Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
Why don't we have underground thermal power plants? I learned from wikipedia that for the production of electricity, the temperature of geothermal sources must be at least 150 degrees Celsius.
Also wiki says that at a depth of 6 kilometers, the temperature of the earth is about 270 degrees Celsius, that is more than en... | The physics reason is that energy isn't generated directly from heat. It's generated from heat reservoirs that are different in temperature*. So if you dig a 6km deep hole and lower a thermal engine down into it, you now have to transport heat from your thermal engine to the surface to cool it.
The practical reason i... | {
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Diy Water park tipping bucket physics So I'm building a tipping water bucket like the ones at a water park, just much smaller. I want the bucket to tip at 500mL. I'm currently using 3 inch PVC pipe cut 11 cm down crosswise. I can have any amount of weight at the bottom, so I was planning on 50 grams. I want to know, wh... | To get a quick "dumping" motion you need to place your tipping axis below the centre of gravity of a full bucket. As the bucket fills, the centre of gravity rises, and once it is above the axis the bucket becomes unstable and will tip over. Once the water has emptied the weight at the bottom of the bucket will bring it... | {
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Can we say that the center-of-momentum frame is the frame in which the center of mass is at rest? Isn't the center-of-momentum frame is same as the frame in which the center-of-mass is at rest? Since the position of the center-of-mass of a system of particles is defined as $\vec R=\sum_i m_i \vec r_i/M\Rightarrow\frac{... | In newtonian mechanics, center-of-momentum frame is the same as the frame in which the center-of-mass is at rest as showed in your derivation.
| {
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Is an electron in an hydrogen atom being measured by the nucleus? In an hydrogen atom, the electron interacts with the nucleus by multiple forces, for example the Coulomb force. Does that mean that the nucleus makes quantum measurements of the electron?
EDIT: I became aware that the word ‘measurement’ is not present in... |
I would like to know whether the wave-function of the electron is
influenced by the interactions with the proton, in a way that is not
part of the Schroedinger equation.
In all interpretations without wavefunction collapse the wavefunction can be influenced only by Schroedinger's equation. Only two things affect the ... | {
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Different definitions for effective potential in static spherically symmetric spacetimes – which is right? In the paper Existence and stability of circular orbits in general static and spherically symmetric spacetimes authors define the effective potential as
$$ V\equiv \frac{1}{g_{rr} g_{tt}}~[E^2-g_{tt}~(1+\frac{L^2}... | The short answer is yes, all above definitions are admissible!
To explain it I would like to quote professor Tiberiu Harko, who kindly answered my question in private communication as follows:
"The potential in static general relativity is rather arbitrary. As opposed to general relativity, the potential is an effectiv... | {
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Gauge symmetry and Gauge Transforms In QFT or CFT, say the action is invariant under some local transformation. Can we call that transformation a Gauge transform?
There is a specific notion of gauge transform in math which is defined as $G$-equivariant diffeomorphism from some principal bundle to itself with some speci... | No, what physicists and mathematicians mean by gauge theory are not "the same thing", but of course there is a reason the mathematical subfield is named after the physical subfield:
You can phrase many physical gauge theories in terms of the language of principal bundles and mathematical gauge theory more generally. Fo... | {
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Are there everyday materials that change color, depending on illumination spectrum? Initially I wanted to ask this exact question about color change due to "white" light source spectrum change, and the accepted answer satisfies me fully.
However the answer says that most of objects have "rarely a nice clean notch" in t... | Here is how to do it.
There are certain semi-precious gemstones which, when illuminated with a certain color of incident light, appear a different color from the incident color- or from their color when illuminated with full-spectrum light. When I remember the name of the stone I will edit this response, but it IS mind... | {
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What is so significant about electron spins and can electrons spin any directions? I just want to know what is so significant of with direction electron is spinning. Does it have any effect on the element or on the atom?
Also, does electron must spin up or down or can they also spin sideways or vertically?
| the direction of electron spin is of great significance. For example, to fit two electrons into a electron orbital surrounding an atom, their spins must be pointing in opposite directions. In this sense, electron spin is at least partly responsible for the structure of the periodic table and for the manner in which che... | {
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Equivalence of various definitions of reversibility in classical mechanics I was reading Classical Mechanics: The Theoretical Minimum by Leonard Susskind, and the definition of reversibility in that was:
Given a state of a system, then we know exactly what state it came from, no ambiguity.
I have also heard three oth... | Your quoted definition is not about reversibility, rather about determinism.
1 and 2 are the same. 3 is a specific case to classical mechanics and Newton’s law.
To make things clearer, in a classical setting, you are typically interested in a certain number of observables, that I will regroup as one entity $O$, that de... | {
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Electric field inside open conductor Is the electric field zero inside a metal conductor whose surface doesn't enclose a volume ideally, that is, a conductor that is almost a closed surface, except for a small hole?
|
Is the electric field zero inside ... a conductor that is almost a closed surface, except for a small hole?
No. Consider for example a hole that is $5 \mathrm{\ \mu m}$ in diameter. This is “small” but it is still large enough for visible light to pass through. When visible light is inside then the E field is non-zer... | {
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Is it possible to statically generate lift with the difference in pressure like wings? If I understood it correctly, the shape of the wings and/or propellers generates lift/thrust with the difference in pressure in both sides of the wings/propellers; where the lower side has higher pressure airflow and the uper side ha... | The cartoon is missing a key feature: the flow beyond the wing is downward. This is necessary to create lift. The lift force is balanced by a force on the air, Newton's third law in action. This force accelerates the air downward. So, no, you cannot cannot generate lift statically.
| {
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What causes blood flow to become turbulent after a constriction? If a blood vessel is narrowed from the middle, the blood flow turns turbulent (see image below).
Why does this happen? Does it have to do with Reynolds number becoming high because velocity has increased since $Re=\frac{\rho v D}{\eta}$ the higher Re is,... | It depends a bit on what level you are seeking for an answer:
*
*One can say: Because a simulation says so. After all, the simulation is nothing but a bunch of calculations, which you could theoretically do by hand. (Mind that I have no idea whether your particular statement is based on a simulation.)
*One can cons... | {
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Proof for equal eccentricity in a binary star system What is the proof that the orbits of two stars orbiting around a common center of mass have equal eccentricities?
