Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
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What Keeps a Pendulum Moving In a Circular Path?
From the figure, we know that $F_{net} = mg\sin\theta$. Now, this force $\vec{F_{net}}$ is in the direction of the velocity $\vec{v}$ of the bob, both are tangent to the path. Therefore, the net acceleration $\vec{a_{net}}$ has no component perpendicular to the path, th... | Note that a (simple) pendulum performs (simple) harmonic motion and oscillates about it's mean position. For circular motion, the force must always be directed towards the center (centripetal). If you draw the force vectors at different positions (one position is shown in the image) as the bob moves note that the force... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/245223",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
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When thermal IR gets reflected from an object, does it change its wavelength (frequency) I'm working with thermal infrared (IR) cameras to detect human thermal radiation.
I notice I can easily distinguish non-human objects throughout the camera's field of view, though all are at same room temperature.
I assume this is ... | Reflecting from an object doesn't change the frequency.
However the object may not reflect all frequencies equally - that's why things appear colored in white light.
The human is at a different temperature to the room and so will emit at a different wavelength than the other objects at room temperature. It is possible ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/245314",
"timestamp": "2023-03-29T00:00:00",
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If gravitons are 'real' and analogous to photons are they also being 'stretched' by the universe's expansion? Since photon wavelengths are stretched by our expanding universe, appearing to us as a redshift, would graviton wavelengths similarly be stretched? For that matter, do gravitons even have a wavelength like pho... | What has just been proven is the existence of gravitational waves, not gravitons. Beside, if graviton exists, they are likely to be a "pseudo particle" like the photon, i.e., mostly a quantized emission of wave packet.
As a wave, by construction the downstream part is late compared to the upstream part, and because of... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/245520",
"timestamp": "2023-03-29T00:00:00",
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Swing: why does the body position modify the amplitude? When a person swings, why does the amplitude of oscillations increase if the person changes the body position ?
That is, when descending and approaching the vertical position, if the person extend his legs and changes the body position almost horizontally, the amp... | You are not adding energy, but changing the rotational inertia (of the person relativ to the center point of the rotation/swinging).
In the bottom point of the swinging, where the locational energy is 0, the total energy of the system is calculated as rotational inertia * rotational speed, and that is constant (without... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/245632",
"timestamp": "2023-03-29T00:00:00",
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Force with different distance dependence Recently, I have learnt about the central force motion. I came across some forces with different distance dependence, for example, k/r^2(common) one, k/r^4, or even the combined one k/r^2 + k'/r^4 (k, k' are force constant).
I remembered when I first learnt the inverse square l... | While electric forces are 1/R^2, that's only between two charges. If you look at distributions of mixed charges, the force laws include dipole, quadrupole,
octopole... and while those are a bit more complex than simple central forces (because they depend on orientation), the scaling with distance includes 1/R^N
terms ... | {
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How to interpret the units of the dot or cross product of two vectors? Suppose I have two vectors $a=\left(1,2,3\right)$ and $b=\left(4,5,6\right)$, both in meters.
If I take their dot product with the algebraic definition, I get this:
$$a \cdot b = 1\mathrm m \cdot 4\mathrm m + 2\mathrm m \cdot 5\mathrm m + 3\mathrm m... | While being admittedly geometric (um..... they're trig functions), this diagram concisely shows the relationship between the units. This has been said in a variety of different ways in the comments and other answers:
The same principal applies to the cross product. The diagram would be similar but look even more "geom... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/245829",
"timestamp": "2023-03-29T00:00:00",
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Questions about Biot-Savart law and Ampere's law A textbook I'm studying with described finding vector magnetic potential $\vec{\text{A}}$ from Biot-Savart law as below.
$\vec{\text{H}_2}=\int_{\text{vol}}\frac{\vec{\text{J}_1}\times\hat{\text{a}_{\text{R}12}}}{4\pi\text{R}_{12}}dv_1$
$=\frac{1}{4\pi}\int_{\te... | Those two points are in the same coordinate system, but different and independent points nonetheless. Taking the derivative $\nabla_2$ with respect to point 2 means "how much does the given expression change if I vary point 2". And point 1 does not change if you vary point 2, therefore the derivatives $\nabla_2 x_1, \n... | {
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Are objects generally neutral or charged? Basically the questions relates to what is taught about electroneutrality and has 2 parts:
*
*Is the overall charge in the universe zero?
*Are objects the world generally neutral or slightly charged, constantly passing a little bit of charge from one to the other on contac... | The answer involves the current model of the universe. The current model, the Big Bang model, assumes that everything is charge neutral from the beginning of he appearance of the universe almost 14 billion years ago.
The model incorporates the standard model of particle physics and uses it to project the generation of ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/246057",
"timestamp": "2023-03-29T00:00:00",
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No buoyancy inside liquid
The answer to this question is given as (a) 5m.
Due to the difference in pressure at the curved parts there will be a net upward force of buoyancy.
And how can that be calculated without knowing the shape?
The reasoning is further illustrated in the image below.
Am I missing something?
| The idea is to find the force that the water exerts on the object. If the object were not glued to the bottom, but were instead surrounded by water, the upward force would be given by Archimedes principle. However, since there is no water under the contact area, the Archimedes force of the water is reduced by the co... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/246253",
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Airplane on a treadmill - Variant Thought Experiment This thought experiment is in a way related to the (in)famous airplane on a treadmill problem.
If you take a ball and place it on a treadmill, will the ball:
*
*Move backwards relative to the ground at the same speed as the treadmill (as if placing any other non-c... | CASE I : No friction
If the surface is friction less then, the ball wont move neither rotate, it will be motion less
CASE II : Friction
We know that
$$
\tau= I\alpha
$$
$$\tau=\mu mgr $$ and $$ I \alpha K$$
K is radius of gyration
Above statements provide the equations related to ball's rotational motion
Regardin... | {
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How does an electron move through a metal or a semiconductor? I understand that atoms in a metal or semiconductor are bonded and that they will have several eigenstates where electrons can reside. When a voltage is applied electrons in eigenstates will move to other eigenstates and generate a current. However, what is... | To expand the previous answer.
You can model conduction properties with the Kubo formalism, but also using the Landauer-Büttiker formalism. It basically states that every conduction problem can be considered as a scattering problem. A key difference between Kubo and Landauer is that the first one is time-dependent prob... | {
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Einstein-Infeld-Hoffmann (EIH) Lagrangian for a Test-Particle as Limit of Schwarzschild-Geodesic Consider a test particle of mass $m$ which is in orbit around a spherical-symmetric body with mass $M$. It therefore has a position as described by the coordinates $r,\phi$, and its motion can be described by the Lagrangian... | TL;DR: OP's Lagrangians (1) & (2) are not directly related beyond the 0PN approximation.
