for a Material

    In something I was reading, I saw the "stiffness" of a material measured in Newtons per meter which makes sense to me because something that is stiff would require more force to change its length. In other words, "Stiffness is the number of Newtons required to condense a material by one meter."...
  9. T

    How do you derive the units for momentum for photons

    So if i have a photon of some energy and i want to find the magnitude of the momentum, i can get the right answer but the units don't make sense. So i derive p = E/c since i know the energy of the photon and i used f=E/h and substituted this into p=hf/c This means for units of the equation p...
  10. P

    Numerical Evaluation of the Kirchhoff Integral (Flux Pattern) (Units?)

    Naturally there are vector equivalents of the Kirchhoff Integral. Taken from Jackson (10.113) ##\vec{E} \left( \vec{r} \right) = \frac{ie^{ikr}}{r} a^2 E_0 \cos \alpha \left( \vec{k} \times \vec{\epsilon}_2 \right) \frac{J_1 \left( \sin \theta \right)}{\sin \theta}##Where I just let ##\alpha =...
  11. HappyFeynman

    Can Angles Be Assigned Dimension in Scientific Calculations?

    (6.7 rad^2 * m) + (3 m) = 9.7 m
  12. Z

    I Gravitational Redshift: Units, Velocity & Ratio Explained

    I noticed in physics papers that gravitational redshift is expressed in m/s or km/s. I assume that this must be the equivalent velocity to produce that same redshift. So for example, if the gravitational redshift was measured as 3x10 ↑ -4 then; z= v/c 3x10^-4= v/c v = 9x10^4 m/s v=90km/s...
  13. Ashelion

    Kinematics Question with weird units

    Would the A = 2.0 ms^(-5/2) affect my answers for parts (a) to (c)? Since I basically ignored the unit ms^(-5/2) and integrated the function to get the ones for velocity and displacement. And as for part (d) explain the dimensions of A; I assumed that the ms^(1/2) comes from the square root due...
  14. core1985

    Which system of units is coherent for mechanics?

    So, should i write All of these as accodring to wiki pedia maxwell applied coherence concept to FPS, CGS, and SI is already coherent so answer will be All of these ?? Am i right or MKS because question is restricted to mechanics only ?
  15. F

    What units are used in spin factor and coefficient of lift formulas?

    I am trying to add the magnus effect to a golf simulator program but am getting stuck on the coefficient of lift. What are the units I should be using in these calculations? The rest of my program I use SI units. But I seam to get weird results when using them with these equations. Spin factor...
  16. E

    B A question about quantities vs units in physical laws

    A quantity ##p## can be expressed as the product of a dimensionless number, ##\lambda_p##, and a unit, ##u_X##:$$p = \lambda_p u_X$$When we write the equation of a physical law, do the symbols represent the physical quantities ##p## or their dimensionless coefficients ##\lambda_p##? That is to...
  17. P

    Can Mixed Units in Ratios Be Converted and Used Accurately in Calculations?

    I have the solution, but my question is that in my first attempt, I simply calculated a_c = 60^2/150 ignoring the fact that the units for 60 are km/hr and the units for 150 are meters. Is this ever salvageable? Is there any way to retain the 'mixed' units of km with meters in the ratio and...
  18. sophiecentaur

    Is R0 an oversimplified measure of disease spread?

    I understand that R0(The reproduction number) is dimensionless but I cannot help feeling that the timescale for these things must be relevant, at least in some instances, when trying to compare the performance in different countries. R0 is arrived at from the statistics and I think it assumes...
  19. C

    How can units of heat ever be ft^3/s?

    Please and thank you!
  20. Zouatine

    What units for the wave function of a string?

    hello , hope all of you are doing well , i have question about the unit of the function of waves of string fixed in both boundary , the function of waves is function of two variables x and t , so it's function describe the displacement in function of place and time , Ψ(x,t)=φ(x)*sin(ωt+α)...
  21. R

    Does the Flow Coefficient Cv have any units?

    Flow coefficient in valves doesn't have any units. why?
  22. mesa

    What does "....10000 units per 15 min (quick reduction)" mean?

    Hello, looking to find out what the term "10000 units per 15 min (quick reduction)" from the WHO means in terms of using heat to kill a virus.
  23. E

    New unit of mass in a different system of units....