You can use: $m_1r_1 = m_2r_2,$ then say that $r_1= a(1+e_1)$, $r_2=a(1-e_2)$ and from the condition of the center of mass: $m_1a_1=m_2a_2.$
The problem ... | Crudely you can transform this type of problem into the center of mass frame then the total $r$ can be written as $m_1r_1+m_2r_2$ where $r_1=\frac{m_2}{\sum m}$ and similar for $r_2=\frac{m_1}{\sum m}$, hence you will get a definite value of $e$ in the com frame since total energy and angular momentum is conserved
| {
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Does rate of acceleration change as object gets closer or further to center of a mass? I learnt that newton's law of universal gravitation F = G(m1m2)/R^2, and thought if the R is distance and determined gravitational strength, why do we use 9.81 as default acceleration of earth's gravity when it is not even constant a... | Who is "we"? If we're students solving physics problem, $9.81 \,\rm{m/s^2}$ (at the surface) is close enough to the nominal average of
$9.80665 \,\rm{m/s^2}$.
If we're a hydrologist, cartographer, or geodesy-interested person we may use a geoid, such as EGM96 (or EGM08, or the new one), e.g.:
https://cddis.nasa.gov/926... | {
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Linear Harmonic motion (simple oscillator) We know that for a simple harmonic linear oscillator, the displacement is given by $x(t)=A\sin(\omega t + \phi)$, where $\phi$ denotes the phase angle. Now as per my understanding this $\phi$ is only significant when considering SHM in form of a sinusoidal wave. Is there any p... |
$x(t)=A\sin(\omega t + \phi)$, where $\phi$ denotes the phase angle... Is there any physical meaning in reality... Is there a way to measure the phase angle in reality just by virtue of the particle's (which is oscillating) position with respect to the mean position?
The mean position of what you wrote above is zero.... | {
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How is the slit material not considered an observer? As far as I can tell, the essential process of "observation" is that there is an interaction with something else, providing a means by which any part of the universe noticed something about the thing being observed.
In the double-slit experiment it is absolutely obvi... | Not only in the double-slit experiment, but for every edge, it is the edges that influence the electron. Behind each edge you find a wave-shaped distribution of electrons on a screen. And as you say, it is the perfectly functioning mathematical solution that seems to make deeper explanations superfluous.
If something i... | {
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Wave operator in Kerr spacetime: change of coordinates The wave equation for a scalar field, in Kerr geometry and in Boyer-Lindquist coordinates, reads:
$$-\left[\frac{(r^2 + a^2)^2 }{\Delta} - a^2 \sin^2\theta \right] \partial^2_t \Phi - \frac{4Mar}{\Delta}\partial_t\partial_{\phi}\Phi + \left[\frac{1}{\sin^2 \theta} ... | It would have been good if you had shown some of your calculation in detail so that we could see where you might have followed the wrong path (which may also be helpful to others).
The solution to this problem comes back to expressing the partial derivatives in the old coordinates $(t, r, \theta, \phi)$ in partial deri... | {
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How is this child able to move their crib while standing inside of it? I was scrolling Instagram and saw this Reel which at first was normal but when I started to think how the child was able to move then I got confused. The video shows a child standing inside of a crib and repeatedly bouncing their body against the ra... | You're right that this would not work in space. However the crib is attached to the floor via static friction. When the crib is bumped, the floorboards are pushed in the opposite direction, which is eventually transferred to the house/earth.
| {
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Seeking expression for radial velocity of an object in an elliptical orbit (with focus at coordinate origin) as function of radius Consider a planar, elliptical orbit in a simplified two body, $\frac{K}{r^2}$ central attractive force problem (i.e. assume m1 >> m2 so focus $f1$ is effectively at m1, with m2 at point $p\... | The polar equation of the ellipse
$$\mathbf r=r(\theta)\begin{bmatrix}
\cos(\theta) \\
\sin(\theta) \\
\end{bmatrix}\quad,
r(\theta)={\frac {p}{1+e\cos \left( \theta \right) }}$$
thus $~\dot{\mathbf{r}}\cdot\mathbf e_r~$ equal to
$$\dot r(\theta)=\frac{\partial r}{\partial \theta}\,\dot\theta=\frac{\partial r}{\par... | {
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Wavefunction of distinguishable spin 1/2 fermions Does the total wave function for distinguishable (i.e. not identical) spin 1/2 fermions need to be anti-symmetric under particle exchange? Or does the Pauli exclusion only hold for indistinguishable fermions?
| Quoted from Pauli exclusion principle (emphasis by me):
A more rigorous statement is that, concerning the exchange of
two identical particles, the total (many-particle) wave function
is antisymmetric for fermions, and symmetric for bosons.
This means that if the space and spin coordinates of
two identical particles ar... | {
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How would the universe look like without matter? I was wondering how the universe would look like if it would have been perfectly symmetrical in terms of matter and antimatter. If I understand correctly, there would be no "particle" but the energy released by matter-antimatter annihilation shouldn't just disappear, thu... | Nothing new would replace the matter. Asymmetric matter was a tiny contribution to the energy density of the early universe, of order one part in a billion. Moreover, if we were to add the energy of the asymmetric matter to the primordial radiation bath, that would essentially just shift the exact same cosmic evolution... | {
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Why can you hear loud TV in the next room despite the wall and door? In a house, when two rooms are next to each other, why can you hear the loud TV on the next room, despite the wall between them and despite that their two doors are closed. (I don't know a lot on physics, but isn't there something like sound travellin... | Sound can travel through walls and other solid objects because it is a pressure wave that travels through air, liquids, and solids. The sound from a loud TV can penetrate a wall and reach the adjacent room, especially if the wall is thin or has gaps in it.
Additionally, low frequency sounds like bass can travel further... | {
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Linear combination of atomic orbitals (LCAO) justification for molecular orbitals (MO) done in this MIT solid state chemistry lecture In this series on solid state chemistry by MIT, 11:00-13:00 in this lecture, LCAO for molecular orbitals is justified by the fact that Schrodinger's equation is linear (and therefore the... | LCAO is an approximation, partly because we usually only add up a finite number of orbitals. E.g. for the molecule LiH, we could combine the H 1s orbitals with the Li 2s and 2p to get some wavefunction. But we have ignored the Li 3s for example, because it's high in energy and won't overlap well with H 1s.