*
*OP's eq. (2) is (up to normalization) a gauge-fixed version of the square root action
$$\begin{align} L_0~=~~~~&-mc\sqrt{-g_{\mu\nu}\dot{x}^{\mu}\dot{x}^{\nu}}\cr
~\stackrel{\text{static gauge}}{=}&-mc\sqrt{g_{00}-v^2 +{\cal O... | {
"language": "en",
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Forces that exert torque on a rigid body in rotation when angular momentum is not parallel to angular velocity I'm confused about the rotation of a rigid body, when the angular momentum $\vec{L}$ is not parallel to the angular velocity $\vec{\omega}$. Consider a barbell with two equal masses that rotates around a verti... | You have to approach problems systematically, and not intuitively. Like I stated in a previous (accepted) answer, resolve everything on the center of mass, and only in the end transfer the quantities to a different point (like P) to get the results you want.
I start with the kinematics. Use $\ell_1$ and $\ell_2$ for th... | {
"language": "en",
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Reversible process, equivalence of two definitions? There are two common definitions of a reversible process:
A reversible processes is quasistatic with no dissipation.
And
A process where an infinitesimal change in conditions would reverse the direction of the process.
Is it possible to show the equivalence of t... | The second definition includes involvement of both the system and the surroundings. For this definition, I like to envision that the surroundings has available the following sets of tools:
*
*Tiny weights that can be added or removed from a piston, when the piston is at different elevations
*An array of constant ... | {
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Angle of a pendulum as an explicit function of time What would be the solution for the angle $\varphi(t)$ and angular speed $\omega(t)$ on pendulum without the small angle approximation - not as differential, but as an explicit function of time?
The pendulum starts at angle $\theta_i$ with the arbitraty initial angula... | The differential equation to start with is
$$\ddot{\varphi}(t) = -\frac{g}{r}\sin(\varphi(t)) ~ , ~ \dot{\varphi}(0) = -\omega_0 ~ , ~ \varphi(0) = \theta_0 $$
where $\varphi$ is the angle of the pendulum, $\omega_0$ the initial angular velocity and $\theta_0$ the initial angle. The angles are aligned to have 0° on th... | {
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Microscopic Definition of Heat and Work If I am given a statistical system, then I can define state-variables like Energy, Entropy or other observables, and then I can (at least for equilibrium states) give the infinitesimal change of energy as:
$$ d E = T dS + K dx $$
Here x means any observable and K means the depe... | Let's consider an exchange dE of energy. Using the statistical definition of $E=\sum p_i\epsilon _i$ as the average value of the energies of the microscopic states :
\begin{equation}\label{eq:dE}
dE = \sum\epsilon_i dp_i + \sum p_i d\epsilon_i
\end{equation}
We can see that the change in average energy is partly ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/247001",
"timestamp": "2023-03-29T00:00:00",
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Is Newton second law covariant or invariant? Is Newton second law covariant or invariant between two inertial frames, moving with uniform traslational motion with respect to each other?
If it is invariant then, indipendently from the frame, $\vec{a}=\vec{a'}$ and $\vec{F}=\vec{F'}$ (of course $m=m'$) and this means th... | To add more details to jimjo's answer, I would like to explain the "at most" in my comment
Vector, at most, can be covariant.
Three-vectors are only covariant under rotations, but if you include boosts then three-vectors transform in a non-covariant way. Therefore, Newton's second Law is non-covariant under the full ... | {
"language": "en",
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Why don't we hear sound reflecting from buildings, mirrors, etcetera? We can see buildings, doors, cars etc. as light falls on it gets reflected to us.
but why doesn't the same thing happen with sound? I mean why don't we hear sound reflecting that much?
| It is because of our reaction time ,speed of sound and disturbance in atmosphere. As our reaction time is 1/10th of a second and speed of sound is about 343.2 metres per second. So sound travels 34.32 metres in 1/10th of a second. To distinguish between 2 sounds the distance between them must be 34.32 metres and if we ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/247487",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Why must we use mode locked lasers? Quick question:
If I have a laser cavity with a bunch of harmonics under the gain curve:
Why do I not always get a pulsed laser? EG:
I mean since these harmonics exist in the cavity arn't they always in phase because the ends are closed and they create standing waves in phase to ea... | I think you are asking if the various harmonics in a laser cavity are in phase with each other, and if not then why not.
It's pretty straightforward. Consider two laser wavelengths in a cavity of length L: one is $\frac {L}{N}$, and the other is $\frac {L}{N+1}$. Though the two can have the same phase at the ends of ... | {
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Energy of electrons in wire The Drude Model helped me quite a lot to visualize how current could flow in a circuit. However, there is still a point that I cannot grasp in the explanation given by some people when they talk about energy. When considering a round trip of one electron along a circuit, it is usually said t... | 1) Battery constantly moves electrons from the positive terminal to the negative terminal creating non-zero positive and negative charges at the respective terminals. These net non-zero charges create electric field inside the conductor (E=V/d).
2) This electric field inside the conductor accelerates electrons which ... | {
"language": "en",
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Is it always possible for an observer to realize to be in a non-inertial frame? Galilean relativity principle states that two frames moving with uniform linear motion cannot be distinguished. But is it always possible to realize to be in a non-inertial frame?
In a rotating frame it is surely possible for the observer t... | It is possible as soon as one is sure to be very distant from every body (gravitational source) in the universe. This is because all inertial forces behave as gravitational forces. If one is confined to stay in a closed room and observes the motion of bodies therein, he/she cannot decide whether the observed accelerate... | {
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Understanding role of friction in block on block problems Consider two blocks stacked on top of one another. There is friction between the blocks but there is no friction between the lower block and the table. So the only frictional force that tends to retard the lower block is due to the friction between the blocks ri... | If the force being applied is small, it can be applied to either block, and the static friction force will keep them accelerating together. If it is large then the maximum static friction force will be exceeded, and the upper block will slide along the lower with constant kinetic friction force f = μN, where N is the ... | {
"language": "en",
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Photoelectric effect on charged plate As far as I know, to observe the photoelectric effect, one has to expose a metal surface to high-energy radiation. But what happens if the surface has a surplus of electrons? What is the energy needed to knock of those? Would a lower frequency be sufficient to produce the same effe... | When light falls on the photoelectric surface, photoelectric radiations are emitted, when the energy is more than the work function i.e when more electrons are made to fall on the photoelectric surface,it will go beyond the work function and electrons will be emitted normally.
No,there will be not the same effect becau... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/248168",
"timestamp": "2023-03-29T00:00:00",
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Why does limiting friction have to act when a block tied to a wall is pulled? A block lying on a rough surface, is connected to a wall by a mass less, inextensible string and an unknown amount of force is applied to the block opposite to the side of the wall. Now, if it is given that there is some tension in the string... | If I'm understanding your question properly, the block in that situation is over-constrained (i.e. statically indeterminate - you cannot calculate the unknown forces from the laws of statics alone) - you cannot assume that F equals the limiting friction - both it and the tension are unknown.
When you apply a sideways f... | {
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Why do constants have dimensions? I am just a beginner in dimensional analysis, and I see that $G$, the universal gravitational constant, is independent of everything. Speed, for example, depends on distance and time, but $G$ does not depend upon anything.
