Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
Do moving singularities crack/tear space-time? Question: The tip of a crack in a continuum like glass is a singularity.
If we
A) set QM and Planck's volume arguments aside and stick to GR only
B) assume that at least some* blackholes contain a singularity and
C) assume black holes move in space-time,
then shouldn... | You are thinking classical physics + General Relativity.
In the centre of a black hole is a gravitational singularity, a one-dimensional point which contains infinite mass in an infinitely small space, where gravity become infinite and space-time curves infinitely, and where the laws of physics as we know them cease t... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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The counterfactual possibility to explain discrete energies in Thomsons plumpudding model In the german edition of "Tipler: Physics for scientists and engineers" there i a small text about the plum pudding model of the atom. i give a direct translation:
"Thomsons plumpudding model of a atom: The negatively charged ele... |
how could a oscillating electron emit descernible lines as the acceleration - the cause of radiation - would be continuous and not discrete or so.
The membrane of a speaker, playing a pure sinusoidal musical note, moves back and forth with a continuous acceleration. Despite this, the sound it produces has a single fr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/218351",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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Visualising gas temperature and gas pressure Gas pressure is created when gas molecules collide with the wall of the container creating a force.
Gas temperature is a measure of how fast the molecules are moving / vibrating.
However, they both seem to be concerned by "kinetic energy" of the molecules, or in other words,... |
*
*A gas is hot when the molecules collided with your measuring device.
Not quite. Gas heats your measuring device when the collisions are mostly such that the colliding gas molecule has more kinetic energy than the colliding measuring device molecule.
It's instructive to think colliding molecules as sumo w... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/218418",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
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Why don't galaxies orbit each other? Planets orbit around stars, satellites orbit around planets, even stars orbit each other. So the question is: Why don't galaxies orbit each other in general, as it's rarely observed? Is it considered that 'dark energy' is responsible for this phenomenon?
| There are plenty of satellite galaxies orbiting larger galaxies. The question is how long are you willing to wait for an orbit?
The Milky Way has a mass $M$ of something like $6\times10^{11}$ solar masses, or $10^{42}\ \mathrm{kg}$. The small Magellanic Cloud is at a distance $R$ of $2\times10^5$ light years, or $2\tim... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/218539",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "48",
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Why aren't all objects transparent? I know that for an object to be transparent, visible light must go through it undisturbed. In other words, if the light energy is sufficiently high to excite one of the electrons in the material, then it will be absorbed, and thus, the object will not be transparent. On the other h... | For an object to be transparent, the light must be emitted in the same direction with the same wavelength as initially. When light strikes a brick, some is reflected in other directions, and the rest is re-emitted in longer, non-visible wavelengths. That is why a brick is opaque to visible light.
Some materials we cons... | {
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FWHM increase with energy (gamma spectra) Below I have two plots from a gamma spectrum which I've been analyzing. The first plot is between a low energy range, the second between a significantly higher energy range. It is clear that the FWHMs (Full Width Half Maxima) of the peaks in the spectrum increase with energy. T... | The gamma photon energy is a function of the energy levels in the nucleus. There is some uncertainty in these levels. However, the data that you present in the chart is the amount of energy deposited in a detector, where the uncertainty is determined by the uncertainty in the interaction of the photon with the materi... | {
"language": "en",
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What is the relation between Nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR)? It seems to me that the basic principles are exactly the same, right? Then I am puzzled that the former was awarded a nobel prize while the later not.
I noticed a similar question here What's the difference between ... | I would say the maths and equations are pretty much identical except in H NMR you would use the gyromagnetic ratio for a proton, while in EPR you use the data for an electron. Both are spin 1/2 systems.
In terms of medical imaging it is easier to pick H2O via pulse NMR (rather than continuous field i.e what chemists d... | {
"language": "en",
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Time dilation on Satellites due to GR I am trying to determine the time dilation onboard a satellite (say GPS @ 20,000km) w.r.t an observer on the earth. I have already determined the special relativity component using:
$$
t' = \frac{t}{\sqrt{1 - \frac{v^2}{c^2}}}
$$
And I got the correct answer for the time dilation ... | The equation you quote:
$$ t' = t\sqrt{1-\frac{3GM}{rc^2}} \tag{1} $$
gives the time relative to an observer at infinity. You want the time relative to an observer on the Earth's surface. You need to calculate:
$$ t_\text{satellite} = t\sqrt{1-\frac{3GM}{r_\text{satellite}c^2}} $$
and:
$$ t_\text{Earth} = t\sqrt{1-\fra... | {
"language": "en",
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Is the number of turns in a loop part of the magnetic flux? In all both my physics textbooks, the number of loops $N$ is left out of the equation for magnetic flux $\phi_B = \int_s \vec B \cdotp\hat n dA$ and only when calculating the Emf induced by an inductor is the number of turns taken into account.
That leads to... | Yes, if you go around N times then the emf will be N times greater than if you went around once.
Why? Suppose one loop bounds a surface with area A, then N loops will bound a surface with area perpendicular to the field NA. Imagine you're looking directly through the solenoid and the surface it bounds is made out colou... | {
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Conflicts between Bernoulli's Equation and Momentum Conservation? The well known Bernoulli's equation states that
$P+\frac{\rho V^2}{2}=c$
However, a simple momentum conservation considering $P_1$ and $P_2$ acting on two sides, and velocity changes from $V_1$ to $V_2$, yields
$P_1+\rho_1 V_1^2=P_2+\rho_2 V_2^2$, which ... | I think I figured it out. Bernoulli's assumption is incompressible flow. The equation yielding from momentum conservation always holds. When velocity is low (incompressibility holds), the two equations yields similar results.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/220084",
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"source": "stackexchange",
"question_score": "2",
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Does the second law of thermodynamics imply a spacetime beginning of the universe? Recently I have been studying thermodynamics and I noticed a article by a religious person which says that the second law of thermodynamics proves that the universe had a beginning. A spacetime one. He says that the universe should be in... | No. A low entropy "initial state" could be the result of a so-called anthropic fluctuation in a (past) eternal universe.
Fluctuations about equilibrium could, fortuitously, create the initial conditions for life as we know it. This was proposed by Boltzmann and his assistant Schutz in the late 19th century, though ult... | {
"language": "en",
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"source": "stackexchange",
"question_score": "3",
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Working of electric-tester How does the circuit gets completed when we put an electric-tester glow when we put it in the +ve terminal, when actually the electrons flow from the -ve terminal ?
| The electric tester displays a light when electrons flow through it. When you touch a negative terminal electrons flow from the terminal through the tester and into you. When you touch a positive terminal electrons flow from you through the tester and into the terminal. Either way the light illuminates.
The current flo... | {
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Work Done On Circular Motion I am trying to find the total work done on a ball, $m=0.8kg$, tied to a rope of r=1.6m length and swung in a vertical circle.