Another re... | {
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How did Enrico Fermi compute when the Chicago Pile-1 nuclear reactor would become critical? I'm trying to understand the first nuclear reactor, the Chicago Pile-1, specifically the math Fermi did to figure out when the reactor would go critical. There's a nice report available from Fermi, where he tracks the value of $... | I cannot be sure I understand it correctly, but this is how it looks to me at the moment. The report that you quote says:
In a spherical structure having the reproduction factor 1 for
infinite dimensions the activation of a detector placed at the center
due to the natural [my emphasis] neutrons is proportional to the
... | {
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Coherent unpolarized laser light I notice that in semiclassical treatments of laser light absorption by particles, they treat the laser beam as a coherent oscillating electric field over the form $E_0\cos(kx-\omega t)$, sometimes with a factor added in to account for the Gaussian spread of wavelengths. However, my unde... | All lasers are polarized .... where did you hear otherwise?
We can never directly observe the superposition of EM waves in the EM field ..... we can only observe a photon when it excites an electron in a CCD or in your eye. It does not matter if the photons are out of phase (net zero E,M) .... the electrons are able to... | {
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Mass-Energy Equivalence and First Law of Thermodynamics Einstein showed mass can be converted into energy and vice versa.
$E=mc^2$
However, in school we are taught that according to the First Law of Thermodynamics, energy can neither be created nor destroyed.
Are they not contradicting each other? I already tried findi... |
Are they not contradicting each other?
Yes, there is a contradiction, but not between $E=mc^2$ and thermodynamics. The contradiction is between the actual meaning of $E=mc^2$ and its usual pop-science description. Unfortunately, although $E=mc^2$ is very famous, it is also very misunderstood. The usual English descri... | {
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What exactly does a Lorentz transformation provide?
The primed reference frame is moving relative to the unprimed frame. So if we were to take the lorentz transformation of point P from the unprimed to primed, would it be the point A or B that it returns ? Assuming that the Lorentz transformation is passive i.e. we ar... |
Look at this Minkowski diagram point P move parallel to the x‘ axis , the event point is then $~E(ct‘,x‘)$
| {
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Why does the double slit experiment not prove that the wave function is ontological? To me, it seems that the interference pattern is the evidence that the wave function is a physical aspect of reality, but people still seem to be trying to decide whether or not it's ontological or just a mathematical construct.
Why is... | The wave function is a projection of the more abstract state vector onto position space. If you want to say that the wave function is a "physical aspect of reality", then naturally you have to say that any other space one can project the state onto (momentum, energy, angular momentum, etc.) is also a "physical aspect o... | {
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First mode of vibration for a glass window How to calculate the first mode of vibration for a glass window?
I have a window of the size 57 cm by 106 cm and 4 mm of width. I hear the loud noise from it on about 166 Hz and want to realize if it is window's self first mode of vibration that intensify this frequency (maybe... | I am a mechanical engineer who does full time work with finite element analysis including modal analysis which involves finding the natural frequencies of structures.
Knowing the exact window dimensions is a good first step, but to calculate the exact first mode with FEM will require the two additional items listed...
... | {
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Why can we always find $\vec A$ such that it satisfies Coulomb (or Lorenz) gauge and Maxwell's equations? I have a short question about the Coulomb potential.
Let $\vec{E}$ and $\vec{B}$ be the electric field and magnetic field respectively.
The electric field and magnetic fields are described by the scalar potential $... | The short answer is yes.
The general proof of this does not make any assumptions about the fields and thus is valid in general. In particular, if $\vec{A}_1$ is a vector potential for $\vec{B}$, i.e. a potential such that $\nabla \times \vec{A}_1 = \vec{B}$, then $\vec{A}_2 = \vec{A}_1 + \nabla f$ describes the same ma... | {
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Why does magnetic force only act on moving charges? I don't understand why the magnetic force only acts on moving charges. When I have a permanent magnet and place another magnet inside its field, they clearly act as forces onto one another with them both being stationary. Also, I am clearly misunderstanding something.... | After reading some of the other answers I think there is a simpler more fundamental explanation.
According to Field Theory, rather than considering the interactions between particles and field, we can consider the interactions between the fields associated to the particles.
Under this understanding, Classical gravitati... | {
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Why is it not advisable to jump onto the shore from a boat? Just to clarify, I have done this several times without facing any consequences.
But I have heard that we are not recommended to do so. Why? Is it because the boat may move backwards (water is fluid) and we will lose balance and fall into the water? Does this ... | I think the problem is that the boat recoils: when jumping from a hard surface one's force goes fully into accelerating the one's body, which determines how far one jumps/lands. When jumping from a boat, one's force is expended on accelerating oneself and the boat, which moves in the opposite direction. In other words,... | {
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What is happening at the particle level in the Bernoulli Principle? One might think that increasing the speed of particles would increase pressure -- if I understand what the Principle states, it is very counterintuitive.
My guess is, the pressure has something to do with particles moving perpendicular to the fast moti... | The Bernoulli principle comes from the second law of Newton. If we take a slice of fluid with cross section $A$ of a horizontal pipe, and with length $\Delta x$, the net force acting on it is:$$\Delta F = -\Delta P A$$ The minus sign indicates that the pressure must be decreasing along $x$, for a positive force on the ... | {
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Why is electromotive force in magnetohydrodynamics a vector quantity? In the mean-field dynamo theory in magnetohydrodynamics, I frequently came across a quantity;
$\langle v'\times B' \rangle$, which is termed as the mean electromotive force. I want to know that why is it termed as electromotive force, if it is a vect... | $\left\langle \mathbf{v}' \times \mathbf{B}' \right\rangle$ has dimensions of electric field, rather than potential. Therefore, it is different from the standard definition of electromotive force. In a highly conductive fluid it would be equal to $-\left\langle \mathbf{E}' \right\rangle$ (by Ohm's law). It could be con... | {
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Uncertainty Calculation: Applying Product Rule instead of Power Rule I use $\delta$ to represent absolute uncertainty. The power rule for the calculation of relative uncertainty in $t^2$ is
$$\frac{\delta (t^2)}{(t^2)}=2\left(\frac{\delta t}{t}\right).$$
But if I treat the power as a product and apply the product rule,... | The product rule assumes that the things being multiplied vary independently of one another, which is clearly not the case when multiplying something by itself. As such, the power rule is the correct one here.
| {
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Why do some objects tend to sink after some time in water even if they float at the start? I have observed this phenomenon in swimming pools: I have seen many dead insects floating on the surface, but after some time some they tend to sink down without any external influence. Why does this exactly happen? It even happe... | There are different mechanisms that can possibly be relevant here:
*
*The object is floating because its less dense than water on account of enclosed or separated air (or other gases).
*
*Due to contact with water it slowly fills up with water making it more dense and thus sinking.