    I'm finding it slightly tricky to just get a hold of where to start. I try $$\text{m}\text{s}^{-1} = \frac{c}{3\times 10^8}$$ If we then set ##c = 1##, that would seem to imply $$3\times 10^8 = \text{s}\text{m}^{-1}$$For ##G = 1##, I might also write $$\text{kg}\text{m}^{-3}\text{s}^{2} =...
  24. M

    I De-dimensionalizing/rescaling units

    Sorry if this is the wrong place to put this- I had no idea what category my question should fall under. Does anyone possibly have good resources or good explanations on how to re-scale units? Something with concrete examples instead of just abstract explanations? For example, something like if...
  25. naushaan

    Graphing f(x)=√x but shifted 2 units to the left and then reflected

    I drew the graph for f(x)=√x, and then shifted it 2 units to the right. I then drew another graph to reflect it from both x and y axis. Not sure if this is correct though? I'm also quite stuck on trying to write the equation for this final graph. I think the equation after shifting and...
  26. TammyTsang

    Engineering Scaling of units for equations of motions

    In the picture, there is a problem where the t is in units of square root(l/g), and V in square root(gl) I am wondering 1. What it means when time is in units other than time? Does it mean that when solving I have to take time/squareroot(l/g) 2. How did they get square root(l/g). Thank you...
  27. redtree

    I Fourier conjugates and inverse units

    Why do Fourier conjugates take inverse units?
  28. A

    Units of Fourier Transform (CTFT) vs spectral density

    I'm confused on how units work with regards to the Fourier Transform (CTFT). I was reading the Wikipedia article on spectral density. In an example, they use Parseval's equation, along with the units calculated on the time side, to determine the units on the frequency domain side. The units of...
  29. T

    Engineering Help With Mass and Weight Units

    Hello all I was hoping someone could clear up the units & difference between Mass and Weight. I live in the UK. I weighed myself today on the scale and the scale told me that I weighed 95kg. What bothers me is that if:- Mass - Is the amount of "stuff" I am made from which is measured in kg...
  30. T

    Understanding Units of Force (Newtons and Pascals)

    Hi all I am trying to get my head around some units of force. 1) I have units of:- Kg * m / s^2 Am I correct in thinking that these are units of Newtons. So if I had 560Kg * m / s^2, then can I say 560 Newtons - would this be correct? 2) Secondly I have units of:- kg/s^2 I have no idea...
  31. L

    What Are the Units for Precession Rate (Ω)?

    The precession rate is given as ##Ω = \frac{Mgr}{Iω}##. What are the units here?: I'm calculating it as ##\frac{1}{rad . s}##. Am I supposed to interpret this as revolutions per second, sort of like frequency, and ignore the ##rad##? Also, period is calculated as: ##T = \frac{2π}{Ω}##. So...
  32. A

    A Converting JJ Thomson's q/m Units to Modern Standards

    Summary: How to convert from the units used to report q/m by JJ Thomson to the units used in the now accepted form of q/m. Referencing the below paper... The q/m measurement was first done in 1897 and reported by JJ Thomson. Numerous others measured the same in the early 1900's. The paper...
  33. D

    I Get Standard Units Flux Multiply c^2: Perfect Fluid/Einstein's Dust

    The T^0i the components of the energy momentum tensor that correspond to mass flux and/or momentum density. If I wanted to get standard units of flux I would just multiply by c^2 correct? It is the ith component of momentum across the kth surface? I am thinking of an example in particular that...
  34. E

    B Manipulating quantities with natural units

    I'm only really just learning how natural units work so forgive me if this seems like a silly question. I was just wondering if someone could verify whether the following line of reasoning is valid (I will use joules instead of electron volts just so we can ignore the e conversion factor for...
  35. F

    I The new definition of kg and the mass-energy equivalence

    Hi, today I stumbled upon a 2016 article in Scientific American about the (then) possibility of re-defining the kilogram through Planck's constant. The article is really a very quick review of the topic. At some point the author states the following "So for years, physicists have chased an...
  36. A

    I Calculate Retinal irradiance - from photometric units

    I have been trying to calculate what retinal irradiance value I get with a 1 blue LED system. Since the manufacturer didn´t give the spectral distribution information, I will approximate LED as a monochromatic one (using the 460 nm peak). From the datasheet, the LED intensity range goes from 6...
  37. B

