Questions tagged [units]
Units are standards of measurement used for different types of quantities.
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questions
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1answer
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Error Analysis - Units and Dimensions [closed]
I have a question regarding error analysis in Units and dimensions chapter in physics.
It is regarding finding equivalent resistance in parallel
I have a attached 2 images of the same question solved ...
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votes
1answer
40 views
If a model car is made of 9.35 kg iron, what is the mass of gold required to recast the car using gold? [closed]
The detailed question is
An automobile company displays a die-cast model of its first car, made from 9.35 kg of iron. To celebrate its hundredth year in business, a worker will recast the model in ...
0
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0answers
17 views
Cgs to Si with Force
So i just have a Problem with the transition from Cgs to SI. I want to Calculate the Force of the Plates of a Capacitor. The Formula is the following:(k=1,2,3 and Einstein Notation is used and T is ...
0
votes
1answer
35 views
Making the distinction between position and time in a gravitational system
Say we have a large spherical shell of thickness $R$ floating in space. The shell has a hole of negligible radius punched into one side that allows for a ball of mass $m$ to pass through. Say now that ...
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1answer
40 views
Why do we need to convert revolutions into radians before we multiply with meters?
When we are calculating tangential acceleration, I notice that we have to convert the $\alpha$ that was in unit of rev/min^2 into the unit of rad/min^2 before we are able to multiply this value with a ...
1
vote
4answers
80 views
Basic Question About Canceling Units
This is a very basic question but, here it is. In a given equation, where I have a term such as 4 m/s and I multiply this by the variable t, which is time, why is it incorrect for me to cancel out t ...
0
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1answer
51 views
Boltzmann constant in atomic units
I was just wondering what value the Boltzmann constant $k_B$ takes when we are using Hartree atomic units (i.e $\hbar=e=a_0=m_e=1$) where the unit of energy is 1 hartree. Should we we convert $1.38 \...
0
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1answer
25 views
Subatomic natural units
In High Energy Physics it seems to be common use to measure everything in terms of eV powers, by assuming $\hbar = c = 1$ (dimensionless). Often times this system of units is referred as Planck units, ...
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1answer
35 views
Converting light-year to $m\cdot y/s$
For the solution to question $a$ of this problem, I'm confused with what that $3.0\times 10^8 m/s$ conversion factor is. I thought that light year is a measurement of distance, so why are we ...
2
votes
1answer
54 views
SVD of $2\times 2$ matrix where entries have different units
I have the following matrix:
$$
M = \begin{pmatrix}
1 + xy & y \\
x & 1
\end{pmatrix}
$$
where $x$ has the unit $m^{-1}$ (per meter) and $y$ has the unit $m$ (meter). This matrix acts on real ...
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votes
2answers
96 views
Negative Celsius Temperature [closed]
If the Celsius scale is from 0 (freezing point) to a 100 (boiling point) then how is it that we can have negative numbers like -3 degrees Celsius? And can we also have temperatures like 120 degrees ...
2
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1answer
53 views
Units of Dirac delta potential constant
Suppose we have a Dirac delta potential correction of the form $$ V(x)=V_0\delta(x-x_0)$$ What would be the units of $V_0$ ? I think it should be units of energy as calculating first order ...
1
vote
1answer
54 views
Why plane and solid angles have units?
I have learnt that the dimensionless quantities have no unit. Such as the refractive index have no unit. But plane angles and solid angles have their radian and steradian units, although they are ...
0
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0answers
29 views
Natural units in Monte Carlo simulations
I'm wondering how, if one sets the Boltzmann constant $k_B=1$ in an MCMC simulation used to sample the Boltzmann distribution $P(x,p) \propto \exp\left(- \frac{\frac{p^2}{2m}+\frac{1}{2}m\omega^2 x^2}{...
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0answers
24 views
Notation of physical quantity with applied operator
I'm writing my Bachelor's degree at the moment. I have one equation expressing the force generated by a rotor depending on the angular velocity $\omega_m$. At the moment it is written as
$$
F_m(\...
0
votes
1answer
82 views
Does $E=mc^2$ have a constant? If not why? [duplicate]
When I Google the units for $E=mc^2$ it says m is mass in kg speed of light is in m/s and energy is in joules. Why does there not need to be some sort of constant? Why does the total energy in joules ...
0
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1answer
68 views
If gram isn't part of the SI system, then how is it that SI recognizes its symbol?