Typical examples are a sponge or ... | {
"language": "en",
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Invariant nature of mass and particle annihilation Since mass is a Lorentz invariant, it can never change to preserve the vectorial nature of the four-momentum and the other four vectors. Thus the only interpretation of the energy-mass equation that I can give is that in
$$E^2=m^2+p^2$$
the first term, $m^2$ is a const... |
Since annihilation means a change in mass, how is mass invariant?
In this context, mass being invariant means it is Lorentz invariant, which means it is a concept that has the same numerical value in all inertial frames of reference. It does not mean it cannot change in time. If a small part of a rocket (judging by m... | {
"language": "en",
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What is sound and how is it produced? I've been using the term "sound" all my life, but I really have no clue as to what sound exactly is or how it is created. What is sound? How is it produced? Can it be measured?
| Sound is basically produced by vibrating by either blowing through something like a flute or a recorder, hitting something like a drum, shaking something like maracas, or strumming something like a guitar when you do all sorts of these things the air goes through the instrument and hits each side of it to make it vibra... | {
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Where do magnets get the energy to repel? If I separate two magnets whose opposite poles are facing, I am adding energy. If I let go of the magnets, then presumably the energy that I added is used to move the magnets together again.
However, if I start with two separated magnets (with like poles facing), as I move them... | As is said in a comment, the reasoning in the first paragraph is correct but the one in the second paragraph is wrong.
If you apply a force on something without "moving" the work is null and there is no energy exchange involved (this is not the same thing than doing that with your muscles, but that's another story :p).... | {
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Mnemonics to remember various properties of materials I'm trying to figure out how to remember that
*
*hardness: how resistant it is to deformation
*toughness: how resistant it is to brittle failures
*stress: force on a surface area
*strength: ability to withstand stress without failure
*strain: measurement of d... | I always used to confuse stress and strain: most of my mnemonics involved making words out of initial letters.
When you're stressed, you show the strain.
Stress is what is applied to the material, strain is what it does in response - I always used to get these the wrong way around.
E equals Fl/ea
Young's Modulus = ... | {
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Is Newton's Law of Gravity consistent with General Relativity? By 'Newton's Law of Gravity', I am referring to
The magnitude of the force of gravity is proportional to the product of the mass of the two objects and inversely proportional to their distance squared.
Does this law of attraction still hold under General ... | May be the case that Gerber could not give an exact explanation for his formula, 18 years before GR, on the advance of Mercury's perihelium as we can see at mathpages. After reading the fine explanation on Lienard & Wiechert retarded potentials in the Hans de Vries online book I think that the treatment of the subject ... | {
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Book about classical mechanics I am looking for a book about "advanced" classical mechanics. By advanced I mean a book considering directly Lagrangian and Hamiltonian formulation, and also providing a firm basis in the geometrical consideration related to these to formalism (like tangent bundle, cotangent bundle, 1-for... | I can't believe nobody's mentioned Arnol'd's book "Mathematical Methods for Classical Mechanics" - it covers everything you ask for in the first paragraph quite elegantly (though sometimes somewhat tersely).
| {
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Law for tap water temperature I was wondering if anyone put together a law to describe the rising temperature of the water coming out of a tap.
The setup is fairly simple: there's a water tank at temperature T, a metal tube of length L connected to it and a tap at the end where temperature is measured. The water flows ... | We can consider the following model: a tube of constant temperature $T_e$ of lenght L, radius $r$ where water is flowing uniformly at a speed $v$ (that you can obtain from your flow $P$).
A "slice" of water travels an interval $dx$ in a duration $dt = \frac{dx}{v}$.
The tube will contribute to the "heating" of the wate... | {
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Why can you "suck in" cooked spaghetti? We all know that there is no "sucking", only pushing. So how are cooked spaghetti pushed into your mouth? The air pressure applies orthogonal on the spaghetti surface. Where does the component directed into your mouth come from?
| The component directed into your mouth comes form the different pressure between the outside and the inside of the mouth. If you create a difference in pressure of $\Delta P$ the force pushing the spaghetti in will be $\Delta P \cdot S$ where $S$ is the section of the spaghetto (or spaghetti.. depending how hungry you ... | {
"language": "en",
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Finding the volume of this irregular shape I have I have an approximately basketball-sized non-hollow piece of aluminum sitting in my house that is of irregular shape. I need to find the volume of it for a very legitimate yet irrelevant reason.
What is the best way I can do this? In fact, what are all the ways I coul... | Since the object is basketball size, it would displace a significant volume and weight of water when submerged. Weigh a container with some water and take a reading. Then attach a wire to the object, suspend it in the container of water till it's submerged without touching the container, and take a second reading. The ... | {
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Why don't spinning tops fall over? One topic which was covered in university, but which I never understood, is how a spinning top "magically" resists the force of gravity. The conservation of energy explanations make sense, but I don't believe that they provide as much insight as a mechanical explanation would.
The hyp... | The quick answer is that, for the top to fall over due to gravity, each fragment of the top that is moving around the spin axis has to change its individual direction of movement.
They are already changing direction around the spin axis, due the rigidity of the top keeping them moving in a circle. But gravity is operat... | {
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Why does water make a sound when it is disturbed? When I disturb a body of water, what causes the familiar "water moving" sound?
| The major source of sound when water is disturbed is the creation of underwater air bubbles, which oscillate in shape and size, producing damped sinusoid sound waves. The resonant frequency of the bubble depends on its size, so many bubbles of different sizes and different resonant frequencies produce the "burbling" s... | {
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Planet orbits: what's the difference between gravity and centripetal force? My physics teacher says that centripetal force is caused by gravity. I'm not entirely sure how this works? How can force cause another in space (ie where there's nothing).
My astronomy teacher says that gravity is (note: not like) a 3D blanket... | I think a key element that this student is missing is actually a very common misconception about centripetal force. We hear of various types of forces, like normal forces, frictional forces, and gravitational forces, and then we tack on "centripetal force" as if it was another type of force like that. But it's not-- ... | {
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Home experiments to derive the speed of light? Are there any experiments I can do to derive the speed of light with only common household tools?
| I can't think of a way to do it with "common household tools" but if you have an oscilloscope, a laser diode, a couple of photo-sensors, a beam splitter, you can do it. All of these things are readily available from science supply/hobby stores online, but not usually in most homes.