I understand the total work done by both the tension in the string and gravity is 0 for a complete circle, but I am having trouble trying to find the work done by gr... |
If $\Delta S = r cos\theta$
then
$dl=ds=rd\theta$
and $F_g=mg$
$$W_{g}=\int_0^\pi mgrcos\theta d\theta $$
If you're taking the angle from the center of the circle (which you are, since you said that $\Delta S = r cos\theta$, then the initial position of the ball is $-R$, since displacement is a vector quantity (and t... | {
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Meaning of components of Maxwell's stress tensor $T_{ij}$ I am learning about Maxwell's stress tensor and what I understood is that the components, say $T_{ij}$ is something like a force parallel to the $j$th-direction acting on the surface with its normal in the $i$th-direction.
I was working on a problem which is to ... | To find the total force in the z axis you should sum over the z vector embedded in the field's matrix, which is the
The integral should be (for the net force in the z-axis):
$$ F_{z} = \sum_{i = 1, j = 3}^{i=3} T_{i}^{j} \cdot \hat{n}dS $$
With
$$ T_{ij} = \left( \begin{array}{ccc}
xx & yx & zx \\
xy & yy & zy \\
xz &... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/220616",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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When sugar is added to water, how does the mass change, and how does that affect the water's density and boiling point? I can't find a good answer anywhere. How does the amount of sugar added change the boiling point, mass, and density of water? Does it affect the mass or the volume? Or both?
| The mass doesn't change at all, it will be just the sum of the water mass and the mass Added, what happens is the change of density because the mixture, in general the molecules get closer to each other ( through the intermolecular forces) and, this way, the volume become lower to the same mass quantity, what increase... | {
"language": "en",
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"question_score": "1",
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Compton effect in photo-electric? In photo-electric effect Einstein said that photons incidents on material and gives their energy which will gives kinetic energy to electrons. But i also want to know that why Compton's effect not works in this situation. In my view when photon incident on material it should eject a el... | In some cases it does eject a photon with a lower wavelength, if it did not do this then the laws of conservation of momentum would not be supported thus disproving many aspects of modern physics. The problem with this is without the experimental evidence or data, it is hard for someone to calculate or predict the new ... | {
"language": "en",
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How to keep a helium balloon between 1 to 5 meters above ground? (without it being tied) I understand that helium balloons rise because their density is less than air, so they can rise up to a point where the air surrounding it has the same weight as the balloon.
I was thinking to fill it with something like half air a... | You might be interested to have a look at my answer to Why do helium balloons rise and fall? where I answer a closely related problem.
The change in bouyancy with pressure depends on how rigid the skin of the ballon is. If the skin is very rigid, i.e. the balloon volume doesn't change as the external pressure changes, ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "14",
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Magnetic field at the center and ends of a long solenoid A long solenoid has current $I$ flowing through it, also denote $N$ as the turns per unit length. Take its axis to be the $z$-axis, by symmetry the only component of the magnetic field inside is $B_z$. Find the magnetic field at the center of the solenoid (on the... | Notice, the magnetic field at some internal point on the axis of a solenoid is given by general expression
$$B=\frac{\mu_0 NI}{2}(\cos\theta_1-\cos\theta_2)$$
where, $\theta_1$ & $\theta_2$ are the angles between axis & the lines joining the extreme-points of both the ends of solenoid to the concerned point.
1) magn... | {
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Compute affect of a shower on density altitude As a pilot I have a basic understanding of density altitude, how temperature affects the effective air pressure:
I noticed recently that I have difficulty breathing when I take a shower in Santa Fe, NM, which is at an altitude of 8000 feet. According to the chart above i... | To account for the humidity, you need a chart (or equation) that relates the effective altitude to the humidity of the air. For example, if the chart you show here is for 50% humidity, then since the air density increases(lower altitude) as the humidity of the air increases, at 100% humidity, the effective density alt... | {
"language": "en",
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What's the bubble's wall made up of in false vacuum decay? It is well known that for some kind of double well potentials, there are two minima with one is unstable called the false vacuum while the other stable one called the true vacuum. The tunneling is allowed by quantum mechanics which is also called the false vacu... | That depends on what you mean by "made up of". Is static electric field "made up of" photons? Is the stream of water "made up of" waves?
To me these are semantic games that have no connection to reality. But if you are willing to answer "yes" to the questions above, then you can safely say that those domain walls are ... | {
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Infinite pulley system Infinite Atwood Machine Harvard Solution
Hi, I've been trying to solve this question for a while. I understand the first solution and also the solution to the second problem but I don't understand how to apply the second problem to solve the infinite pulley system.
In particular, I don't understa... | Here's a simple diagram of the Atwood machine you describe.
For reference:
$F_{total} = m_{total} \cdot a$
Let us call $g$ the gravitational acceleration, $m$ the mass of the less massive "block," and $a$ the acceleration of the "duel-block-system."
$$F_{total} = m_{total} \cdot a$$
$$a = \frac{F_{total}}{m_{total}}... | {
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Principle behind electrostatic shielding? If we have a solid conducting sphere with charges around it, then the electric field inside the sphere is zero, otherwise the electrons of the sphere would not be in equilibrium as there would be a net force acting on it. However, if its a hollow sphere, then why does the elect... | $E$ is necessarily zero inside hollow sphere because,
Inside hollow sphere
$Q = 0$
from gauss’s law
$$\phi=\frac{Q}{\epsilon}$$
$$E.A = \frac{Q}{\epsilon}$$
Since,
$E.A = 0$
$E = 0$ or $A = 0$ but $A\ne0$ so $E = 0$
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/221618",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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Can the hole concentration of cuprate superconductors generally be determined from lattice parameters? As it is known, in YBCO superconductors, lattice parameters are strongly dependent on hole concentration, so hole concentration can be determined by measuring the c-axis parameter, see e.g. "Oxygen determination from ... | Here is paper where lattice parameters dependence on x is shownn:
https://journals.aps.org/prb/pdf/10.1103/PhysRevB.49.4163
This graph I extact from this paper:
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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The anticommutator of $SU(N)$ generators For the Hermitian and traceless generators $T^A$ of the fundamental representation of the $SU(N)$ algebra the anticommutator can be written as
$$
\{T^A,T^{B}\} = \frac{1}{d}\delta^{AB}\cdot1\!\!1_{d} + d_{ABC}T^C
$$
where $\delta^{AB} = 2\text{Tr}[T^AT^B]$ is the normalization c... | I am not sure what you are asking. For every antisymmetric d-dimensional matrix $T$ you can extract the trace part and hence have
$$T=\frac{I}{d}\cdot tr{T}+(T-tr{T}).$$
You can check out that the first term is really the trace part and the second term is traceless. So in your equation, it is simply a definition of $d_... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
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Induced EMF dependent on terminal wire connection? Figure(a):
When a conductor moving inside magnetic field, of a given length at a certain velocity the induced EMF is:
$$\epsilon = vBL$$
However, what if we changed the position where the bottom wire is connected to the wire like so:
Is the induced EMF now:
$$\epsil... | The magnetic flux intercepted by the conductor will be reduced by connecting the wire at intermediate point of conductor because the effective length of the conductor will now $L_2$ & induced E.M.F. is given as $$\epsilon =vBL_2$$
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
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Null geodesic equations If one is constrained to the $xt$ plane, one can define the intersection with that plane of the null hypersurfaces originating at some point $P$ as
$$ g_{tt} \frac{d P^t}{d \lambda}\frac{d P^t}{d \lambda} + g_{xx}\frac{d P^x}{d \lambda}\frac{d P^x}{d \lambda} = 0,\tag{1}$$
$$ \sqrt{ \frac{g_{xx... | Comments to the question (v3):
*
*In 3+1D, the 1D-intersection of a null-hypersurface with the constraints $y=0=z$ does not need to locally be a geodesic, as simple counterexamples show.
