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Q&A

General Q&A about physics of any type and at any level

81%
+7 −0
Unusual way to write spacetime coordinates/metric: Is there any downside?

In special relativity, spacetime coordinates are normally given as $$(ct, x, y, z) \tag{S}$$ with the metric being either $$g = \operatorname{diag}(-1,1,1,1) \tag{+}$$ or $$g = \operatorname{d...

1 answer  ·  posted 2y ago by celtschk‭  ·  last activity 2y ago by dmckee‭

80%
+6 −0
How do constraints work in Lagrangian systems?

I have a question about the discussion of constrained Lagrangian systems in the book Mathematical Aspects of Classical and Celestial Mechanics by Arnold et al. (section 1.2.5). The Lagrangian syst...

1 answer  ·  posted 2y ago by Ezekiel‭  ·  last activity 2y ago by Derek Elkins‭

80%
+6 −0
What is inflation in cosmology?

I am having trouble understanding what a cosmic inflation is. Please try to suffice an explanation for non physicists / non formal students for physics / general audience. I understand that an ...

2 answers  ·  posted 2y ago by deleted user  ·  last activity 1y ago by deleted user

77%
+5 −0
How does probability conservation work in Dirac's original formulation of relativistic QM?

I asked this question on Stack Exchange, and didn't get an answer, but maybe someone here will be able to help. In non-relativistic quantum mechanics, the normalization condition for position eige...

1 answer  ·  posted 2y ago by Ezekiel‭  ·  last activity 2y ago by Derek Elkins‭

77%
+5 −0
Should I always write units in equation no matter if it looks like variable?

I am not sure if it is suitable in the site. I am asking the question here cause Olin and Celtschk said it here. They told me to use unit in every "single line" (Nope! They didn't say like it. Oli...

2 answers  ·  posted 1y ago by deleted user  ·  last activity 1y ago by Canina‭

75%
+4 −0
What is the meaning that the universe is flat?

I have heard the physicist Laurence Krauss claiming that he believes that the universe is flat. Okay, but if we take four astronomers and we put one in the north pole, one in the south pole, one i...

2 answers  ·  posted 1y ago by deleted user  ·  last activity 1y ago by celtschk‭

75%
+4 −0
How are the assumptions behind two ways of deriving the Rayleigh-Jeans law related?

The Rayleigh-Jeans law does a good job of describing the spectral radiance of a black body at low frequencies: $$B_{\nu}(T)=\frac{2kT\nu^2}{c^2}$$ with $T$ the temperature and $\nu$ the frequency...

1 answer  ·  posted 1y ago by HDE 226868‭  ·  last activity 1y ago by Derek Elkins‭

75%
+4 −0
Is it possible to derive the Dieterici equation starting from assumptions about microstates?

I was introduced to a somewhat novel derivation of the ideal gas law that starts by thinking about the number of microstates of an ideal gas, $\Omega$. Say we have a gas with a single particle in a...

0 answers  ·  posted 1y ago by HDE 226868‭

71%
+3 −0
If you're stopped and about to be hit from behind, should you brake or release the brake?

The title is a framing for a theoretical question; I'm not asking for practical advice. A friend was recently in this situation and my attempts to apply what I remember of a couple semesters of co...

1 answer  ·  posted 5mo ago by Monica Cellio‭  ·  last activity 5mo ago by Olin Lathrop‭

71%
+3 −0
What is virtual work?

$$\sum_i F_i \cdot \delta r_i$$ is virtual work when internal force is $0$. For that reason, $$\sum_i F_i \cdot \delta r_i = 0$$ Here internal force stands for what? When a object's displacement ...

1 answer  ·  posted 1y ago by deleted user  ·  last activity 12mo ago by deleted user

71%
+3 −0
What is "order" and "disorder" in entropy?

What is "disorder" in entropy? Entropy is measurement of "disorder". (Some says it's not "disorder") I had read "disorder" and "order" of entropy in my book. But I was wondering what "order" ...

2 answers  ·  posted 1y ago by deleted user  ·  last activity 1y ago by Derek Elkins‭

71%
+3 −0
How exactly do eddy currents slow down objects moving though a magnetic field

When a pendulum made of a conducting material moves through a magnetic field, it's a well-established fact that it experiences a retarding force, thus slowing it down, however, I'm unable to unders...