Then why is $G$'s dimensions not $M^0 L^0 T ^0$, as it is not ... | G is actually not independent of everything.
It does depend on factors like density of space, rate of universe expansion etc.
Its just scientists have not found this dependence.
For time there have been many constants which were later were found to be dependent.
For example at time of Kepler
$T^2 =k R^3$
Later,
k was f... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/248376",
"timestamp": "2023-03-29T00:00:00",
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Tree Level Feynman Diagrams of Electron Positron Interaction Consider the interaction $$e^-+e^- \rightarrow e^-+e^-$$ The following is a tree level Feynman diagram for this: We can also make the paths of the two electrons on the right hand side cross over and this is a distinct diagram.
However lets consider the inter... | 'Crossover diagrams' exist when the final products are indistinguishable. When using Feynman diagrams for computations, you need to add all distinct diagrams that depict the same process, i.e., have the same external lines. Both the crossed and uncrossed diagrams for the electron-electron process have the same final pr... | {
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Can lasers lift objects? I have been fascinated by a very intriguing question - Can lasers push objects up?
I have done the below math to find out
Lets say we have a $1000~\text{mW}$ laser and we would like to lift an object of weight $100~\text{g}$.
By definition: $1~\text{W} = 1 \frac{~\text{J}}{~\text{s}}$
That mean... | Lookup Optical Tweezers.
The limitations of the technique are due to the damage thresholds; see laser ablation.
This idea has been used extensively in science fiction, especially when implemented as solar sails. It's even practical for some applications, as noted in the article. But laser propulsion from the ground ... | {
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Quick question on weight/mass (In the US, just to clarify)So, from a physics perspective weight and mass are different, but when people are talking about weight in everyday(non-physics) situations ("how much do you weigh" etc.), are they actually talking about mass and it's just common to refer to it as 'weight'?
Expan... | your mass is independent of gravity (ignoring relativity). in space (or on the moon) your scale would not show what it shows on Earth, so it can't be measuring your mass. a scale measures weight, but sometimes converts to mass via
$$ Weight = 9.8 N/kg * Mass$$
(near surface of Earth)
| {
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Why do positron generators use solenoid magnets to focus positrons instead of FODO structures? When positrons are generated from a particle beam hitting a dense target, why do we use solenoid electromagnets to focus the resulting positrons? As far as I can see, a FODO (focus-open-defocus-open, focus means quadrupoles ... | Solenoid magnets can focus in both the X and Y planes, unlike quadrupoles. They are also simpler devices that don't require the precise alignment that a FODO array would require. The limitation on solenoid magnets is that they are only effective at focusing low-energy beams, like the positrons from your spallation sour... | {
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Relation between the electromagnetic wave and quantum wavefunction I have been thinking about this for a while. I think I misunderstood something about the basics of quantum waves.
Let's look at light diffracted in conditions similar to the double slit experiment. The intensity pattern we can observe on the screen is ... | Photons don't have an associated wave function. You either use the (fully classical) Maxwell's equations for the electromagnetic field (without photons) or Quantum Electrodynamics (which doesn't work with wave functions at all).
For more details, see What equation describes the wavefunction of a single photon?.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/249116",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Were the Michelson-Morley results a surprise? How unexpected were the Michelson-Morley experiment results?
Did physicists have theoretical reasons to predict that the speed of light would result to be invariant?
| As far as I know, the only clue at the time that the speed of light would be invariant were Maxwell's Equations where "something" shows up as a constant. However, speed of light being invariant in all inertial reference frames is very counter-intuitive. One might rather expect physics to be slightly different in differ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/249507",
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Spin Orbit Coupling Hamiltonians I am really struggling with something fundamental.
I keep coming across two versions of the hamiltonian for spin orbit coupling:
$H_{soc}=\frac{\mu_B}{2c^2}(v \times E) \cdot \sigma $
$\mu_B =$ bohr magnetron
$v =$ velocity
$E = $ Electric Field
$\sigma = $ pauli matrices
and
$H_{soc}... | The first formula is generally valid while the second applies to spherical symmetry.
| {
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Do any elements form stable doubly-charged negative ions? It is perfectly possible for an atom - particularly on the electronegative end of the periodic table to form negatively-charged ions by attracting an electron, and these species can be stable, requiring (a small but positive amount of) energy to detach the extra... | Assuming I understand you correctly the quantity you refer to is the second electron affinity i.e. the energy absorbed in the gas phase reaction:
$$ X^- + e \rightarrow X^{2-} $$
(it's the energy absorbed because a negative second electron affinity means energy is released)
If so, there are no elements for which the se... | {
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Rolling without slipping in absence of friction force I'm confused about a rolling without slipping situation. Suppose to have a disk of radius $R$ on a floor, and exert a horizontal force at a certain distance $r$ from the center of mass.
I would like to see in which situation I obtain that the disk rolls without sli... | I think that I understand what you are asking and I think that it is a good question.
If the centre of mass of the disc and the point mass moves a distance $x$ then the work done by the force is $Fx$ in both cases and yet the disc has gained some extra kinetic energy because it is rotating.
If the centre of mass of t... | {
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Israel-Wilson-Perjés Solutions I'm searching for a reference that gives explicitly the field strength (or at least the gauge fields) of the Israel-Wilson-Perjés Solution, using complex harmonic functions for the metric.
In "Gravity and Strings" by Ortin, one can find the required formula at pag. 280, but I'm not able t... | There was a misprint in the first edition of the book, now corrected in the second edition.
| {
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How much load does an aqueduct support? Recently, I found out about /r/InfrastructurePorn, and I found a particularly interesting photo of the Gouwe Aqueduct in Gouda, NE:
It seems like the bridge that is supporting the boat wouldn't be able to do it. Is the weight of the actual boat being supported by the aqueduct? ... | Another way to look at this is to ask yourself "how is the water at the bottom to know about the ship above it?" It can't; a given water volume "communicates" only with its immediate neighbors. Now it's possible that the water volume at the bottom experiences different pressures from different sides. This will just pus... | {
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What is the QED model about half wave plate? Surely classical electrodynamics (CED) has a good and well-known answer about the change of light polarization in a HWP (or Quarter wave plate). I tried to find how does this look like from point of view of QED but couldn't find anything by googling. And nevertheless as QED ... | A wave plate is a passive component, and can be modelled as a unitary operator on the quantum state. The state is a superposition of left and right circular polarized photons, and the operator gradually alters the relative phases. The total distance then determines the final polarization state.
| {
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Non coherence of Fermions and Bosons through $U(1)$ I "know" the textbook answer why we cannot write,
$$
|\psi\rangle = a|j=\tfrac{1}{2}\rangle + b|j=1\rangle
$$
as "each term in the quantum superposition transforms differently under $U(1)$",
$$
U(2\pi)\ |\psi\rangle = -a|j=\tfrac{1}{2}\rangle + b|j=1\rangle
$$
and "he... | This is the prototypical example of a superselection rule.