*The analogous 1+1D question is more interesting: In 1+1D, the eq. (1) is locally always a non-affine parametrized geodesic. It ... | {
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Relativistic acceleration in sinusoidal electric field Consider a relativistic charge $q$ moving with an oscillating electric field $E_z$ with phase velocity $v_p=c$ in direction $\hat{z}$ (e.g. radially polarized laser coprogating with electron). What is the energy gain of this charge as a function of time?
I set thi... | Since I can't comment yet, here some thoughts to your initial question:
A changing electric field will always generate a magnetic field due to Maxwell:
$$\nabla \times {\bf H} = \varepsilon_0 \frac{\partial {\bf E}}{\partial t}$$
So you can't consider an electric field changing with time seperately! And you want to con... | {
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Does the proton wobble due to the electron's orbit in a Hydrogen atom? In a hydrogen atom the nucleus only has one proton and no neutrons so the electron to nucleus radio is higher for hydrogen than that for any other atom. Does the orbiting electron induce a wobble on the proton similar to the way a star wobbles due t... | Perfunctory quantum comment #1: "orbital", not "orbit". There are no little balls moving in a circles in there. These things are quantum objects.
But the sort answer is "yes", the proton has a non-zero momentum distribution that mirrors the electron's. Now, because the proton is nearly 2000 as massive, the proton's po... | {
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"source": "stackexchange",
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Exact meaning of radial coordinate of the Schwarzschild metric In this answer as well as on Wikipedia the radial coordinate of the Schwarzschild metric is described as follows:
...the r co-ordinate is the value you get by dividing the circumference of the circle by 2π.
This circumvents (no pun intended) the problem o... | The circumference is the distance you would measure if you laid out a (very long!) tape measure along a circle centred on the black hole. Or if piloted your spaceship round the black hole it's the distance your navigation computer would record.
Extracting these distances is less work then you might think. Suppose we us... | {
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Velocity of a leak in a closed water tank Bernoulli's equation states
$P_1+{1\over2}\rho v_1^2+\rho g h_1 = P_2+{1\over2}\rho v_2^2+\rho g h_2$
In a classic "water tank with an open top and a leak" scenario, "point 1" is the surface water in the tank, and "point 2" is the leak. The equation could be rewritten for $v_2$... | If the leak is closed, the velocity goes to zero. So instead of the water down near the leak having extra flow velocity ($v_2 > v_1$), it has extra pressure ($P_2 > P_1$).
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/222656",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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Chiral current VEV below the QCD scale Let's have pure QCD. I know that after spontaneous symmetry breaking quark bilinear form are replaced by their averaged values:
$$
\bar{q}_{i}q_{j} \to \langle \bar{q}_{i}q_{j}\rangle \approx \Lambda_{QCD}^3, \quad \bar{q}_{i}\gamma_{5}q_{j} \to \langle \bar{q}_{i}\gamma_{5}q_{j}... | By the chiral anomaly equation
$$
\partial^\mu \bar{q}_f\gamma_\mu\gamma_5 q_f
= \frac{N_f}{16\pi^2}
\tilde{G}^a_{\alpha\beta}G^{a\,\alpha\beta}
$$
this correlator is proportional to the topological susceptibility
$$
\chi_{top}=
\frac{1}{V}\frac{1}{(16\pi^2)^2}\int d^4x \int d^4 y \; \langle T\,
\tilde{G}^a_{\alpha\b... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/222775",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Moment of inertia of solid cube about body diagonal How do I find the above mentioned moment of inertia?
Steps I've tried:
1.) Triple integrations that proved to be to big.
2.) I noticed that the if we split a $2\times 2\times 2$ into individual $1\times1\times1$ components, the body diagonal of the $2\times 2\times 2$... |
If the moment of inertia of the 1x1x1 about body diagonal be I, then the moment if inertia of the 2x2x2 about its body diagonal will be 8I because it has 8 times as much mass.
This is the source of your confusion. The moment of inertia of a solid, uniform density cube about any axis that passes through the center of ... | {
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"timestamp": "2023-03-29T00:00:00",
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How do opaque materials have an index of refraction? The index of refraction defines how much light is bent as it passes through an object, right? So how can opaque objects, which don't transmit light, have a value for the index of refraction? (e.g., Blender Docs lists the IoR of steel as 2.5)
| Index of refraction refers to the speed of light in a material, which comes up when determining how much light is reflected vs. refracted. In an opaque material the refracted light is absorbed, but the intensity of reflection still depends on the illumination angle.
For example, light reflected at Brewster's angle is ... | {
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How do you determine if the spin is up or down? Fundamental particles such as quarks and leptons can have a spin either up or down. These spins are (obviously) opposite of each other. But what differentiates them? Let's say you examine a pair of electrons and you find out they're opposite (one up and one down). How do ... | Let's say you examine a pair of electrons and you find out they're opposite (one up and one down). How do you know which one is up and which one is down?
There is no way of knowing, or more accurately, there is no answer to your question. The wave function of the state you describe is a super position of the two optio... | {
"language": "en",
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Can we speed up the evaporation of black holes manually by accelerating it? If we throw an object to pass near a black hole, to bypass it, it will change the speed of the black hole, just like gravitational assist for a space probe. Does an accelerating black hole evaporate faster because:
*
*When object accelerates... | The question related to speeding up evaporation of Black Holes manually has no basis ever in the science of Physics of the universe. The General Theory of Relativity had predicted the presence of Black Holes as regions in space, in which space-time distorted in such a way that nothing, not even light can escape.
Howeve... | {
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How can I prove that D'Alembert operator is invariant under Lorentz transformations? I'm currently taking a course on Classical Electrodynamics and I'm trying to prove that the D'Alembert operator ($\square=\eta^{\mu \nu} \partial_{\mu} \partial_{\nu}$) is invariant under Lorentz-like transformations.
My professor does... | Let $\square = \eta^{\mu\nu}\partial_{\mu}\partial_{\nu}$ and let $\Lambda$ be any operator leaving the metric invariant, i. e. $\Lambda\eta\Lambda^{-1}=\eta$.