Set up the laser diode to hit the be... | {
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Advantages of high-energy heavy-ion collisions over proton-proton collisions? Some high-energy experiments (RHIC, LHC) use ion-ion collisions instead of proton-proton collisions. Although the total center-of-mass energy is indeed higher than p-p collisions, it might happen that the total energy per nucleon is actually ... | The main goal of accelerating heavy ions in the LHC is to produce the quark gluon plasma as a result of large energy concentration in a small volume in case many nucleons of the two nuclei interact producing a collective effect of hadronig heating to a temperaturę of milliards degrees which melts all hadrons. In heavuy... | {
"language": "en",
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Would a magnet attract a paperclip indefinitely? Let's say we have a magnet stuck to a metal bar, suspended above the ground. If I attach a paperclip to the magnet, where is the energy to hold the paperclip coming from (against the force of gravity), and for how long will the paperclip remain there - will it remain the... | The paperclip is not moving relative to earth - this means that no energy is being spent by magnet to hold it, so it can hold the paperclip as long as the magnet has magnetic properties.
As a contrast, when you hold something with your hands, energy is being spent by your muscles not to hold the object, but to remain c... | {
"language": "en",
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Why is it thought that normal physics doesn't exist inside the event horizon of a black hole? A black hole is so dense that a sphere around it called the event horizon has a greater escape velocity than the speed of light, making it black. So why do astronomers think that there is anything weird (or lack of anything In... | For large enough black holes, space is still weakly curved at the event horizon, so of course we should expect that normal physics still exists there. An infalling observer wouldn't experience anything out of the ordinary when crossing an event horizon.
What is true is that for an outside observer, it's impossible to p... | {
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Notation of plane waves Consider a monochromatic plane wave (I am using bold to represent vectors)
$$ \mathbf{E}(\mathbf{r},t) = \mathbf{E}_0(\mathbf{r})e^{i(\mathbf{k} \cdot \mathbf{r} - \omega t)}, $$
$$ \mathbf{B}(\mathbf{r},t) = \mathbf{B}_0(\mathbf{r})e^{i(\mathbf{k} \cdot \mathbf{r} - \omega t)}. $$
There are a f... | I'd call it the initial complex field since it's {E(t=0)}. No reference though.
| {
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Why does kinetic energy increase quadratically, not linearly, with speed? As Wikipedia says:
[...] the kinetic energy of a non-rotating object of mass $m$ traveling at a speed $v$ is $\frac{1}{2}mv^2$.
Why does this not increase linearly with speed? Why does it take so much more energy to go from $1\ \mathrm{m/s}$ to... | Throw 3 balls up with equal weight with no air friction.
The ball have up velocity
$$V_{1}$$
$$V_{2}$$
$$V_{3}$$
And let's for simplicity sake we have
$$V_{2}=2V_{1}$$
$$V_{3}=3V_{1}$$
Let's for simplicity sake ball 1 travels
$$S$$
And stop after time
$$T$$
How far will ball 2 travel?
Notice that $$V_{2}=2V_{1}$$
Also ... | {
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Material resistency to lasers beam Keeping the average power constant, why some materials are more eager to be damaged by pulsed laser with respect of C.W. lasers, or viceversa?
When i talk about pulsed lasers i think for examples of duty cycles in the order of $10^5$.
For example optical elements (such as a vortex pha... | One thing that ought to matter is how much laser light gets reflected versus how much gets absorbed and transmitted.
I don't know much about this, but my naïve guess is that materials get damaged by lasers primarily because they are heated by the intensity of the light, and then they melt (or burn!). Therefore ther... | {
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Equilibrium and movement of a cylinder with asymmetric mass centre on an inclined plane A cylinder whose cross section is represented below is placed on an inclined plane. I would like to determine the maximum slope of the inclined plane so that the cylinder does not roll. The mass centre (CM) of the cylinder is at a d... | The effect of friction is to make the cylinder roll down the ramp rather than slide.
To find an equilibrium angle, use virtual work.
If $\phi$ changes by a small amount $d\phi$, as the cylinder rolls, then everything goes down a little (neglecting at first the small interior cylinder's upward movement) because you're m... | {
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Evolution in the interpretation of the Dirac equation As I understand, Dirac equation was first interpreted as a wave equation following the ideas of non relativistic quantum mechanics, but this lead to different problems.
The equation was then reinterpreted as a field equation and it is now a crucial part of quantum f... | Although not directly relevant to your question, it is helpful to compare the different editions of Dirac's Principia. He revised his treatment of QED every time, and such an evolution must shed light on the evolution of the Dirac eq. from a one-particle eq. to its current status as a QFT equation. I cannot quite rec... | {
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Suggested reading for renormalization (not only in QFT) What papers/books/reviews can you suggest to learn what Renormalization "really" is?
Standard QFT textbooks are usually computation-heavy and provide little physical insight in this regard - after my QFT course, I was left with the impression that Renormalization ... | Regarding "providing unifying insight about renormalization in QFT, statistical physics or pure mathematics", this is what I tried to do in my detailed answer to Wilsonian definition of renormalizability
| {
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"source": "stackexchange",
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Books that every physicist should read Inspired by How should a physics student study mathematics? and in the same vein as Best books for mathematical background?, although in a more general fashion, I'd like to know if anyone is interested in doing a list of the books 'par excellence' for a physicist.
In spite of the ... | What is life? E. Schrodinger
The origin of life. F. Dyson
How nature works. P. Bak
Because physics is not only particle physics.
And the trilogy of Weinberg, that with the Landau course forms the holy bible of theoretical physicist.
| {
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "29",
"answer_count": 24,
"answer_id": 8
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Relativistic Cellular Automata Cellular automata provide interesting models of physics: Google Scholar gives more than 25,000 results when searching for "cellular automata" physics.
Google Scholar still gives more than 2.000 results when searching for "quantum cellular automata".
But it gives only 1 (one!) result when ... | Some cellular automata, like the basic rule 110 are universal, i.e., Turing complete. What this means is that you can simulate/emulate any mathematics on them, including any physical theory, including non-local ones. Many people make the mistake of thinking that because cellular automata have local and discrete rules t... | {
"language": "en",
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"source": "stackexchange",
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Lightning strikes the Ocean I'm swimming in - what happens? I'm swimming in the ocean and there's a thunderstorm. Lightning bolts hit ships around me. Should I get out of the water?
| Most probably the current just spreads in all directions and weakens quite fast (at least like $r^{-2}$, not counting resistance), so I don't think the hazard is much (in magnitude) larger than on land in similar conditions.