If gram is of the CGS system of measurement (which is a variant of the metric system, which is still used), then how does SI recognize its symbol? When it is not part of SI. What does this statement ...
2
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0answers
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What does it mean for $2\pi = 1$ in a “private system of units”?
I saw the following image of an excerpt from Robert Mill's Tutorial on Infinities in QED, floating about the internet:
The book is available here, however I don't think I have access to it, unless ...
0
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1answer
16 views
How does an accuracy specified in vol% translates to ppm?
I need to setup some CO2 concentration measurement. The datasheet of the sensor is:
Range: 0 to 25 vol%
Accuracy < 0.5 vol% + 3% of measured value
vol% is the percentage by volume. So the range ...
-1
votes
1answer
77 views
Convert metres to the 1.25 to metres [closed]
I have been given a constant, of value $\beta=3$ x $10^{-30} Jm^{1.25}$. I need to use it in an equation that gives a value in SI units (metres). How can I convert the value of the constant $\beta$ to ...
1
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2answers
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Problem calculating the ground state radius of Toponium using strong force potential for short ranges
If it were true that the top quark were stable we could, in theory, have a hydrogen-like bound state consisting of toponium $T \bar{T}$. Looking at the strong force, for small ranges it takes the form ...
0
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1answer
41 views
Why is the decimal system a more convenient system? [closed]
Can you give reasons as to why decimal system is a more convenient system? I got this in the chapter Units and Dimensions.
1
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2answers
119 views
Why don't we take the universal gravitational constant $G$ to be equal to 1 in $F= \frac{Gm_{1}m_{2}}{r^2}$?
In the derivation of Newton's Second Law, we get to an equation $F=kma$. Since this equation is essentially defining force, Newton could have taken the value of $k$ to be anything. For the sake of ...
0
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3answers
67 views
When and why are units measured in degree?
I was always under the impression that a unit is measured in degree if it is a relative unit. While this is true for temperatures and angles, I cannot find any reference to this definition or any ...
0
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1answer
42 views
Symbol of quintal
Depending on the source, I have come across the symbol of quintal as 'q', 'qt'and 'qtl'. Is there a standard symbol for quintal. If yes what is it?
2
votes
1answer
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Why do the sensitivities of magnetometers and gravimiters have such strange units?
It looks like the sensitivities of magnetometers and gravimeters are usually reported with the units of $\text{Tesla}/\sqrt{\text{Hz}}$ and $\text{Gal}/\sqrt{\text{Hz}}$, respectively (where "Gal&...
0
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0answers
26 views
Ambiguities regarding dimensions of Planck length
There is a couple of posts circulating around about how to express higher dimensional (Newtons) gravitational constants in terms of each other. In these discussions, summarized below, there is one ...
1
vote
3answers
66 views
Are absolute and relative temperatures really the same quantity?
Different units for the same quantity usually only differ by a multiplicative constant, such as inch and meter, but °F and K differ by both a multiplicative constant and an additive one. Does that ...
2
votes
0answers
33 views
Going from 4d Planck mass to 3d Planck mass
I have an expression where I have converted all units such that the answer is expressed in 4d Planck mass, $m_p^{(4d)}$.
In David Tong's lecture notes, Newtons gravitational constant in 4d is given by ...
2
votes
6answers
3k views
What is the definition of unit vector?
In many physics textbooks it is given the following definition of unit vector: "A unit vector is every vector whose magnitude is 1 unit". I don't like this definition.
On one hand, it is ...
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4answers
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Do units have to make sense in Physics?
For a lack of a better title and phrasing of my question, I am asking: do units have to make sense in physics?
Let me give an example of what I mean. Let's say that I have some arbitrary units (for ...
3
votes
2answers
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Why do we set the number of degrees of freedom of an electron in a gas to 2 rather than 3?
I was reading up on basic properties of plasmas and came across this description in the Wikipedia on how to convert between eV and Kelvin units to describe the amount of energy of the electrons:
The ...
0
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1answer
33 views
How do exponents of a unit of physical quantity affect converting between different prefixes of the same unit?
Lets say I have some 'quantity' in units (kg)^-1/2, I now want to express the quantity in g^-1/2. Do I simply multiply the quantity by k^-1/2 which is approximately 0.0316? The way I am going about ...