From the above it follows that the components of a $(2,0)$ tensor transform with twice the matrix $\Lambda$, whereas the vector components transform with $\Lamb... | {
"language": "en",
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Deriving the equation of motion for a rigid system I want to derive equation of motion for the system shown in picture. How do I choose a generalized coordinate in order to calculate kinetic and potential energy of the system? I need the aforementioned quantities to implement Lagrange equations for determining the equa... | Consider the net forces $F_x$, $F_y$ and moments $M_A$ at a point A, let's say the corner of the beam
now consider the center of mass at a point C located at $(c_x,c_y)$ relative to A. This location changes in each time frame because it is measured from an inertial reference frame (and not in the body frame).
The thre... | {
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How do I solve for $v_2$ where $mv_1^2 + MU_1^2 = mv_2^2 + M U_2^2$ and $MU_1 - Mv_1 = MU_2 - mv_2$ by eliminating $U_2$? I was trying to solve the head on collision slingshot problem where the rocket moving with speed $v_1$ approaches a planet which is moving with speed $U_1$. I wanted the final speed of the rocket ($... | I am getting $ U2=-U1-v1-v2 $ (Remember, these are added according to vector rules)
writing the two equations as,
\begin{equation}
M(U1)^{2}-M(U2)^{2}=m(v2^{2})-m(v1^{2})
\end{equation}
\begin{equation}
\implies M(U1-U2)(U1+U2)=m(v2-v1)(v2+v1)
\end{equation}
\begin{equation}
M(U1-U2)=m(v1-v2)
\end{equation}
Divide last... | {
"language": "en",
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How many particles can a particle accelerator accelerate at once? Does a particle accelerator lose its accelerating effectiveness as the number of particles being accelerated increases? According to Wikipedia, the mean acceleration of a proton in the Large Hadron Collider is 190,000,000 g's. Could the LHC accelerate on... | In general the total voltage $V$ seen by a particle (for a 1 TeV proton must be 1 trillion volts), multiplied by the beam current $I$, gives you the beam power:
$ P = VI $. So in principle you may have some trade off between the two of them, but often there are other limitations both to $V$ and to $I$. The designs of m... | {
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Is there a relation between complexity of a system and entropy? Disclamer: I'm not a physics professional, so pardon me if the question is stupid/incomperhensible/generally doesn't make sense. And I've googled it, but didn't find an answer.
Getting to the point, I would love to know if a bigger, more complex system exp... | Complexity behaves as the time derivative of entropy. In a closed system, entropy and complexity increase together initially, in other words the greater the disorder the more difficult it is to describe the system. But things change later on. Toward the end, as entropy approaches its final maximum where there is also m... | {
"language": "en",
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Is it possible that a person with myopia will see a blurry picture as normal? I am trying to process an image in good quality to appear blurred to a normal person and good to a person suffering from myopia
as seen in this source.
Is it possible that a picture that is blurry will appear normal to a person suffering from... | I think that you can do something like that, but you have to exploit a caveat.
Myopia affected people have the same depth of field of normal-viewing people, with the exception that instead of going from tens of cm to infinity, it goes from few cm to tens of cm. This forces myopics to go close to the objects in order t... | {
"language": "en",
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How do I figure out the totally airborne height for a given machine? Technically "airborne" can just mean to move through the air, but I would like to know how high you have to be before you are entirely supported by air in a helicopter-like machine, as opposed to benefiting from the reaction from the earth (or whateve... | The ground effect is only present in winged craft because in a flight with wings there is a much high pressure under the wings when closer to the ground this increases the normal lift effect. In propeller lifted craft the effect still applies because the propellers are just air foils twisted ,but they are also smaller ... | {
"language": "en",
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Elliptical orbit changing as a star's mass increases I'm studying Kepler's Laws, specifically the orbit of the Earth around the Sun. I know that if the Earth was more massive, the orbit would not be significantly affected. If the Sun was more massive, I know the velocity of Earth's orbit around the Sun would increase, ... | If the existing orbit were already circular then any changes like you describe would immediately result in an elliptical orbit. After that the orbit would tend toward a circular orbit. How long that takes depends on the total tidal forces. A rigid body would take longer than a pliable one as tidal forces are exchanged.... | {
"language": "en",
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How many fixed points does a Kelvin scale have? I have a book that says:
In the absolute Kelvin scale, the triple point of water is assigned the value of 273.16 K. The absolute zero is taken as the other fixed point.
But, then another section in the same book says:
On the Kelvin scale, the lower fixed point is taken... | A linear scale for measuring something - temperature, pressure, whatever - has to have two fixed points, because two points are necessary and sufficient to define a line.
If a scale is defined with more than two fixed points, then calibration using any two of them is expected to produce the same result (up to experimen... | {
"language": "en",
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Magnetic field due to stationary electric dipole As we know from Maxwell's 3rd equation the magnetic field is given as
$$\nabla \times \mathbf{E} = - \frac {\partial \mathbf{B}}{\partial t}$$
Now, if we consider an electric dipole which is stationary, there will be electric field lines
like this:
I want to know what w... | Since everything is stationary, all time derivatives are zero, and there are no currents. Now look at the relevant equations. The curl of $\mathbf{E}$ you wrote yourself: $\nabla \times \mathbf{E} = -\partial \mathbf{B} / \partial t$. And will there be a magnetic field? Well, let's look at the relevant equations:
$$\na... | {
"language": "en",
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Does Quantum Mechanics say that anything is possible? I may be incorrect in this, but doesn't Quantum Mechanics state that everything has a probability of occurring (with some probabilities so low that it will essentially never happen)?
And if I am correct in my thinking, could it mean that, quite literally, anything h... | The short and direct answer to your main question is no, quantum mechanics does not say that anything is possible.
In regards to the "walking through a wall" question, the answer is not as you say, "walk for eternity." All you have to do is live long enough for the wall to crumble, then you walk through. This would be... | {
"language": "en",
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Is time created by movement through a higher dimensional object? In a comment I read on this page, someone mentions a theory where time is created by moving through and slicing "moments" of a higher dimensional object. For analogy, a 2-D creature living on a plane that moves through a 3-D object may experience the diff... | I don't think time is even a well defined concept and how it behaves conceptually can vary between different theoretical frameworks. Asking if time is created or explained by something in particular is assuming a standard definition already exists.
So it would probably be better to just ask if time can be defined as sl... | {
"language": "en",
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Intuitive explanation for subsonic Fanno flow In most situations in physics, the effect of kinetic friction is to reduce the macroscopic kinetic energy of a system and convert it into heat, thereby increasing its temperature.
but in the case of subsonic Fanno flow, the opposite happens: temperature decreases and veloci... | Friction still dissipates kinetic energy from the fluid. However, since we are assuming that the flow is adiabatic, heat is not transferred out of the system and is instead fed back into the fluid.