EDIT: In what I found in Internet salty water has only 10 times better conductivity than wet so... | {
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How relevant is LHC to quantum gravity? Premise: the LHC is obviously mapping unseen territory in high energies, and therefore it's always possible to imagine far out results.
Excluding completely unexpected outcomes - is the LHC performing any experiment that could help with string theory or m-theory? For example:
*... | Well, there's no reason to believe in supersymmetry, beyond some theoretical niceness to it, so if they see THAT at the LHC, then string theory gets a big boost, as there is no way other than supersymmetry to produce fermions in string theory.
The other thing that might be relevant to quantum gravity is that if there a... | {
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Common false beliefs in Physics Well, in Mathematics there are somethings, which appear true but they aren't true. Naive students often get fooled by these results.
Let me consider a very simple example. As a child one learns this formula $$(a+b)^{2} =a^{2}+ 2 \cdot a \cdot b + b^{2}$$ But as one mature's he applies t... | "Summer is when the Earth is closest to the sun, and winter is when it's furthest away."
It's true that the Earth's orbit is slightly elliptical, but the effect of this, as far as seasons, is very small. For one thing, this wouldn't explain why the sun rises and sets at different times in different seasons, and if this... | {
"language": "en",
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How do contact lenses work? I understand how telescope, microscope and glasses work.
But how do contact lenses work?
| Your question is really "how does the human eye work?", since the contact lens is designed to adjust the optics of the human eye.
This image from the wikipedia article on the anatomical lens shows how the cornea and lens focuses incoming light from the left onto the retina (right).
Previously, I'd written that the b... | {
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Home experiment to estimate Avogadro's number? How to get an approximation of Avogadro or Boltzmann constant through experimental means accessible by an hobbyist ?
| Electrolysis. Run a current through a weak acid, and measure the current going in and coming out. Hydrogen ions in the acid will capture electrons and bond to each other to form hydrogen gas. If you accept a measured value for the charge of an electron, you can find the number of hydrogen molecules liberated. Then ... | {
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What's the difference between helicity and chirality? When a particle spins in the same direction as its momentum, it has right helicity, and left helicity otherwise. Neutrinos, however, have some kind of inherent helicity called chirality. But they can have either helicity. How is chirality different from helicity?
| At first glance, chirality and helicity seem to have no relationship to each other. Helicity, as you said, is whether the spin is aligned or anti aligned with the momentum. Chirality is like your left hand versus your right hand. Its just a property that makes them different than each other, but in a way that is revers... | {
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What is the difference between "kinematics" and "dynamics"? I have noticed that authors in the literature sometimes divide characteristics of some phenomenon into "kinematics" and "dynamics".
I first encountered this in Jackson's E&M book, where, in section 7.3 of the third edition, he writes, on the reflection and r... | Primarily, the distinction between kinematics and dynamics is one of causation. What do we mean by this? The etymology of the word kinematics is the Greek kinēma, which means motion. On the other hand, dynamics draws its origin from dunamis, meaning power (though we are better off thinking of it as a power in potentia,... | {
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Are gauge choices in electrodynamics really always possible? If $B$ is magnetic field and $E$ electric Field, then
$$B=\nabla\times A,$$
$$E= -\nabla V+\frac{\partial A}{\partial t}.$$
There is Gauge invariance for the transformation
$$A'\rightarrow A+{\nabla L}$$
$$V'\rightarrow V-\frac{dL}{dt}.$$
Now, we can write:
... | Since, my similar question was closed, I will answer here.
The gauge transformation
$$ \mathbf {A} \rightarrow \mathbf {A} +\nabla \lambda, $$
$$ \varphi \rightarrow \varphi - \frac {\partial \lambda}{\partial t}, $$
(where $ \lambda=\lambda(\vec {r},t) $ is an arbitrary scalar function of coordinates $ \mathbf{r} $ an... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1250",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 4,
"answer_id": 3
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Doppler's effect use While i was in high-school i learn't the Doppler's Effect which if i remember correctly is:
*
*The Apparent change in the frequency of sound caused due the relative motion between the sound and the observer.
This phenomenon seems obvious, but what i would like to know is, what use does Doppler... | It can be used to measure speed - that's how police radar guns and speed cameras work. Radar waves from the gun/ camera are reflected off the moving vehicle, and the wavelength is shifted according to the speed of the vehicle relative to the gun/ camera.
In astrophysics, looking at light from distant galaxies, we notic... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1294",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
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What sustains the rotation of earth's core (faster than surface)? I recently read that the earth's core rotates faster than the surface.
Well, firstly, it's easier to digest the concept of planetary bodies, stars, galaxies in rotation and/or orbital motion.
But, what makes a planet's core rotate? And in the earth's cas... | The earth's rotation is slowing down, due primarily to tidal interaction with the moon (transfering angular momentum to the moon). This frictional loss occurs near the surface. On this basis, if the rotational coupling of the inner core to the mantle is imperfect, it would be expected to lag with respect to the slowdow... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1336",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
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Best example of energy-entropy competition? What are the best examples in practical life
of an energy-entropy competition which favors entropy over energy?
My initial thought is a clogged drain -- too unlikely for the
hair/spaghetti to align itself along the pipe -- but this is probably
far from an optimal example. Cu... | One of the nicest examples I know is the Kosterlitz-Thouless phase transition in the XY model. What is cool is that the transition is driven by the condensation of vortices which have an energy that diverges logarithmically with the size of the system. You would think they couldn't contribute at all because of this, bu... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1354",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 7,
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Searching books and papers with equations Sometimes I may come up with an equation in mind, so I want to search for the related material. It may be the case that I learn it before but forget the name, or, there is no name for the equation yet. In this case, I may be able to recall a reference book. Searching in Interne... | You can perform $\LaTeX$ search - that is, write formula in LaTeX in an appropriate search engine:
*
*http://www.latexsearch.com/
However, as one can type the same expression in different ways and with different symbols, I never used it it practice. (Anyone did?)
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1392",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "15",
"answer_count": 2,
"answer_id": 1
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Is there a name for the derivative of current with respect to time, or the second derivative of charge with respect to time? This measurement comes up a lot in my E&M class, in regards to inductance and inductors.
Is there really no conventional term for this?