The operator $U(2\pi)$ commutes with all observables (because it represents a full rotation, and is hence physically a "do nothing" operator), and yet is not a multiple of the identity (because it is -1 on the fermionic and 1 on the bosonic parts of the Hilbert... | {
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Are there negative energy states in QED? I was reading Weinberg I, when I came upon the following statement$^1$ (slightly edited by me):
\begin{align}
(\not p+m)u=ie\not A\\
(\not p-m)v=ie\not A \tag{1}
\end{align}
The minus sign on the r.h.s. of the equation of $v$ shows that the $v$ are the famous "negative-energy" ... | It does involve bound states. For an electron in the Coulomb field $A(r)=-\frac{Ze}{r}$ that arises from a nucleus (of mass $m$), the lowest energy is $E_0=m\sqrt{1-(2Z\alpha)^2}$ where $\alpha$ is the fine structure constant. This is positive for $Z\le\frac{1}{2\alpha}$, which is what Weinberg means by "moderate" fiel... | {
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Can I re-wind gravity? Suppose I perform an arbitrary simulation where I integrate the motions of a collection of particles which interact only gravitationally. Suppose I use a time reversible integrator (to be specific, let's say leapfrog, which is also symplectic, in case that's important). The fact that the integrat... | TL;DR: Yes. Although in reality you don't have unlimited floating point precision, and this will almost always break time-reversibility.
I should point out that not all integrators are time-reversible. For example, predictor-corrector schemes, and most schemes that deal with constraints. The Verlet method, however, is ... | {
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Where does the energy of a photon trying to escape a black hole go? I've heard "light cannot escape a black hole" explained several ways. One is that if a photon inside the event horizon tries to escape a black hole it loses energy to gravity. As it loses energy its wavelength gets longer and longer until its energy is... | A photon that escapes from black hole's neighborhood does work on the black hole. The photon causes the black hole not to be in the photon's gravity well after the photon has escaped. In other words the photon increases the potential energy of the black hole.
The following paragraph may not be science, I just want to s... | {
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D2O distribution in water I just learned that about 1/1000 of hydrogen is deuterium. Does that mean 1/1000 of water is D2O, which is heavier and sinks to the bottom in a glass of water? And consequently is there a layer of D2O at the bottom of the ocean?
thanks!
| It's rarer than that, more like 1 in 6000. Because each molecule of water has two atoms of hydrogen, then about every 2 in 6000 (1 in 3000) has a single atom of deuterium (DHO). And would be closer to 1 in every 6000$^2$ for a molecule of D2O.
The linked question Deuterium density in seawater gives sources that show ... | {
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According to the doppler effect , if my source is faster than sound , will I hear the sound backwards? In a hypothetical situation,say I have a sound box with a very large volume and let's say it is placed pretty far away from me. If it travels towards me at a speed higher than that of sound in the same medium, air for... | Lets take a simpler case: instead of a song, the fighter pilot emits two pulses separated by some time. After the first pulse is emitted, he will indeed outrun it - he is after all going faster than the sound speed. He will then emit a second pulse in front of the first (and outrun that as well).
As a distant observe... | {
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Light bulbs flashing on when disconnected For the 2nd time I've had a lightbulb flash on in my hand after removing from the power socket. The first time the overhead light in my hallway was out for a couple of days before I got around to changing it. This was a cfl bulb and after I unscrewed it and was making my way ... | Flurescent light bulbs are known for the way they light up when subjected to static electricity. Here is a video of an instructor using a Van de Graaff generator to light up an old fashioned fluorescent light. CFLs are just old fashioned fluorescent bulbs bent into a different shape with a DC power supply tacked on.
| {
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Intuition on Gibbs measures I am (roughly) aware of the way Gibbs measures are used to solve physical systems (e.g. the Ising model). We can basically boil it down to pinpointing a Hamiltonian.
My question is, consider a system with Ising-reminiscent, cascading dynamics (like opinion formation). In that case, how woul... | Many researchers are thinking about this question. I'm afraid that there is no straightforward answer. That is the main reason for non-equilibrium statistical physics to be so hard. We can't even pinpoint a good measure. Think of it the other way around: If the dynamics of your system are Hamiltonian and if we know thi... | {
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How does acceleration feel compared to gravitational pull? I was debating a variation of this Phys.SE question with a friend. The original question is:
"If you had your eyes closed, could you distinguish between standing still on earth and being in a spaceship which is constantly accelerated at 1g?"
Yes. When you are... | Gravity and uniform acceleration ar not the same. and with sensitive equipment the man in a box can tell the difference.
imaging he has two plum bobs, on strings. if he measures the difference between the top of the strings and the bottoms, he will notice in uniform acceleration the strings are parallel, But in Gra... | {
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What are the accelerations of blocks? I've talked with 2 teachers about this situation:
one teacher said he was completely sure that B have twice the acceleration of A, the other said he was completely sure they have same acceleration. Can you have a better look on it? What do think? Consider it has no friction.
| The accelerations are the same. They can't be different, since otherwise $B$ would move faster than $A$ and the rope wouldn't be tight anymore.
The reason your teacher thought that they are different is a common mistake: under the assumption $F_{AB} = -F_{BA}$ and $F=ma$ (force on A should be force from B and vice vers... | {
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Why current in series circuit is the same? I have read in the internet that the charges do not have any other path to go and they must go through the same in a series circuit,hence the current is same.
It was quite convincing but what confused me was:
"A resistor is a passive two-terminal electrical component that impl... | Yes, but where is the problem? :)
If you have a resistor in your series circuit, then the current is reduced - in the whole circuit, of course, not just in the resistor. The current is the amount of charges per time. There are just less, in a certain time, but they still have to pass through all the other elements of t... | {
"language": "en",
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Time taken for a layer of ice to form The book I have gives the following derivation:
Let the temperature of the atmosphere be $-\theta$ and the temperature of the water be $0$.
Consider unit cross sectional are of ice, if layer of thickness $dx$ forms in time $dt$ with $x$ thickness of ice above it,
heat released due... | The heat is continually being released to the atmosphere, and the layer is continually getting thicker. The heat has to be conducted from the water-ice interface to the ice-atmosphere interface through the layer of ice. And, as the ice gets thicker, the rate of heat being conducted slows down. And the rate of ice fo... | {
"language": "en",
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Utilising Black Holes as a potential energy source I'm aware of the Penrose process and the basic physics behind that. Also, I know that the Blandford-Zjanek process (That is potentially responsible for the relativistic jets). Aside from these two, and Hawking Radiation, what other methods or theories are there for ext... | Great question. Black holes are some of the brightest objects in the universe. While we think they require the Blandford-Znajek (BZ) mechanisms to produce things like Relativistic Jets, the bulk of the light (emission) they produce is just the efficient thermalization of gravitational energy when material falls into (... | {
"language": "en",
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Gravitational wave equations worked out Is there a website where gravitational wave equations are worked out numerically?