To conserve total energy, this thermal energy can either increase the temperature and decrease velocity or decrease veloc... | {
"language": "en",
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What does it mean that the Rutherford's cross section is infinite? I'm studying elastic scattering and I read that the Rutherford's differential cross section is defined as:
$$\left( \frac{d \sigma}{d \Omega} \right)_R = \frac{Z^2}{4} r_o^2 z^2 \frac{(m_ec / \beta p)^2}{\sin^4(\theta/2)}$$
And the total cross section i... | That the total cross section is infinite just means that every charged particle that passes by the (bare) nucleus is scattered to some extent. This is a consequence of the Coulomb potential being long range. Classically, it is sufficient for the potential to be nonzero for all radii in order to have an infinite total... | {
"language": "en",
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Current from Middle Battery in a Two-looped Circuit
With this question, as with many tutorials of similar questions I’ve found online, my textbook only mentions three currents: $I_1$, which flows through the left loop from and to the 19 V battery, $I_2$, which flows through the right loop from and to the 19 V battery,... | *
*If only the 19 V battery was present, there would be currents in all branches, right?
*If only the 12 V battery was there, there would also be currents (but of different values) in all branches, right?
When both the 19 V and the 12 V batteries are present, they both contribute to all branches. The $I_1$, $I_2$ a... | {
"language": "en",
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What will happen to a human being exposed to Martian atmosphere? Mark Watney, in the movie The Martian, says that,
If the HAB breaches, I'm just gonna, kind of... implode.
The corresponding novel, by Andy Weir, says he will explode (as pointed out by @MikaelSundberg).
I think he will neither explode, nor implode, bu... | The Martian atmospheric pressure is approximately equal to $0.6\%$ of Earth's mean, at sea level, mostly consisted of $CO_2$ ($98\%$).
This is equivalent of pressure at altitude of approximately $17 km$ in the Earth atmosphere, with boiling point $30 ^oC$.
The pilots use oxygen masks at altitudes $> 4km$, so even if Ma... | {
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Which elementary particles are behind magnetic field, similar as photons behind radio waves? I see, there are photons behind radio waves.
As Wave–particle duality said: the radio waves are waves and at the same time are fluxes of particles called Photons.
I'm wondering, what is behind magnetic fields?
Means, magnetic... | First. Eletric and magnetic field is dependent of the inertia frame. Only when we see this two fields together, that we have an physical object living in space-time, being independent of the inertial frame. Actually, when we do this we learn what is the truly symmetries of our space-time. Einstein is the one that disco... | {
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Decomposition of Spring Can a spring respectively its stiffness in a 2D-system be decomposed into its x- and y- components (stiffnesses)?
Assuming a spring located in an 2D coordinate system with positions p(x,y), length l and angle alpha, describing the orientation of the spring inside the system. Can the spring be re... | If you are asking about the stiffness ( or spring constant $k$ ) , then obviously it cannot split into x and y components, because $k$ is a scalar constant. Suppose in your example, if the spring , at an angle $\alpha$ from x axis in $2D$ travels a distance $r$ then the displacement and force can be split into x and y ... | {
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Dirac equation, $\alpha_i$, $\beta$ hermitian The argument I've seen is the one given here: http://epx.phys.tohoku.ac.jp/~yhitoshi/particleweb/ptest-3.pdf under (3.10):
$$H=\vec{\alpha}\cdot(-i\vec{\nabla})+\beta m$$
$H$ is hermitian, $-i\vec{\nabla}$ is hermitian, so $\vec{\alpha},\beta$ are hermitian.
This is not con... | I think its important to say that the Dirac equation cannot be derived using standard QM: it can be motivated, but ultimalely the definitive argument for/against its correction is a matter of experimental confirmation.
This means that the arguments that the link gives you in favour of the equation are just formal: they... | {
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Reconciling De Broglie wavelength with relativity I have trouble understanding what a De Broglie wavelength means when differing frames of reference are taken into account.
Observers in differing frames of reference will see a particle's wavelength as being different. I think I understand a matter wave as similar to a ... | The De Broglie wavelength corresponds to a free particle's momentum, $\bf p=\hbar \bf k$, and a particle's observed momentum obviously depends on it's velocity relative to the observer.
The localization probability, on the other hand, is given by the wave function $\psi$ as $|\psi|^2$. But for a free particle $\psi \s... | {
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Why does moving air have low pressure? According to Wikipedia lift in an aircraft is due to an area of low pressure formed above the wings of an aircraft due to the fast moving air there. So why exactly is an area of low pressure created due to fast moving air?
| I don't know if it helps or not but give it a try
For this to understand let us consider a room in place of a hose say of length l in which a ball is bouncing up and down and colliding elastically with the ceiling and the floor continuously.
Now if we increase the speed of the ball in this upward or downward direction... | {
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Is all of a black hole's mass situated arbitrarily close to the event horizon? Forgive me if I'm thinking about black holes in completely the wrong way, but since time dilation increases to arbitrarily large amounts the closer you get to the event horizon of a black hole, wouldn't that make it impossible for anything t... | From what I know of black holes, you may get a few theories from this question, but the truth is we don't know presently. Singularities such as this are complete mysteries. We may never know, but as long as we survive our own self destruction, I believe we will figure it out eventually. :)
| {
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If I toss a coin, vertically, on the surface of Mars, will it land back in my hand? When I toss a coin in Mars, is the planets atmosphere rare enough that I'd rotate with the planet (at its angular velocity), but not the coin?
| The coin will come back to your hand just like it would on the earth. The effect of atmosphere is negligible comparing to the coin's inertia, so the horizontal position of the coin relative to your hand will hardly be affected. The rareness of the atmosphere will only affect the vertical motion of the coin, like how qu... | {
"language": "en",
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Will physics ever be able to answer the question: "What caused the universe to come into existance"? Or is this just the flip side of a metaphysical question "why does the universe not exist", if the opposite was the case and nothing existed i.e. a universe devoid of all energy, matter forces, particles of any kind, ma... | There used to be a belief that eventually we would discover/create a Theory of Everything (TOE) that would explain why all the constants have the value they have and why the universe and laws of physics could be no other way than they are.
I doubt whether anyone still believes that is possible.
| {
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Does quantum randomness measurably affect macro-sized objects? I understand that while it is believed that there is no true randomness on the macro scale, there is true randomness on the quantum scale. A previous theory that quantum processes could be determined through "hidden variables" has been disproven (through po... | The stochastic features of QM could leave, in principle, a "trace" at the macroscopic level.