1 answer  ·  posted 5mo ago by esrdtfghjk‭  ·  last activity 5mo ago by Olin Lathrop‭

71%
+3 −0
Calculate Center of Thrust

If I have a rocket with some known number of engines $n$, each producing thrust with arbitrary direction $T_n$ and center of thrust $r_n$, the total thrust amount and direction $T_{total}$ is equal...

1 answer  ·  posted 1y ago by Josh Hyatt‭  ·  last activity 1y ago by Olin Lathrop‭

71%
+3 −0
How can the kinetic energy equation be intuitively understood?

Momentum is proportional to an object's velocity, and kinetic energy is proportional to the square of its velocity $\dfrac{mv^2}{2}$. It's pretty intuitive that if object B is going twice as fast a...

3 answers  ·  posted 1y ago by gmcgath‭  ·  last activity 1y ago by celtschk‭

71%
+3 −0
Why series springs behave like parallel? A comparison between parallel resistance and series spring.

I know that equation for parallel resistance is $$\frac{1}{r_{tot}}=\sum_i \frac{1}{r_i}$$ But i wonder to see equation of series spring constant. If we add multiple spring in series. Then their ...

1 answer  ·  posted 1y ago by deleted user  ·  last activity 1y ago by Olin Lathrop‭

66%
+2 −0
Interaction terms in Srednicki's proof of spin-statistics theorem

In Srednicki's proof of the spin-statistics theorem for scalar fields ("Quantum Field Theory", section 4), he considers interaction terms, added to the free Hamiltonian, that are Hermitian function...

1 answer  ·  posted 2y ago by Ezekiel‭  ·  last activity 2y ago by Mithrandir24601‭

66%
+2 −0
What's the meaning of "outdated" in physics?

I was recently reading some questions here and there saying that "relativistic mass is outdated". I saw someone saying that "outdated" doesn't mean the concept is wrong. My question is what physici...

1 answer  ·  posted 1y ago by deleted user  ·  last activity 1y ago by Mithrandir24601‭

66%
+2 −0
Should Earth's moon colonies being deep underground ensure natural heating?

Should Earth's moon colonies being deep underground ensure natural heating? By natural I mean "non artificial" i.e. not from "artificial" air conditioning. The question is based on the thought th...

1 answer  ·  posted 1y ago by deleted user  ·  last activity 1y ago by Dave Tweed‭

66%
+2 −0
What is Ether theory? (I think the book I read is misinterpreting Ether)

I was reading about Ether, Maxwell and Young-double-slit experiment. I got a question about Ether law. Let me say what I read about Ether Law. Suppose, a boat is traveling with Adverse of Water ...

2 answers  ·  posted 1y ago by A person who doesn't know anything‭  ·  edited 1y ago by A person who doesn't know anything‭

66%
+2 −0
Motion of charged particle inside a magnetic field

We have place a charged particle of 2C with mass 2kg, 1mm above a current-carrying wire of 1A.The charged particle has an initial velocity of 100m/s The magnetic field of the wire for simplicity w...

0 answers  ·  posted 1y ago by MissMulan‭  ·  edited 1y ago by MissMulan‭

66%
+2 −0
Applying Young-Laplace equation on meniscus formed due to rise of liquid on a single plate

Let's say we have a single plate with liquid on both sides rising up due to surface tension. The meniscus formed has a radius of curvature $R$. I'm trying to find the excess pressure, i.e, the pre...

0 answers  ·  posted 2y ago by TripleFault‭

66%
+2 −0
Magnetization as a function of temperature in ferromagnets

Suppose a ferromagnetic material with initial magnetization $M_o$.Is there some specific formula which calculates the total magnetization $M$ as a function of $M_{o}$ and the Curie temperature $T_{...

0 answers  ·  posted 2mo ago by MissMulan‭

66%
+2 −0
What is semiholonomic?

What is semiholonomic is? Holonomic actually means two point can be expressed by a equation. Non-holonomic can be expressed but, not properly. Holonomic e.g. $r^2-a^2=0$ Non-holonomic e.g. $r^2-a...

1 answer  ·  posted 1y ago by Anonymous‭  ·  last activity 1y ago by HDE 226868‭

66%
+2 −0
Calculating Max humidity for a house

I want to write some code to control my whole house humidifier. I want my code to calculate the percentile relative humidity above which dew will form on my windows. I have sensors for air tempera...

0 answers  ·  posted 1y ago by re89j‭

66%
+2 −0
What happens if an electron collides with a proton?

What will happen if an electron and a proton collide? They attract each other inside an atom. But, why they don't collide inside atoms? Does the "boundary" of the nucleus push them away? depends...