If not, is there some historical reason for this omission?
| It is change of current in unit time. If there is a current, there will be a magnetic field. If there is a change in current, the possibility is an acceleration of charge which leads to the production of electromagnetic waves. So it could be an electric field and/or magnetic field.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1421",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 2,
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Can I parameterize the state of a quantum system given reduced density matrices describing its subparts? As the simplest example, consider a set of two qubits where the reduced density matrix of each qubit is known. If the two qubits are not entangled, the overall state would be given by the tensor product of the one q... | Density matrices often admit an interesting geometric interpretations when you map them to the space of generalized Bloch vectors, see for example the book I. Bengtsson, K. Życzkowski, Geometry of quantum states, 2006. I won't be surprised if it turns out that the result has something to do with the coset space $SU(2N)... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1491",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 2,
"answer_id": 0
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Is it possible to obtain gold through nuclear decay? Is there a series of transmutations through nuclear decay that will result in the stable gold isotope ${}^{197}\mathrm{Au}$ ? How long will the process take?
| the alchemists have dreamed about the production of gold (Z=82) from some cheap material and lead (Z=79) was their favorite choice. They were just using a wrong science - namely primitive chemistry instead of nuclear physics. But otherwise their choice of lead was OK. And indeed, lead became the element that was transm... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1530",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 7,
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Which experiments prove atomic theory? Which experiments prove atomic theory?
Sub-atomic theories:
*
*atoms have: nuclei; electrons; protons; and neutrons.
*That the number of electrons atoms have determines their relationship with other atoms.
*That the atom is the smallest elemental unit of matter - that we ca... | The history of atoms is definitely intertwined with quantum mechanics. There are many features of the quantum theory that make atomic nature of our world apparent. But here I'd like to state an earlier result.
Thomson's 1897 discovery of the electron not only showed that atoms exist but also that they have substructure... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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What's the difference between running up a hill and running up an inclined treadmill? Clearly there will be differences like air resistance; I'm not interested in that. It seems like you're working against gravity when you're actually running in a way that you're not if you're on a treadmill, but on the other hand it s... | (Running up a treadmill) = (expend energy to keep feet moving at a constant speed) + (other effects)
(Running up a hill) = (expend energy to keep feet moving at a constant speed) + (energy to lift center of gravity by hill height) + (other effects)
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1639",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "27",
"answer_count": 10,
"answer_id": 7
} |
Why doesn't air freeze? I am in no way experienced in the Physics field so this question may seem a bit silly but i'd appreciate an answer :)
Why doesn't air freeze?
| Air does freeze just at temperatures and pressures we don't often experience.
There is an entire industry around producing and distilling liquid air. You take air and compress it. As a result the air increases in temperature; the air is allowed to cool. Then the air is expanded by venting it into a new chamber. The r... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1768",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
"answer_count": 6,
"answer_id": 1
} |
Why are physicists interested in graph theory? Can you tell me how graph theory comes into physics, and the concept of small world graphs?
(inspired to ask from comment from sean tilson in):
Which areas in physics overlap with those of social network theory for the analysis of the graphs?
| Richard Feynman reformulated quantum mechanics (and quantum field theory) in terms of a path integral, meaning that in order to find the likelihood of some process occurring, you take a kind of weighted average over all potential trajectories. The weighting function is the exponentiated "action," $\exp(iS/\hbar).$ and... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1876",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "24",
"answer_count": 5,
"answer_id": 2
} |
Buckyballs in vacuum I've read about the idea that buckyballs and other nanostructures could be used to hold drugs and things until they reach certain places in the body and then get released.
So I was wondering, if you created a buckyball in air, so that some molecules that are in air (such as oxygen and nitrogen) wer... | It is just that the buckyball's faces are not holes as in popular view; there is pretty much electron probability density there forming quite a strong barrier. Also the size of the faces is comparable to the size of a, for instance, oxygen molecule, so I'm pretty sure that it is impossible for a buckyball to release a ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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Why does holding something up cost energy while no work is being done? I read the definition of work as
$$W ~=~ \vec{F} \cdot \vec{d}$$
$$\text{ Work = (Force) $\cdot$ (Distance)}.$$
If a book is there on the table, no work is done as no distance is covered. If I hold up a book in my hand and my arm is stretched, i... | In my humble opinion, I don't really think this is much of a problem that needs so much clarification. You must understand that the "energy" you know in Physics has absolutely, I mean absolutely nothing to do with the energy that your body cells expend. You can actually expend energy while doing physical (of Physics) w... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/1984",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "172",
"answer_count": 14,
"answer_id": 13
} |
Can I levitate an object without using an electromagnet? I know that it's possible to make an object levitate using an electromagnet to hold it up.
But is it also possible to do this with regular magnets? Is there a special kind of magnet I need in order to have one powerful enough to hold an object up?
I'm asking bec... | Answer is NO.
I would summarise the question as below.
'Is it possible to make a system with 2 parts made of permanent magnets so as to have an arrangement of one part floating due to magnetic repulsion of the other part above it ( effectively the gravitational pull gets balanced by the magnetic repulsive force )
As we... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2029",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "17",
"answer_count": 4,
"answer_id": 2
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Why do we think there are only three generations of fundamental particles? In the standard model of particle physics, there are three generations of quarks (up/down, strange/charm, and top/bottom), along with three generations of leptons (electron, muon, and tau). All of these particles have been observed experimental... | As the Gluons don't have any charge, the position of Gluon 7 and 8 takes the free Spinaxis on the PionLeft and Right answered Oct 20 Hsch31
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2051",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "76",
"answer_count": 7,
"answer_id": 6
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How long a straw could Superman use? To suck water through a straw, you create a partial vacuum in your lungs. Water rises through the straw until the pressure in the straw at the water level equals atmospheric pressure. This corresponds to drinking water through a straw about ten meters long at maximum.
By taping se... | I think we can most easily consider the problem from the perspective of energy. For a unit area column the external energy put in equals the volume of the column times the air pressure. The gravitational energy is the mass of water raised times the average height of the water. So
as we pull the water up, we the water i... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2111",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "66",
"answer_count": 12,
"answer_id": 6
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Why did this glass start popping? I remember a while ago my father dropped a glass lid and it smashed. It looked something like this. When that happened, for about 5 minutes afterwards, the glass parts were splitting, kind of like popcorn, and you could hear the sound. I was just wondering why this happened, and the pa... | Due to internal stress in the material.
This stress might be there due to fabrication technology or due to heat cycling while usage (less likely).
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2156",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Is it possible for information to be transmitted faster than light by using a rigid pole? Is it possible for information (like 1 and 0s) to be transmitted faster than light?