This is because not every $\hslash$-dependent family of quantum states yields, in the limit $\hslash\to 0$, a completely deterministic classical state (phase space point). As a matter of fact:
Every possible classical phase-s... | {
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Sliding along a circular hoop: work done by friction
Assume a point object of mass $m$ slides along a hoop of radius $R$, starting from a position which makes 90 degrees with the line of radius connecting the center and the ground. Let the coefficient of kinetic friction between the hoop and the object be $\mu$. Assum... | You are almost there. All you need to do now is realize that the velocity can be deduced from the kinetic energy, which is the difference between the potential energy lost and the work done by friction. And you have expressions for both of those. See if that gets you there.
| {
"language": "en",
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"source": "stackexchange",
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How is $ \left(1-\frac{p^2}{2mE}\right)^{3N/2-2} =\; \exp\left(-\frac{3N}{2}\frac{p^2}{2mE}\right)\;?$ How is
$$ \left(1-\frac{p^2}{2mE}\right)^{3N/2-2} = \exp\left(-\frac{3N}{2}\frac{p^2}{2mE}\right)$$
(Karder, Statistical Physics of Particles, Page 107)
in the large $E$ limit. Here $N$ is particle, of the order of $... | You can use the approximations
$$1+x \simeq e^x$$ and
$$(1+rx) \simeq (1+x)^r$$
You can obtain
$$(1+(-p^2/2mE))^{3N/2-2}$$ which can be approximated in the $N\gg1$ limit as $$(1+(-p^2/2mE))^{3N/2}$$ which is approximately equal to$$(1+\frac{3N}{2}(-p^2/2mE))\rightarrow \exp [1 + (-p^2/2mE)]^\frac{3N}{2}$$ by using t... | {
"language": "en",
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Velocity of liquid molecules in turbulent flow I was solving some questions when I came across this:
what is the velocity of liquid molecules in contact with the walls of the tube? and the answer was given that it can have any velocity and then no further explanation. I tried to search from different sources but could ... | In laminar flow, fluid moves in defined layers. The boundary layer closest to the wall moves with least magnitude of velocity. The direction of its velocity is the direction of flow through the tube.
In turbulent flow, molecules become disorganized and can move in eddy currents that swirl in any direction, including ... | {
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About the ratio of the density of dark energy at the time of cosmic microwave background emission to the current density of dark energy In a question, I am given the current densities of dark energy, dark matter and normal matter and am asked to find the ratio of density of dark energy at the time of CMB and now.
The a... | Assuming dark energy is a cosmological constant and not quintessence or something more complicated, it is a constant and does not change as the universe expands. Well, if it changed it would be a cosmological variable not a constant!
For more on this type of calculation you should read Pulsar's answer to Equation for H... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/227860",
"timestamp": "2023-03-29T00:00:00",
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Do free-electron lasers actually lase? Free-electron lasers are devices which use the motion of highly energetic electron beams to produce bright, coherent radiation in the x-ray regime. More specifically, they start with a high-energy electron beam and feed it into an undulator, which is an array of alternating magnet... | A free-electron laser (FEL) is a parametric amplifier, which operates by transferring energy to the output signal (photon pulse) from an oscillator (electron bunch running down a long undulator magnet). An electron bunch is accelerated to relativistic energies and sent through a periodic magnetic structure (undulator) ... | {
"language": "en",
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How exactly does a bulb light up?
A typical value for the electron drift velocity in a copper wire is $10^3\ \mathrm{m\ s^{-1}}$. In the circuit below, the length of the copper wire joining the negative terminal of the batter to the lamp is $0.50\ \mathrm{m}$.
(i) The switch S is closed. Calculate the time it would t... | We must understand that the internal electromagnetic forces inside the wire constrains the charge movement.
It is not so different from a rigid body as a rocket or a ship. The source of power is at the rear in both cases, but the rear part can move only if all body moves. A ship can have a slow velocity, but as soon as... | {
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How to move a bubble which is trapped by the capillary pressure? I have a question about how to move a trapped bubble in a tube.
If we assume to have a horizontal tube, with water on each side of the bubble. The point to the left of the bubble is point 1, while the point to the right is point 2.
The capillary pressure... | From an engineering POV you could use sound, either in the audible or ultrasonic range to disrupt the adhesion. A transducer could be placed in the liquid at one end of the tube and arranged so some sound travels along the tube. If that is not possible them maybe attach a transducer to the outside. All experimental tho... | {
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Why can the spin of a relativistic particle not be orthogonal to its momentum? I have read that the 3-momentum of a relativistic particle cannot be orthogonal to its spin 3-vector. When thinking about how the spin vector transforms when the particle approaches light speed, it seems clear that it cannot be orthogonal to... | I think you are just referring to a simple kinematical phenomenon. Consider a massive particle carrying a $3$-vector $\vec{S} \equiv (s_x,s_y,s_z)$ in its rest frame, which exists just because the particle is massive.
This vector may describe the spin, but not necessarily it. Now suppose that the particle travels wit... | {
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Direction of static friction in inclined rolling motion I can't seem to understand in which direction static friction faces for inclined plane motion with rolling motion. This considers rolling motion without slipping, how do i find the direction of the static frictional force?
| The way i ended thinking about it is as follows: I convinced myself friction is forcing opposing motion. Now imagining a ball rolling down an incline, I considered the point of contact of the ball with the incline, call it x. This point intends to move in a direction that is opposed to the general translational motion ... | {
"language": "en",
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How to determine the radius of curvature of a convex lens? Suppose there is an equi-convex lens made of glass which has a focal length ($f$) of 30cm. Then, can we not say that the radius of curvature, $R$ of the lens is twice the focal length, i.e. $R = 60cm$?
Why do we need to use the Lens Maker's Formula for the same... | Intuitively, the radius of curvature has to depend on the index of refraction of the glass. If the index were $1$, the lens would have no effect at all. If the index were very high, say $10$, it would not take much curvature to get a given f'ocal length. Clearly we cannot just say the radius of curvature is twice the... | {
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How is the perceived quality of sound measured and analyzed? I am doing a physics experiment researching the effect of temperature inside the clarinet pipe on the quality of its sound. I know that the speed of sound is directly proportional to temperature of the medium it propagates in, I can measure decibels, frequenc... | This question is "turned upside down". The proper direction of research is:
$$
I \ can \ describe \ the \ percieved \ quality \rightarrow \ Let's \ seek \ for \ its \ physical \ (objective) \ description
$$
not the other way.
At this point the question suddenly becomes very broad and will be closed probably. Generally:... | {
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Why is the speed of sound lower at higher altitudes? At sea level the speed of sound is 760mph, but at altitudes like the Concorde would fly at (55,000ft) the sound barrier is at 660mph, so 1000th slower. Does it have to do with lower pressure?
| From a non-technical viewpoint, I would say that the simplest way I understand this is the following one.
Yes, it has to do with pressure. Actually, it might be easier to think that it has to do with density. Consider the dominoes effect. If the dominoes are more apart from each other, that is, if the density of domin... | {
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Hamiltonian of a quantum harmonic oscillator On page 286-287 of Nielsen Chuang's Quantum Information and Quantum Computation (10th edition) book, the Hamiltonian for a quantum harmonic oscillator is approximated as $H=a^\dagger a.$ What are the assumptions involved in such an approximation and why is this approximation... | I have not looked at the book , however the sense in which it is an approximation is that it is neglecting the constant term
The Hamiltonian of a SHO is ,
$H= (a^{\dagger}a + 1/2)\hbar\omega$
This means that the ground state energy of the SHO is $1/2\hbar\omega$. This is what is being neglected since it is only a const... | {
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visible light spectrum Why do we see black objects? Colors of objects are formed when the spectrum of that color is reflected. Example Green objects are green because they reflect the green spectrum of light, red objects are red because they reflect red spectrum of the visible light and white objects because they refle... | As long as there is a discontinuity in the index of refraction ( real or imaginary parts) then there will be a reflection from the "black" object. How your eye-brain perceives it is a different matter..
| {
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What is the metric tensor for? I am wondering how to use the metric tensor, in practice? I read the book and done the exercises in A student's guide to vectors and tensors by Dan Fleisch. The concept of a tensor and their applications are well defined.