I would like to experiment with mass configurations but can't find examples.
| The topic is a bit too broad. If you want to look at gravitational wave waveforms coming from e.g. binaries then Post-Newtonian approach is usually sufficient and there are many codes for that out there.
If you want gravitational wave emission worked out in any system it might become a bit troublesome.
I would suggest ... | {
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Diminishing solar temperature and its effects on earth This is a hypothetical question; considering both the earth and the sun as black bodies. If the temperature of the sun decreased N times, what would be the effect on the radiation intensity received on earth?
(i am inclined to say it would decrease N^4 times using... | Yes, the amount of radiation (i.e. power) would decrease $N^4$ times, since the whole geometry of the system does not change, only the power emitted by the sun.
But the temperature of the earth would decrease only $N$ times, since the ratio of the temperatures remains equal - again because of the unchanged geometry. T... | {
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gradient strength units in MRI This may not be the appropriate forum for this but seemed to be the closest.
I am trying to understand some concepts around MRI physics and it is common to use external magnetic fields created using gradient coils to manipulate the main magnetic field strength at different locations.
Now,... | The term "gradient" implies a change in some quantity versus a change in second quantity, usually over a distance. It's very much like a slope. For example, the gradient of a roof line on a house is given as rise/run like 15 cm/m or 5 inches/foot. The gradient of potential is the electric field magnitude, with SI unit... | {
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Is it possible to low pass filter the amplitude of a sound wave? Is it physically possible to block or attenuate noise above a certain amplitude, but leave other lower amplitude noises unhindered?
| Remember that noise is unwanted frequency signal superimposed into the original frequency. This means to reduce noise, you need to block the unnecessary frequency components. The effect of noise increases while increasing the amplitude because amplitude is a measure of loudness. So the amplitude (loudness) of the noise... | {
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Why does imaginary periodic time of the Rindler space of black hole give Hawking temperature? The metric of a Schwarzschild black hole in Rindler coordinates is
$$ds^2= -\frac{\rho^2}{(4MG)^2} dt^2 +d\rho^2 + d\mathbf x_\perp$$
where $\rho$ gives us the distance to the horizon.
If we switch to imaginary time $\tau=it$,... | It is not a coincidence. It has to work like that. The deficit angle has to be zero.
It's most convenient to see it in the Feynman's path integral approach to quantum mechanics. One works in the Euclideanized spacetime to calculate the temperature $T=1/\beta$ partition sum. Let us consider the full finite-size black ho... | {
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Question about the apparent loophole in principle of least action: boundary condition vs initial condition In Lagrangian formalism, given two points $(x_1,t_1)$ and $(x_2,t_2)$, we ask the question which paths $x(t)$ make the action $S=\displaystyle \int_{t_1}^{t_2}L\ \mathrm dt$ stationary and satisfy the boundary con... | The issue is that the underlying classical physics is determined by equations of motion (EOMs) (i.e. Newton's 2nd law), which are common for initial value problems (IVPs) and boundary value problems (BVPs). For BVPs , the EOMs can often alternatively be formulated as Euler-Lagrange (EL) equations of a stationary action... | {
"language": "en",
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Why does a pot start rotating when coffee is stirred inside? I usually make Turkish coffee as my morning coffee. I have a small somewhat rounded pot with handle on one side. I noticed that when I pour water in and start stirring, pot has a tendency to start rotating in the same direction as I'm stirring.
Why is that?
| A probable explanation for this effect is simply that the bottom of the pot might be a bit bulged out, as to form only one point of contact around which the pot then can rotate relatively freely (with little friction).
As you stir the water inside the pot, the moving water molecules exert a frictional force on the wall... | {
"language": "en",
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Is static friction the only force causing a car to move (without sliding)? A car is moving without sliding means that the friction between wheels and the ground is static friction. This is the force causing an object to move forward, therefore, its direction is the same as the moving direction of the car.
My question i... | I agree that friction in the drive mechanism reduces thrust, rather than opposing the motion of the car. However, this is not the case for wheels which are not in the drivetrain - ie where there is front/rear wheel drive instead of 4-wheel drive. Friction in non-drivetrain wheel mechanisms are then sources of resistanc... | {
"language": "en",
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What ds>dQ/T mean? I read the derivation on page 216 over here.
First, it considers an irreversible process between states 1 and 2, followed by a reversible process between states 2 and 1.
From my interpretation, equation 8.31 means that the entropy change of a reversible process is greater than that of an irreversibl... | I think you have a typo in your question. A reversible process will have a smaller entropy change than an irreversible process. Your interpretation that the equality refers to a reversible process, while the inequality refers to in irreversible process is correct. Looking at the specific equations in that notes documen... | {
"language": "en",
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Intensity fluctuations at the output of a single mode fiber coupled to a He Ne laser I have coupled a Thorlabs HNL050L-EC - HeNe, 632.8 nm, 5 mW, Polarized Laser to a 2 meter long single mode fiber patch chord using a Thorlabs F230-FC-B aspheric lens. While I am certainly able to obtain a pure single mode Gaussian at t... | My bet is that your fiber is very short (something like one meter or so) and that the fluctuations you see on the output mode are due to cladding modes, i.e. a part of the injected light propagating into the cladding of the fiber instead of the core. The resulting fluctuations are due to external perturbations of the f... | {
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Finding relation between angular acceleration and liner acceleration directed in one direction
In the above given configuration what could be the relation between the angular acceleration of the rod $\alpha$ and the acceleration $a$ of the $2m$ block.
My attempt : What I thought is that the string connected to the ro... | The rod will only rotate upwards due to the force that is normal to it's length, acting from the tether on the rod,
$$F_{t,\perp} = F_t\cos\theta$$
Here $F_t = (2m)g$ is the force from the tether, equal to the weight of the block, and $\theta$ is the angle. The rod also feels a downward force, gravity, that has a nor... | {
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If we threw a baseball from the ISS, could we deorbit the ball? Clearly this is a hypothetical question.
Say we bring a star baseball player into NASA, prep them appropriately for a mission in space, and fly them up to the International Space Station. They go on a spacewalk with a baseball, and at the apoapsis (highest... | You don't need to throw the ball!
At the altitude of the ISS the atmosphere is thick enough that it loses 50-100m of altitude every day due to the drag. At that rate over your ten year timescale the ISS would lose 180 to 360km. When you take into account the increased drag at lower altitudes ten years is enough to brin... | {
"language": "en",
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Momentum conserving delta-function in the transfer matrix of quantum-field-theoretic scattering theory The $S$-matrix vanishes unless the initial and final states have the same total $4$-momentum, so it is helpful to factor an overall momentum-conserving $\delta$-function:
$$\mathcal{T}=(2\pi)^{4}\delta^{4}(\sum p)\mat... | Yes, there is an integral, which comes from the LSZ reduction formula,
$$
\langle f|i\rangle\sim \int \mathrm dx\ \mathrm e^{ikx}\square_x G(x)
$$
where $x=(x_1,x_2\cdots,x_n)$, $k=(k_1,k_2,\cdots,k_n)$ and $G$ is the $n$-point function. If you go to momentum space you'll get that integrand depends on $x$ only through ... | {
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Will the two capacitors be charged in this circuit? Is such a circuit possible to exist with the two capacitors charging?