2 answers  ·  posted 1y ago by deleted user  ·  edited 1y ago by deleted user

66%
+2 −0
Why we can't find a particle accelerating unless there's some other particle accelerating somewhere else?

I was reading "Introduction to classical Mechanics" by David Morin. In that book they wrote that The third law says we will never find a particle accelerating unless there’s some other particle ...

3 answers  ·  posted 1y ago by deleted user  ·  last activity 1y ago by Olin Lathrop‭

62%
+3 −1
What does Lagrangian actually represent?

$L=T-U$ Here, $L$ is Lagrangian. T is kinetic energy. U is potential energy. But, what Lagrangian actually is? I know what Holonomic and non-holonomic is. But, I was thinking what the Lagrangian re...

2 answers  ·  posted 1y ago by deleted user  ·  last activity 1y ago by deleted user

62%
+3 −1
Why should there not be water below 6kms in the Earths crust?

In 1970, the USSR began drilling at the Kola Super-deep Borehole. The target depth was 15,000 meters; The stated areas of study were the deep structure of the Baltic Shield, seismic discontinuit...

0 answers  ·  posted 2y ago by Conrado‭  ·  edited 2y ago by Conrado‭

60%
+1 −0
Direction of vector of electric field

Suppose we have a arc of charges with some charge density $\lambda(\theta) = sin\theta$.I am using polar coordinates for convenience.But how can I find the direction of the unit vector of the net e...

0 answers  ·  posted 3mo ago by MissMulan‭  ·  edited 3mo ago by samcarter‭

60%
+1 −0
Clear up confusion on Minkowski signature

All given metrics are for orthonormal-basis. 2 dimensional spacetime : I saw that Minkowski Metric looks like this : $$\pmatrix{-1 &amp; 0 \\ 0 &amp; 1}$$ or $$\pmatrix{1 &amp; 0 \\ 0 &amp; -1}...

1 answer  ·  posted 1y ago by deleted user  ·  edited 1y ago by deleted user

60%
+1 −0
Find position of a particle at a time

If we have a force which changes depending on the position of a particle, how can we find the position of the particle at some time $t$? We can find its velocity if it has travelled a given distan...

1 answer  ·  posted 1y ago by MissMulan‭  ·  last activity 1y ago by celtschk‭

60%
+1 −0
Double slit experiment with 2 electrons fired from a gun

Suppose we perform the double slit experiment , but we fire instead 2 electrons instead of 1. In the double slit experiment performed in the 1920s a interference pattern was observed at the screen...

0 answers  ·  posted 6mo ago by MissMulan‭

60%
+1 −0
Calculating Surface temperatures of a thermal insulator

I'm interested in knowing the surface temperature of both sides of a double-pane or triple-pane window. Given the R-value of the window, and the air temps outside and inside, how can I calculate t...

1 answer  ·  posted 1y ago by re89j‭  ·  last activity 1y ago by Olin Lathrop‭

60%
+1 −0
What does Laplace operator represent?

I was wondering what's the physical meaning of Laplace operator. $\vec\nabla$ actually represent a field. I had seen that Laplace operator is written as $\vec \nabla \cdot \vec\nabla=\nabla^2=\Delt...

1 answer  ·  posted 1y ago by deleted user  ·  last activity 1y ago by deleted user

60%
+4 −2
Would we need Alternating Current if superconducting wires existed?

The major advantage of Alternating Current is that it can be transmitted to large distances without significant losses, which is not possible in Direct Current. Had economical superconducting wire...

2 answers  ·  posted 2y ago by theabbie‭  ·  last activity 2y ago by celtschk‭

60%
+1 −0
Maxwell equations EM wave confusion

The general formula for a EM wave (solving for the E field) is: where $\varepsilon = \varepsilon _{r}+j\frac{\sigma }{\omega }$ My professor told me that the conductivity of vacuum is 0 so we...

0 answers  ·  posted 4mo ago by MissMulan‭

60%
+1 −0
What's the importance of Poisson brackets?

$$F=F(q,p,t)$$ $$\frac{dF}{dt}=\frac{\partial F}{\partial q}\frac{\partial q}{\partial t}+\frac{\partial F}{\partial p}\frac{\partial p}{\partial t}+\frac{\partial F}{\partial t}$$ $$=\frac{\part...