In that book, is explained how to get the metric tensor for coordin... | The metric is a rank two tensor that defines several features of a differential manifold. It defines how to relate changes in distance to changes in coordinates, how to take take the inner product of two vectors. Given an inner product we have a way of measuring angles.
More indirectly, the metric describes geodesics ... | {
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What came first, the Universe or the Physical laws that govern the Universe? This sounds like the Egg and the Hen question but I am curious about this. If universe came first and created physical laws for itself, then what created the law or the principle as a consequence of which the universe came into existence in th... | As far as I understand, Physics is not able to awnser this question, because the physical laws we use to describe the Universe are not valid up to the exact event of the Big Bang (the Big Bang is said to be a singularity of spacetime).
Physics attempts to describe the Universe at a moment when it already existed, but d... | {
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Kinetic energy of a rotating object in an exercise, a linear molecule is being subject to a force applied on the edge in its axis. Then $K_1=\frac{1}{2}mv^2$, all is well.
Then in the second point of the exercise, the force is applied on the same edge but in an orthogonal direction to its axis. Then the molecule begins... | The energy of the system is not only proportionate to the force, applied to it, but is actually the work done by that force ($F\Delta s$) on the path that your system has travelled ($\Delta s$). In the first case the molecule has only the translational motion. In the second case, in addition to translational motion the... | {
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What's the reason double-slit experiment can't be explained by edge effects rather than quantum interference? Say we had exactly this...
But instead, it was a PING PONG GUN (imagine as table tennis players use to train),
throwing out PING PONG BALLS. The two slits are say 20 cm wide, and the observing screen is say 5... | Your explanation makes no sense. To see why, suppose you have two slits and you record a particular interference pattern as a result: a series of light and dark bars. If you then cut an additional pair of slits half way between the first pair of slits, the resulting pattern may have some dark bars where formerly there ... | {
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What would be the view like from inside a black hole looking towards the event horizon? Ignoring the fact that we would be torn apart by gravitational gradient and assuming we get some time to make some observations before hitting singularity, what would we see looking towards the event horizon or in any other directio... | There are actually some nifty simulations that show what you would see:
http://jila.colorado.edu/~ajsh/insidebh/intro.html
(Had to post as 'answer' because I don't have enough reputation to comment)
| {
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Why wavefunction is sometimes multiplied by the radius to get probability density? When solving 1d particle in a box, the probability density is said to be proportional to $|\psi|$, but when solving 3d orbitals, the probability density is said to be proportional to $|\psi|^2 r^2$. Why this difference?
| It's not "multiplied by $r^2$ to get the probility density". The issue is that the volume element in spherical coordinates is
$$ \mathrm{d}V = r^2\sin(\theta)\mathrm{d}r\mathrm{d}\theta\mathrm{d}\phi$$
and since the probability to find a particle in a subspace $X\subset \mathbb{R}^3$ is
$$ P(X) = \int_X\lvert \psi(r)\r... | {
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Derive drag coefficient of plate There is any analytical way to derive drag coefficient of flat plate aligned perpendicular to the flow? Wikipedia says it's between 1.98~2.05 but I want to get this value in calculation, not experimental value.
| Not really. For laminar flows, the solution will be the Blasius solution, but the solution is still numerical. There are analytical functions that can approximate it fairly well.
For turbulent boundary layers, there is even less hope for an analytical solution. The flow is non-linear and time dependent, but mean equati... | {
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Why are four-legged chairs so common? Four-legged chairs are by far the most common form of chair. However, only three legs are necessary to maintain stability whilst sitting on the chair. If the chair were to tilt, then with both a four-legged and three-legged chair, there is only one direction in which the chair can ... | The four-legged chair is the chair with the minimum number of legs (for whatever form of the sitting surface) to let you move in two perpendicular directions with equal effort. Leaning back, leaning forward, leaning right, leaning left.
One leg will make you tumble
Two legs will do that too
Three legs are coming closer... | {
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Total number of primary maxima in diffraction grating I am trying to determine the total number of primary maxima that can be observed when light of wavelength 500 nm is incident normally on a diffraction grating, with the third-order maximum of the diffraction pattern observed at 32.0 degrees.
Rearranging the diffra... | The diffraction pattern is essentially infinite on the screen on which it appears. We regard the location on the screen with a single coordinate, $\theta$, which is the angle between the perpendicular line stretching from the center of the grating to the prime maximum (we can call this point the center of the diffracti... | {
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Can one write down a Hamiltonian in the absence of a Lagrangian? How can I define the Hamiltonian independent of the Lagrangian? For instance, let's assume that i have a set of field equations that cannot be integrated to an action. Is there any prescription to construct the Hamiltonian of a such system starting from t... | The field equations must be conservative in a fairly precise sense in order that this can be done in a physically appropriate sense.
Then there are several Hamiltonian approaches to field theory: the De Donder-Weyl formalism and the multisymplectic formalism. Although both formalisms can accommodate Lagrangians, the c... | {
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Why are these equations valid despite seemingly inconsistent units? I am having quite a difficult time in trying to understand what units are used in this paper and how to convert things to SI. For example, look at equation (1):
$$T_M \approx 1500 \rho^{1/3}\ \mathrm{K}\tag{1}$$
It seems to be showing that temperature ... | The units are not consistent. Or in less precise terms, wrong.
Here's the only way I can think of for this to make some sense: just after equation (1), the paper says
...where $\rho$ is the density in $\mathrm{g\,cm^{-3}}$.
My guess is that they intend you to take $\rho$ as a pure number. For example, if the density ... | {
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When sunlight bounces off the Earth, why isn't the entire spectrum reflected rather than just the infrared portion? I've read that greenhouse gases absorb and reemit sunlight, and that the infrared portion is what bounces off Earth back to space. When sunlight bounces off the Earth, why isn't the entire spectrum reflec... | The reflectivity of the atmosphere, and of the surface itself, is strongly wavelength-sensitive. So while some percentage of any given wavelength is reflected -- and some percentage is absorbed rather than transmitted, the variation over wavelength is what leads to the somewhat misleading statement you refer to. Here... | {
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What is the minimum force required to move this block Please don't report. It's not a homework question. Yesterday on my physics test there was this question. there is a block of mass $m$ connected to a spring as shown in the figure. the spring constant is $k$ and the friction coefficient between the block and the floo... | What would the force be if there was no spring? Each side of the spring feels the same force - so if you put a black box around the spring and only saw the string "going in" and a string "coming out" of the box, with the same tension on each, the force needed to move the box would be the same.