Because if we considered the outer loop using Kirchhoff's law we get:
ℰ = q1/C1 + q2/C2
But since the two capacitors are initially uncharged, the two terms of the equation increase, that is, q1 and ... | No, that circuit cannot exist in that regime. You are neglecting the internal resistance of the wires between the voltage source and the capacitors, and if the capacitors are discharged (in which case the voltage over them is zero) that's no longer a good approximation. You therefore need to insert a small resistance o... | {
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Is a falling leaf an example of a chaotic system? Let´s assume is a wind still day in autumn. When a little change is made in the initial motion of a leaf at the time it falls off a tree, the resulting path of motion of the leaf is very different from the path that would develop if these changes wouldn´t have been made... | Yes. With the qualification that chaos describes the behaviour of an ideal (continuous?) mathematical model, whereas leaves and the air through which they fall are real, I think it is a chaotic system. Leaves falling from the same place but with a small change in orientation can land in very different places, and inte... | {
"language": "en",
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What is beyond gamma rays and radio waves in the electromagnetic spectrum? The electromagnetic spectrum is commonly referred to as consisting of;
radio-waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays - of increasing frequency from left to right.
But is it possible to get radiation of higher ... | Wouldn't a wave smaller then the smallest be "in the same theoretical universe of discourse" mathematically negative? Looks like a good explanation for a time traveling Sci-Fi xD
Also, wave may be able to reach a higher speed then it's correlation to length. For example if traveling on a referent that is also traveling... | {
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Why doesn't orbital body keep going faster and faster? If we consider the change in velocity during an infinitesimal interval of an orbit:
where body B is orbiting body A, we can see that the magnitude of the resultant vector (the green arrow) is greater than the magnitude of the original tangential velocity. Why does... | Your vector sum drawing is incorrect. Considering the simple case of a perfectly circular orbit, there is a tangential velocity vector and a perpendicular (inward) acceleration vector. Strictly speaking, you can't add them because they are different quantities (acceleration vs velocity).
At some point in the orbit, th... | {
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Resistance from $I$-$V$ graph
I thought that the answer was obviously A as the gradient remains constant...however, the answer is B...who so?
| Remembering that resistance = $\frac V I$ work the resistance at $I=1$ and $I=2$.
For the resistance to be constant the current-voltage characteristic must be a straight line and go through the origin.
There is another parameter which is useful in some instances and that is called the incremental resistance $\frac {\D... | {
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What is the difference between metallic hydrogen and plasma of hydrogen? As far as I know, metallic hydrogen and plasma seems share some common properties, eg:conduct electricity, nuclei share electrons, high temperatures...
So my question is, is metallic hydrogen in fact plasma state of hydrogen? If not, what is the d... | The primary difference is that the electrons in metallic hydrogen are nearly completely degenerate.
Degenerate electrons cannot be dissipatively scattered and lead to the "metallic" characteristics of extremely high electrical and thermal conductivity.
To first order, to produce metallic hydrogen you need to make sure ... | {
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Why does the sign of Delta H indicate whether the reaction is exothermic or endothermic? If $\Delta H = Q + W$ (assuming constant conditions), then there are two terms involved in calculating $\Delta H$, only one of which measures heat gained/released. It is possible then for $\Delta H$ to be negative (if $W$ were very... | Your original equation is incorrect. For a process occurring in a closed system, the equation should read$$\Delta U=Q+W$$where U is the internal energy of the system, Q is the heat added to the system, and W is the work done by the surroundings on the system, $W=-\int{PdV}$. If the process takes place at a constant p... | {
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Pressure on the sides of a container? Sorry if this is an incredibly basic question for these categories. Basically, I don't understand these types of problems. I'm sure it's something really simple I'm missing.
Let's say there's an open swimming pool with width $w$, length $l$, depth $d$, and density $\rho$ (equal to ... | At a certain depth a liquid exerts the same pressure on all sides.
So if you want to calculate the force exerted by water on sides w and l then you have to put in the value of depth in the formula.
Whatever the answer will come will be the pressure exerted by water on the sides w and l for that particular depth.
As for... | {
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CFT conformal weight vs. scaling dimension I was wondering if anybody could clarify what the difference between the conformal scaling dimension $\Delta$ and the conformal weight $h$ is?
Is it correctly understood that $\Delta$ is related to the transformation properties of a field and $h$ is an eigenvalue of the Viraso... | Two dimensional CFTs separate into a left-moving sector and a right-moving sectors. The Virasoro generators $L_n$ act on the left-moving sector and ${\tilde L}_n$ act on the right-moving ones. Operators (or states due to the state-operator map) are labelled independently by representations of the left- and right-moving... | {
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estimating deviations from ideal gas behaviour How can one estimate the pressure at which argon atoms show deviations from ideal gas behaviour due to the finite size of the atoms?
I have tried Taylor expanding the hard sphere gas equation:
$$P'(V-b)=NkT $$
to get $P'=P(1+b/V)$ to first order, where $P$ is the ideal ga... | The result is problematic that the compressibility factor is greater than 1.
The well-known Van der Waals equation is, $$(P+\frac {n^2a}{V^2})(V-nb)=nRT$$
And ideal gas EOS is, $$P_{ideal}V=nRT$$
where n is molar number and R is universal gas constant. You can use particle number if you prefer that way.
From VDW equa... | {
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What do we mean by speed of light dependent on direction? I have a statement in textbook saying:
When the speed of light is independent of direction, the secondary waves are spherical.
When is it dependent on direction and how will the secondary waves defer?
| There are some materials, called Birefringent in which the index of refraction depends on the direction in which light is moving (often also dependence on the particular polarization of the light). This is due to the crystallin structure of the material which allows faster propagation in one direction or another.
Anot... | {
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Maxwell velocity distribution, in 1D or otherwise I learned from my textbook that Maxwell's velocity distribution gives: $$v_{rms} =\sqrt{\frac{3kT}{m}}$$ $$v_{avg} = \sqrt{\frac{8kT}{\pi m}}$$
Presumably this is for a three dimensions. This confuses me because for one-dimensional distributions the values are $$v_{x,rm... | The one dimensional Maxwell distribution for the $i-$component of the velocity vector is
$$f_{1D }(v_i) = \left(\frac{m}{2 \pi k T}\right)^{1/2} \exp\left(-\frac{m v_i^2}{2kT} \right)$$
Let's drop the $i$ and $1D$ subscripts for simplicity. You are looking for the average of the absolute value of $v$, $|v|$. To find $\... | {
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Model/formula for bouncing ball I'm programming an animation of a bouncing ball, and I want it to be as realistic as possible.