0
votes
3answers
79 views
Degrees vs radians [duplicate]
I’ve been messed over on a test (at least one that I can clearly remember, probably at least a few more) (and it was only a few points, but still very frustrating) because I forgot to switch my ...
0
votes
1answer
96 views
What does 'seconds squared per meter' mean?
I was wondering about the meaning of seconds^2 / meters (more specifically, period^2 / length - of a pendulum, after the square root graph of period vs length is linearized). I know that acceleration ...
1
vote
0answers
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What is the value of the universal gas constant in atomic units? [closed]
On Wikipedia the value of the universal gas constant is given for various units but not for atomic units. It should be easy to figure it out but I keep getting it wrong.
0
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1answer
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Can I safely replace the speed of light by 0.5 in equations?
In natural unit we set (for example in QCD) c->1, hbar->1, k->1.
All quantities can be expressed in terms of MeV. After the calculation, one can simply add c=3*10^8 properly.
However, if I ...
17
votes
9answers
3k views
How are physics theorems so accurate, relative to fixed measurement systems? [duplicate]
I admit this sounds like a dumb question. I know very little about physics, but I've always wondered that given the fact that:
A unit of measurement is a very specific constant value.
Then how can a ...
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votes
1answer
37 views
Transform lbm.in/min² into Newton [closed]
The units of physical quantities are important for the transformation of units, achieving coherence. So I need to transfom 10000 lbm.in/min² into ...
0
votes
1answer
36 views
Dimensional analysis physics [closed]
In a certain system, the units of length, mass and
time are chosen to be 1 cm, 100 g and 10 s. The
unit of kinetic energy according to above sys-
tem of unit, will be N erg. Find the value of N.
I did ...
0
votes
2answers
81 views
What does a.u. mean as unit (not astronomical unit) in graphs in physics publications?
For example, in this paper (also arXiv), it uses a.u. as the unit in multiple graphs for the quantiy $\frac{dI}{dV}$, for example in Figure 3. I am suspecting a.u. means "appropriate unit," ...
5
votes
0answers
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Are there any units of measurement (other than the ohm) that are represented by non-latin characters? [closed]
We have the prefix μ and the unit Ω, but other than that we seem to not use any other greek letters (or any non-latin symbols for that matter) for specifying units.
All other units – meters, seconds, ...
0
votes
1answer
64 views
How to define “unit spatial length” in general relativity?
Suppose I have a metric with time-independent $g_{\mu\nu}$. Suppose I have a small measuring rod at some spatial position and I define it to be of unit length. Will it still have unit length when it ...
0
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2answers
85 views
How does a proper motion displacement of $x$ arcseconds/year mean a star is moving at $x$ AU/year?
I trying to understand the author's answer to this question:
I don't understand the part underlined red. How does 3.6pc mean 3.6AU? It says a "distance of 3.6AU", but doesn't say what is ...
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0answers
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Is it possible to extract real physical values from multiphase LBM simulation with Shan-Chen force?
Is it possible to extract real physical values from multiphase Lattice Boltzmann method simulation with Shan-Chen force?
SC force is of the form
$$\vec{F^{SC}} = -\psi(\vec{x}) G \sum_i w_i \psi(\vec{...
0
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0answers
23 views
Derived units described with multiple 'per's
In a dimensional analysis problem, a rate was given textually as so many "Milliliters per Kilograms per Day". This seems ambiguous, as one could either interpret it as
A:: Ml / (Kg / Day) = ...
0
votes
1answer
34 views
Meaning of a product of two physical units [duplicate]
I am having a hard time to understand the meaning of measurements that involve a product of physical units, like $10\, \rm N\cdot m$ ($10$ Newton-meters), $3.33\, \rm J\cdot s$ ($3.33$ Joules-second),...
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0answers
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Why did the scientific community stop using hydrogen as the main measure for the atomic mass unit?
The only thing I could find is that the community switched form one hydrogen atom to 1/16 of Oxygen-16 (which they thought to be the only isotope that exists at time because they had no concept of ...
0
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1answer
59 views
Understanding the meaning of the integral of energy of an $\rm H$ atom
This is probably very basic but my notes are confusing and not clearly written so I would appreciate some help in trying to clarify the following points:
If we consider the expression $$\left\langle \...
0
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1answer
41 views
How to express the force of a GPU in Pound-hours?
I am training a neural network on a small cluster of GPU-equipped computers. Out of curiosity, I would like to express the force of the running GPU's in terms of the force required for a car to remain ...