    PID loop with a different command and feedback units / types

    I have a laser and I am adjusting it's position using a motor, and I am reading the laser light intensity at the top and bottom using a sensor. then I divide the top from the bottom and the goal is to get around 0.97%. This is called delta, if delta is too high, you move the motor lower and if...
  38. M

    B Unit of Gravitational Constant Explained

    We say the unit of gravitational costant is m3/kg.s2, but what does it mean? I mean, I think we define units when we need to measure a quantity and thereby compare it with other same kind of quantities. So what does it mean to define a unit for a constant, when it doesn't change?
  39. person123

    Units For a Power Spectral Density (PSD)

    (Just as clarification on what I am doing, I am attempting to use MATLAB to create PSD plots of time series from pressure and accelerometer sensors. Wave gauge sensors might still be helpful for me. I did not discuss accelerometer sensors in the post but maybe I could figure it out if I get the...
  40. CivilSigma

    Finding 50-Year Return Period Maximum Temperature

    Problem Statement: I obtain monthly maximum temperature values for the last 30 years, and I fit the data to a statistical model. Now, I want to find the 50-year return period maximum temperature. Relevant Equations: P(X>x) = 1/R I was having a discussion with my "Prof" for one of my classes...
  41. L

    A Understanding Lyaponov Time & Units

    In wikipedia text Lyaponov time is defined as inverse of the largest Lyapunov exponent. I have some difficulties with the units. Lyaponov exponents are dimensionless? So Lyaponov time is also then dimensionless? Right? How then in wikipedia article we get Lyaponov time in time units? Could you...
  42. T

    Understanding Units: Calculations with Units

    Hi I am trying to figure understand what happens to the units when I perform calculations on them. For instance, given the equation:- T = L^2 If the value of L was 78mm then what would the value of T be? Would it be: (78mm)^2 = 78^2 mm^2 = 6084mm^2 or would it be:- (78mm)^2 = 78^2 =...
  43. pvorisek25

    Calculating pressure with U.S. customary units

    (Sorry for my bad English.) Hello, I am trying to understand U.S. customary units (don't ask why I am trying to do this) but when I was calculating pressure to lb/in2 I went into problem. I remember that p = F / S but when you have mass you have to multiply mass by gravitational acceleration...
  44. R

    How is moment of inertia calculated and translated between units?

    Calculating moment of inertia and translating it between units, I've become confused. The example is a mass of 1kg at 2000mm from the pivot. The force is applied at 1000mm from the pivot. Basics as far as I'm aware: Moment of inertia = mass * Distance to center of rotation^2 Torque = Moment...
  45. sysprog

    I Can all fundamental constants be set to 1 in natural unit systems?

    This is a question arising from the discussion in another thread. It's my understanding that among systems of natural units, the Planck units system sets not only "all three of those to 1", but also does the same with the Coulomb and Boltzmann constants, leaving the elementary charge not...
  46. B

    Simple units conversion question

    as it wants the units kWh does that not mean you times by 3600 to go from s --> h and to go from W to kilowatt --> you divide the value by 1000 yet they divided by 3600 to get answer
  47. A

    B Is there any physical quantity having two different units?

    Is there any physical quantity having two different units or can it be possible that a physical quantity have two different units?
  48. IzitoI

    Physical units problem for a DC Motor with viscous friction

    Hi everyone, I'm trying to derive DC motor differential equations. I have some doubts: I have a viscous friction coefficient in terms of N*m*s. Is it possible to express it in terms of N*m*s/rad? And how? Some exercises show this value in Newton-meter-second and others in...
  49. M

    What are the units of the argument "x" for this cos(x) function integral?

    Show that the value of ##\int_0^1\sqrt(1-cosx)dx## is less than or equal to ##\sqrt2## ##1\ge cos x\ge-1## The problem is a worked one but I am just confused by a simple thing. We integrate the function f ##\int_0^1\sqrt(1-cosx)dx in the interval [0,1] but I don't understand that what stands...
  50. S

    Textbook says V=E/w but units don't match

    In my textbook, it is talking about the Hall Effect on a flat conductor with width w carrying a current i in a uniform magnetic field perpendicular to the plane of the strip. It says that this will create a potential difference of V=E/w where E is the induces electric field from the electrons...
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