I fully understand the physics while the ball is rising and falling: It accelerates downward at 9.8 meters/second/second.
But once it hits the ground, I'm lost. I know that it experiences some ... | Visual Solutions (Now Altair) makes VisSim Software. Here is a demo block diagram that they have used to simulate a bouncing ball:
The $1/s$ blocks are integrator blocks from the VisSim library.
The plot of the bouncing ball with these parameters is shown below.
If you are interested in running this, the demo comes wi... | {
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How to imagine WiFi signal propagation? When thinking about how the WiFi signal propagates through a household, can I use the following thought experiment?
Assume absolute darkness. Place a strong lightbulb where the WiFi access point is. The illumination that reaches various places in the house is approximately propo... | It's hard to access how 'accurate' an analogy is (i.e. how is this being quantified?). But, I think, there is a simple - better analogy:
WiFi is more like sound in a house. The transmitter is a speaker. If its a good, loud speaker, you will still easily be able to hear it in the next room - through a wall. A few wa... | {
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Metric that is Minkowski plus sum of null vectors In GR exercises I've often seen metrics of the form $g_{ab} = \eta_{ab} + k_ak_b$ where $k_a$ is null with respect to $g$ (or equivalently $\eta$). I'm happy doing calculations with such metrics, but I wondered what the physical interpretation of such a metric is?
If $k... | The reason to consider such metrics was not a particular interpretation, but that for this ansatz one could hope to find some exact solutions. It is known as the Kerr-Schild metric, and its role in the process of finding exact solutions has been described by Kerr in Wiltshire, Visser, Scott (eds.), The Kerr Spacetime. ... | {
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What are the differences between the differential and integral forms of (e.g. Maxwell's) equations? I would like to understand what has to be differential and integral form of the same function, for example the famous equations of James Clerk Maxwell:
How to know where to apply each way?
Excuse the ignorance, but alwa... | The equations are entirely equivalent, as can be proven using Gauss' and Stokes' theorems.
The integral forms are most useful when dealing with macroscopic problems with high degrees of symmetry (e.g. spherical or axial symmetry; or, following on from comments below, a line/surface integrals where the field is either p... | {
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Two Black Holes held stationary by EM forces If two black holes with large enough mass (so that the tidal forces are minimal and the intersection is large) that are held apart by like charges (saddle point stability). Imagine the black holes in a vacuum placed such that there event horizons are overlapping. Is this ove... | If the event horizons overlap you get one big horizon. EM forces can not counteract gravity if the curvature is too large since the force required to counteract gravity becomes infinite at the horizon. You can see this in the equation
$$F=\frac{G\cdot M\cdot m}{r^2\cdot\sqrt{1-r_s/r}} $$
which becomes infinite at the h... | {
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Modern interpretation of wave-particle duality As far as I understand, in the early days of quantum theory there was quite a lot of debate over how to interpret what it meant for a quantum mechanical object to exhibit both wave-like and particle-like properties.
Is it correct to say that the modern interpretation (as ... | It probably depends what interpretation of quantum physics you subscribe to. That sounds approximately right for the Copenhagen interpretation, in which you aren't allowed to analyze where the wave function comes from.
For those who appreciate more what de Broglie, Einstein, Bell and others have put into quantum physic... | {
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How to deal with zero uncertainties? Suppose you measure quantity $x$ with an uncertainty ${\rm d}x$. Quantity $f$ is related to $x$ by $f=x^2$ . By error propagation the uncertainty on $f$ would be ${\rm d}f=2x{\rm d}x$. If a certain point $x$ equals zero then the uncertainty on $f$ would be zero, even if $x$ carries ... | Use the second derivative (or third, or whatever). The reason we use that formula is that
$$
df \approx \frac{df}{dx} dx
$$
is the first order Taylor approximation to df. If the first order term vanishes, you should include higher terms:
$$
df \approx \frac{df}{dx} dx+\frac{1}{2}\frac{d^2f}{dx^2} dx^2+...
$$
In your ca... | {
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What does it mean that the laws of physics are time reversible?
The Universe, as far as we can tell, only operates according to laws
of physics. And just about all of the laws of physics that we know are
completely time-reversible, meaning that the things they cause look
exactly the same whether time runs forwar... | The standard reply is: get your video camera out, take a movie of dropping a (empty) coffee cup on the kitchen floor, then play the movie backwards. There is no physical way of telling which way time is running from your movie or the equations behind the event (Newtons Laws).
Rather than thinking of an absolute arrow o... | {
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Where does $\hat{P}\psi(x) = -i\hbar \partial_x \psi(x)$ come from? It's a very basic question, where does the relation $$\hat{P}\psi(x) = -i\hbar \partial_x \psi(x)$$ for any square integrable $\psi(x)$ come into existence? Some texts I found states that the above relation comes as a consequence of momentum being defi... | Momentum and position are conjugate variables in classical mechanucs, which means they satisfy the Poisson bracket relationship. When quantum mechanics was invented the Poison bracket relation was replaced by the operator commutation relationship which results in the relation under consideration.
| {
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How is the Rayleigh criterion connected to the Abbe limit? I am interrested whether one can derive a formula for the point resolution (like Abbe did) of an optical system from the Rayleigh criterion (without the use of small angle approximation i.e. $\rm{sin}(\alpha)=\rm{tan}(\alpha)$ which is not really suitable e.g. ... | As @Felix and @Caterina are still not satisfied I will add my 2 cents, for which I hope they are correct.
As far as I am aware Rayleigh developed his criterion on diffraction of light on slits, whereas Abbe was working on microscopy. Therefore, you have a refraction index in one and not in the other. However, you could... | {
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Why would a spinning space station create a centrifugal force on an astronaut rather than simply spinning around him/her? We often see films with spinning space station that create artificial gravity by having the astronauts pulled outwards by centrifugal force.
I'd like to know if this would really happen, and if so, ... | All that you have written is correct.
Let's go further to the next point 4:
Once he comes to the cylinder's wall and stands on it, he gets the same angular speed as the cylinder, and he will also get a centrifugal force and rotational gravity as in the films.
| {
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Can we use radiation pressure to push/levitate a human? I want to know, at least in theory, are there any safe em wavelength that could affect human body as net force and do no harm?
To put it bluntly I want to know is it possible for levitate a human on Earth with radiation pressure?
I just think maybe some range of i... | The force of radiation pressure is $F=P/c$ for absorbed radiation or $F=2P/c$ for reflected radiation, with $P$ the power and $c$ the speed of light. If you want to generate 750 N of force, you need (a) a radiation source of 100 GW and (b) a mirror that reflects so well that nor the mirror, nor you will be vaporized in... | {
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Coupling a spinor field to a preexisting scalar field? So I'm not a physicist, but I'm thinking about a mathematical problem where I think physical insight might be useful.