This means your approach ... | {
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Would you save energy by heating the air in a shower stall so that you could use colder water? It is refreshing to take a cool shower in hot weather. And for the sake of discussion, lets assume that one should be "comfortable" with temeratures when taking a shower.
Considering that the vast majority of the heat from a ... | According to a steam shower vendor, a 10KW unit is required to provide enough steam at 118 deg.F in a 6'x8'x 8' enclosure. The timer runs for 20 minutes (about right for a nice shower), so figure the unit would use 3.3KW and 2 gallons of water to make the steam room hot and steamy.
According to numbers pulled from all ... | {
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Charge inside a charged spherical shell
*
*If I were to put a negative charge inside a negatively charged spherical shell, will it move to the center?
*Electric field inside the shell due to the shell is zero (Gauss's Law), would that mean the charge inside the sphere faces no force?
But, that doesn't make intuitiv... | How about this?
1) There is a charged spherical shell. The origin of the sphere must not have any electric field due to symmetry.
$${\bf E}({\bf 0}) = {\bf 0}$$.
2) Now take a point from the to the origin at $\bf r$. Due to symmetry of the problem the electric field has to be radial (points away from the origin), but ... | {
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Could a hydrophobic surface increase a liquid's resistance to compression/displacement? Imagine a quantity of an aqueous (yet slightly viscous) solution is resting on a hydrophobic surface with a contact angle around 100°.
A downward force is then applied as a (repellant) surface is lowered onto it, in order to spread... | I guess that depends on what qualifies as a large force, but I don't think that the shear forces will be significant.
Consider the Couette flow, the shear stress and thus the viscous force along the boundary scales with the velocity. As velocity increases so too does the viscous force. In your problem, the magnitude o... | {
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Dependence of average speed of molecules of gaseous mixture We know that the average speed of gases in a single gas chamber is given by $\sqrt{8RT/\pi M}$
where R is universal gas constant,T is temperature,M is molar mass of gas.
But what if we mix two gases in any ratio say 1:1 and then try to find average speed of an... | If the two gases are NOT interacting, then you can treat them independently and use your formula for each of them, weighting the result according to the mean speed, using your formula, will be
$$v=f \sqrt{8RT/\pi M_1}+(1-f)\sqrt{8RT/\pi M_2}$$
where $f$ is the fraction of gas number 1 and $M_1$ and $M_2$ the two molar ... | {
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Why is the "expansion postulate" a postulate of quantum mechanics? I'm currently reading the following set of lecture notes on quantum chemistry, which includes the so-called "expansion postulate" as a fundamental postulate of quantum mechanics: "The eigenfunctions of a linear and hermitian operator form a complete bas... | It's a postulate, not because it's a self-consistent mathematical property, but rather because it is an assumption about how the physical world may, or may not, be described.
There are many many different types of linear vector spaces other than a Hilbert space, so to make things easy they make an assumption and run wi... | {
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How can energy be negative in a finite square well? Say if the potential $V(x) < 0$ in the well but the sides or the scattered states its zero potential, anyways
*
*How is that the energy in the well is less than zero?
*Is it because the potential is less than zero?
| Energy or the value of $V(x)$ negative means it is a bound system. Think of it in this way, if a particle is free and has no kinetic energy and potential energy then it's total energy is zero. If this particle is not free or otherwise is bounded by a negative potential well then it's potential energy is $-V$. You have ... | {
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Backyard experiments to falsify the Flat Earth theory I recently became aware that the flat Earth theory still exists in the 21st century, and has colored the views of a friend of mine. Roughly speaking, the tenets are:
*
*The Earth is a flat disk, with the south pole blown up into a circular "ice wall" where one wo... | Foucault Pendulum is a great example.
The original purpose of this experiment was to prove that earth rotates relative to the stars and not the other way around, yet nevertheless it proves that the earth rotates in a way which contradicts the "flat earth" theorem.
This experiment can be easily recreated at home, if you... | {
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Mass of proton vs mass of nucleus I have just started reading nuclear physics.I know that the sum of masses of the quarks is less than the proton or neutron itself as a whole .
But why is it that the sum of the masses of the nucleons(protons and neutrons) is more than the nucleus itself?
whats the difference between th... | The difference comes from the kind of force that holds the constituents together. The force on the quarks in the nucleon is the color force, (one of the four fundamental forces), between nucleons, it is the residual color force, which appears as the strong force that binds the protons and neutrons in a nucleus.
Th... | {
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Does potential difference or electric field change with distance between parallel plates? Say you have a set of parallel plates, one is positive and one is negative, if you change the distance between them would electric field strength change or potential difference, given the equation
$E=dV/dx$
From pure intuition, ... | If you treat parallel plates as infinite sheets, then you get a constant field in between that depends only on charge density. This isn't exactly true in reality, since real plates aren't infinite. But it gives a good estimation.
| {
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Why is meteor speed what it appears to be? Is the speed of a meteor through our sky because of the speed of the earth's axis rotation, or because the meteor is speeding towards us at that speed?
| Meteors are essentially bits of rock that are independently in orbit around the Sun and which cross the Earth's orbit. If the Earth happens to be there at the same time then it will enter the Earth's atmosphere and we will see a meteor.
The velocity of meteors is related to how fast they we going in their orbit around ... | {
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Why do materials show plastic behaviour for large stress? As the stress is increased, the strain increases proportionally up to elastic limit and the material regains its original dimension within elastic limit. When the stress is increased further the material shows a plastic behaviour. What change in the internal str... | Plastic behaviour is characterised by there being permanent (non-reversible) deformations. In terms of molecules held together with springs (the bonds) in plastic deformation the springs are broken and the bonds then might the be between different molecules.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/232713",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 0
} |
Torque: The T-shaped stick problem This is a problem that I have been unable to solve for some time.
Imaging a T-shaped stick, as shown in below image, which do not deform in any appreciable way and has pivot point at the tail of the "T".
There are two questions in my problem, the first is: given that the "T" is symme... | The torque is done with a cross product and thus is going to be:$$\tau = |\vec F| |\vec r| \sin\theta$$where $\theta$ is the angle between the position vector $\vec r$ (which points from the pivot to the place where the force is applied) and $\vec F$ (which points however the force points).
You can also express this as... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/233012",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Coefficient of friction and practical experience of sliding The classical model of friction has a coefficient of friction depend only on the materials, but not area, and the force proportional to the normal force and coefficient of friction. So a given object on the same surface has the same friction whether it is supp... | In laboratory experiments with bricks, the sliding angle comes to be the same irrespective of the orientation of the brick. However, when it comes to a child sliding there are some extra factors. The child usually is not sitting at normal to the plane especially at large angles of sliding due to the fact that he can to... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/233094",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 5,
"answer_id": 2
} |
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