0 answers  ·  posted 1y ago by deleted user  ·  edited 1y ago by deleted user

60%
+1 −0
Find jerk of time varying force

This gravitational field we move inside has some distance L after which it becomes 0.Before L it is just like any gravitational field. Suppose we move inside that gravitational field.The accelerat...

1 answer  ·  posted 1y ago by MissMulan‭  ·  last activity 1y ago by Mithrandir24601‭

60%
+1 −0
Is that electron which jumps from one stationary state to another?

In classical physics book of kleppner, i read that An atom can "jump" from one stationary state a to a lower b by emitting radiation with $E_a-E_b$. The frequency of the emitted "package of radi...

1 answer  ·  posted 1y ago by deleted user  ·  last activity 1y ago by Olin Lathrop‭

60%
+1 −0
Superconductivity

In my notes from University the reason a material can exhibit superconducting properties is at really low temperatures electrons form Cooper pairs of which the electrons of the Cooper pairs have lo...

0 answers  ·  posted 1y ago by MissMulan‭

60%
+1 −0
May it be that there are more than 8/9 planets in our solar system?

May it be that there are more than 8/9 planets in our solar system which aren't detectable with the current technologies? Is there any theory suggesting that our solar system has more planets, all...

1 answer  ·  posted 1y ago by deleted user  ·  last activity 1y ago by Canina‭

60%
+1 −0
Signal modeling as only digital, only analogue, or as both

In signal processing, are signals modeled in only one way (digitally or analogously) or are there some signals which can be modeled in both ways?

3 answers  ·  posted 1y ago by deleted user  ·  last activity 1y ago by Lundin‭

60%
+1 −0
What happens if an electron collides with another electron?

We know that electrons always repel electrons. But, if somehow they collide, then what will happen? I found a related question of above one in Quora. An incoming electron can instead make them ...

0 answers  ·  posted 1y ago by deleted user

60%
+1 −0
Conductivity with angle of light

In the lab I changed the angle the light hits a photoresistor and it doesnt obey Lambert's cosine law the conductivity of the photoresistor drops fast from +-20 to +-30 degrees angle.Why?

1 answer  ·  posted 1y ago by MissMulan‭  ·  last activity 1y ago by Olin Lathrop‭

57%
+2 −1
Why would a 1.74 index lens be more prone to chip and be less resilient than 1.67?

My optician counselled me I don't recommend a 1.74 index with a metal grooved frame, like the one you want from ic! Berlin, because the edge will be less resilient and be more susceptible to chi...

1 answer  ·  posted 2y ago by TextKit‭  ·  last activity 2y ago by Olin Lathrop‭

50%
+0 −0
System of ODEs models in physics

What examples of a system can be described by a system of ordinary differential equations?

1 answer  ·  posted 6mo ago by MissMulan‭  ·  last activity 6mo ago by Olin Lathrop‭

50%
+0 −0
How electric displacement is proportional to surface charge?

$$\begin{alignat}{2} &amp;&amp; \vec \nabla \cdot \vec D &amp; = \rho_f \\ &amp; \implies &amp;\int_V \vec{\nabla} \cdot \vec D \mathrm d\tau &amp; = \int_V \rho_f\ \mathrm d \tau \\ &amp; \impl...

1 answer  ·  posted 1y ago by deleted user  ·  last activity 1y ago by deleted user

50%
+1 −1
What's the equation of kinetic energy of charged particle?

I was looking for equation of motion. I came up with a solution but it doesn't satisfy me. Cause I was trying to find motion of that particle using Lagrangian. We know that $$W=\int \vec F\cdot d\...

1 answer  ·  posted 1y ago by deleted user  ·  edited 11mo ago by deleted user

50%
+1 −1
Is $E=mc^2$ true for all frame of references?

Which one is correct? $$E=mc^2$$ or $$E^2=(mc^2)^2+(pc)^2$$ I mostly seen $$E=mc^2$$ from my childhood, and when I was learning problem solving in relativistic mechanics I had seen $$E^2=(mc^2)^2...

1 answer  ·  posted 1y ago by deleted user  ·  edited 12mo ago by deleted user

50%
+0 −0
What is $\mathcal{O}$ in integration of potential?

I saw following equation in Griffiths EM $$V(r)=-\int_\mathcal{O}^r \vec E \cdot d\vec l$$ While the surface was closed but not symmetrical circular. At first I was thinking $\mathcal O$ was repr...

1 answer  ·  posted 1y ago by deleted user  ·  last activity 1y ago by deleted user

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