We're working on a Riemannian manifold $(M,g)$ (positive definite metric) with a distinguished smooth function $f$. The metric and the smooth funct... | Your first equation looks a bit like GR with a dilaton. IIRC, the analogous equations in supergravity will naturally have spinors coupling to the dilaton.
| {
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Does Heat Cause Time Dilation? Since heat is defined as the movement of molecules, and because of relativity time slows for faster moving objects, would a hot object be in a slower time frame then a cooler object, because the hot objects molecules are moving faster?
| No, the hot object itself experiences no relativistic effects. The particles in the object however may well experience time dilation or length contraction. In fact in very hot gases and plasma, ideal gas laws may not always apply due to relativistic effects.
These effects are especially noticeable in gases once the kin... | {
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Conserved quantity in a spacetime with Killing vector I am trying to prove that that the expression $Q=-\frac{1}{\kappa}\int_{S_\infty} \nabla^i \xi^k \mathrm{d}\sigma_{ik}$ is a conserved quantity for a spacetime with Killing vector $\xi^i$ where $S_\infty$ is the 'boundary at infinity' of the spacelike hypersurface $... | I will just sketch this out. The Killing equation $\xi_{a;b} + \xi_{b;a} = 0$ tells you that $\nabla^i\xi^k$ is antisymmetric in indices. I am going to write this as the commutator between $U$ the vector along which we differentiate $\xi$ with $\nabla_U$ so in compact notation we have
$$
Q = -\frac{1}{\kappa}\int[U, \... | {
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Difference between sunrise and sunset? Other than knowing which direction is east and which direction is west, or observing for a sufficient timespan (to determine the direction of motion), is there any way of telling whether what one is seeing is a sunset or a sunrise? A priori it seems not but I was wondering if ther... | It's not something you can directly see, but various amateur radio propagation modes turn on around dawn and turn off around dusk. Read up on the ionosphere and solar ionization.
I imagine this could lead to spectral voids in setting sunlight, but I expect the effect to be too small to see (maybe not too small to meas... | {
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Components of Velocity in polar co-ordinates Consider a point moving along a curve in a plane. The position of a point P on a coordinate system can be specified by a single vector $\vec{r}$=$r\hat{r}$. A rough sketch describing the situation is shown below:
In order to find the velocity of the particle we take the f... | Consider the picture below.
In Cartesian coordinates
$$\hat r=\cos\theta\hat i+\sin\theta\hat j,$$
and
$$\hat \theta=-\sin\theta\hat i+\cos\theta\hat j.$$
Therefore
$$\frac{d\hat r}{d\theta}=\hat\theta$$
| {
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Why does Bernoulli's equation only apply to flow along a streamline that is in viscid, incompressible, steady, irrotational? I am learning about hydrofoil on this website.
In a later video I watched, I learned that in the process of deriving Bernoulli's equation, $$constant=P/d+gh+1/2v^2$$ has to multiplied through by ... | Bernoulli's equation is really like an energy conservation equation: if you multiply both sides by the mass flow $\dot{m}$ (also assumed constant) you get:
$$\frac12 \dot{m}v^2+\dot{m}gh+\dot{m}\frac{p}{d}=C$$
The terms are all energy per unit of time. The first one, $\frac12 \dot{m}v^2$, represents translational kinet... | {
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Positron losing energy but is not destroyed In August 1932, Anderson photographed a positron originating from cosmic rays as it entered a bubble chamber, passed through a 6mm lead plate (in the process loosing energy, as apparent from the changed radius of its path through the chamber).
My question is, as it passed thr... | Likely the dominant process for the positron in the lead plate is simply Bhabha scattering. This is no surprise. We must consider the lab energy of the positron.
At center-of-momentum energies less than twice the muon rest mass, the electron and positron annihilation is limited primarily to radiation of multiple phot... | {
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Does "IF isentropic THEN reversible" only holds for adiabatic processes? I know that: IF adiabatic and reversible THEN isentropic
First question: does the implication IF isentropic THEN reversible hold for adiabatic processes?
Second Question: if yes to the above, are there other processes other than adiabatic for whic... | A process does not have to be adiabatic or reversible to be isentropic. If the difference in entropy between State A and State B is zero, since entropy is a state function, any oddball path (either reversible or irreversible) that can take you from State A to State B will be isentropic.
| {
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Meaning of the phase space in statistical physics I have a silly question about the phase space. I am confused with the meaning of points in phase space. Does the each point in phase space represent concrete particle of the system, or does it represent the whole state of the system? Our teacher told us, that we use the... | A point in phase space represents the state of the whole system.
For example, if you have a system of $N$ particle with coordinates $\vec r_1, \dots, \vec r_N$ and momenta $\vec p_1, \dots, \vec p_N$, its general state will be a point in a $6N$ dimensional phase space:
$$\vec X = (\vec r_1, \dots, \vec r_N, \vec p_1, \... | {
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Why is oil a better lubricant than water? How come mineral oil is a better lubricant than water, even though water has a lower viscosity?
When two surfaces slide over each other with a gap filled with a fluid, the different layers of the fluid are dragged at different speeds. The very top layer touching the top metal ... |
Why Oil is Slippery
Explaining why oil is slippery requires a look at its chemical
properties. First, oil is non-polar, which means it does not have a
positive or negative charge. Some molecules, like water, have a
“charge distribution,” which means the molecule acts almost like a
battery, part of it has a pos... | {
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Physics Chain Problem
A chain of length d lies on a table, $d/4$ of it hangs loose on the side of it. Friction coefficient is 0.2. It is released and begins to slide. What is the final velocity with which it falls of the table?
My attempt:
$Wf = \bigtriangleup Ug = - \int_0^{0.75d} \! \mu_k g \frac{m}{d}(\... | Yes, the work-energy method can be used. The particular difficulties you noticed can be dealt with by considering the chain in 2 parts : that part hanging down (which loses PE but experiences no friction) and that part remaining on the table (which does not lose PE but experinces friction).
Suppose the mass of the cha... | {
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Why do we add intensities for coherent sound sources? When there are N coherent sound sources playing the same note at equal loudness, their sound waves add up to make a sine wave of the same frequency but most likely different amplitude (can be anything from zero to N times the amplitude of one wave), depending on the... | Yes. Let's consider the phase shifts to be random. Then we can consider each amplitude to be an independent random variable $A_i(t)$. Each one has a variance of
$$\text{var}(A_i) = \langle A_i^2 \rangle - \langle A_i \rangle^2 = \langle A_i^2 \rangle \propto I_i$$
because the average amplitude should be zero, and inten... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/259712",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 2
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Can an electron make quadrupole gravitational waves? A gravitational wave is a quadrupole wave. Now when an electron is accelerated it usually emits a photon. But can an electron also emits an gravitational wave? If so how does it 'make' an quadrupole wave?
| The answer by Physics Guy says:
Since the electron moves in spacetime and has mass, it produces gravitational waves. that can be derived from General Relativity.
This is wrong. Motion and mass are not sufficient criteria.
The answer by Árpád Szendrei says:
Now in your case, as an electron alone (as an isolated syste... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/261037",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 1
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