For instance, take a rigid pole of several AU in length. Now say you have a person on each end, and one of them starts pulling and pushing on hi... |
Is it possible for information (like 1 and 0s) tO be transmitted in anyway faster than light.
No.
Born2Smile said the same thing (which I +1'd) but I figured it's worth repeating for emphasis. It'd be a violation of causality. For some more details on why this is not allowed, in addition to Born2Smile's answer, see W... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2175",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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"answer_count": 16,
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Aspherical lenses - perfect analytical shape It's known that single spherical lens cannot focus parallel beam of monochromatic light into single (diffraction-limited) point, so it has to have aspherical shape to achieve that.
Is perfect analytical lens shape is known that is able to focus light into a single point (aga... | The problem you are talking about is called the spherical aberration. Spherical lenses are much easier to make, while from geometrical point of view the ideal focusing surface is a parabola. Since the light on optical instruments goes close to optical axis, one uses the paraxial approximation where sphere and parabola ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2189",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "18",
"answer_count": 3,
"answer_id": 2
} |
Why are materials that are better at conducting electricity also proportionately better at conducting heat? It seems like among the electrical conductors there's a relationship between the ability to conduct heat as well as electricity. Eg: Copper is better than aluminum at conducting both electricity and heat, and sil... | This is true only for metals. Diamond, for example, is barely a semiconductor. But it has a better heat conductivity than any metal.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2245",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "14",
"answer_count": 3,
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} |
How cold does it need to be for spit to freeze before hitting the ground? What is the dominant form of heat transfer between warm water and cold air?
If a $100 mg$ drop of water falls through $-40 C$ air, how quickly could it freeze?
Is it credible that in very cold weather spit freezes in the half a second it takes ... | Yes, I just did it. Arced a small spheriod of spit (no phlem) and it hit the ground and rolled instead of splattered. -15 degrees F, light wind in parking garage (not sure if that matters) attempted with numerous quantaties of spit and trajectories but small quantity with upward trajectory is what got the tiny frozen d... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2363",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 2,
"answer_id": 1
} |
Do high/low pass lenses exist? For an experiment I will hopefully be soon conducting at Johns Hopkins I need two different lenses.
The first needs to allow all wavelengths above 500 nm to pass (thus a high pass filter) and cut off everything else.
The second needs to allow all wavelengths below 370 nm to pass (thus a l... | You may consider to use a prism to separate the different frequency light (essentially you are performing a Fourier transform). Then physically blocked those light with wavelength higher than 500 nm and lower than 370 nm. Finally, you merge these light ray together. This will be the device you want and you can freely a... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2398",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
"answer_count": 4,
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Primer on Liquid/Ink Splatter Physics I'm looking for primer material on the modelling the physics of liquids. In particular I want to make a small simulation (I'm a programmer by profession) of throwing ink at a board, much like this.
http://www.pond5.com/stock-footage/670934/ink-splatter.html
I'm researching ways to ... | There are two separate physical considerations regarding the video you posted:
*
*The physics of a spherical drop hitting a dry surface
*Multiple spherical drops hitting the surface at different times
Once the first is solved, you should be able to simulate the second easily.
Regarding the first point, a quick ar... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2442",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Would you be weightless at the center of the Earth? If you could travel to the center of the Earth (or any planet), would you be weightless there?
| Correct. If you split the earth up into spherical shells, then the gravity from the shells "above" you cancels out, and you only feel the shells "below" you. When you are in the middle there is nothing "below" you.
Refrence from Wikipedia Gauss & Shell Theorem.
{I am using some simplistic terms, but I don't want to br... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2481",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "39",
"answer_count": 5,
"answer_id": 2
} |
Is there a limit to loudness? Is there any reason to believe that any measure of loudness (e.g. sound pressure) might have an upper boundary, similar to upper limit (c) of the speed of mass?
| Well, the short answer is: there is, when the hydrodynamic approximation (that fluid is composed of small "fluid particles" i which real particles move in the reference frame of the "fluid particle" like in stationary fluid) breaks.
The upper bound can be approximated with wave amplitude equal to ambient pressure, so ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2523",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 4,
"answer_id": 1
} |
How is the classical twin paradox resolved? I read a lot about the classical twin paradox recently. What confuses me is that some authors claim that it can be resolved within SRT, others say that you need GRT. Now, what is true (and why)?
| Its easy to resolve from the person on earth - he took the doppler effect out of the equation during his observations of the space ship and observed that the clock on the rocket ship was running slow, and confirmed this when the ship arrived back in port, by comparing the two clocks.
The observation according to the tr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2554",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "52",
"answer_count": 8,
"answer_id": 7
} |
How does mass leave the body when you lose weight? When your body burns calories and you lose weight, obviously mass is leaving your body. In what form does it leave? In other words, what is the physical process by which the body loses weight when it burns its fuel?
Somebody said it leaves the body in the form of h... | Weight loss occurs when a large amount of calories or fat is turned into energy through excercise or activity. That waste is then released from the body in one's urine, stool, & sweat. When your body converts fat into accessible energy, the process generates heat that is used to regulate body temperature, according ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2605",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "119",
"answer_count": 18,
"answer_id": 12
} |
What temperature can you attain with a solar furnace? A solar furnace is a device that concentrates the sun's light on a small point to heat it up to high temperature. One can imagine that in the limit of being completely surrounded by mirrors, your entire $4\pi$ solid angle will look like the surface of the sun, at ab... | There is no limit to the degree of concentration. In theory, the entire output of the Sun could be concentrated into a small point.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2679",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "15",
"answer_count": 3,
"answer_id": 2
} |
How fast a (relatively) small black hole will consume the Earth? This question appeared quite a time ago and was inspired, of course, by all the fuss around "LHC will destroy the Earth".
Consider a small black hole, that is somehow got inside the Earth. Under "small" I mean small enough to not to destroy Earth instant... | If the black hole simply swalled matter, and didn't lose any energy, it probably isn't too hard a calculation, just assume the earth is unsupported mass that falls into the BH, which grows in mass as it adds more stuff. The problem, is we know this isn't how it would happen, and some significant fraction of swalled mas... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2743",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "37",
"answer_count": 7,
"answer_id": 2
} |
Why does my wife's skin buzz when she's using her laptop? When my wife uses her laptop, if I touch her skin, I can feel a buzz. She doesn't feel the buzz, but she can hear it if I touch her ear.
So I'm guessing it's a faulty laptop, and she's conducting an electrical current.
But why would she not feel anything, and w... | Its due to residual transductance of the live AC current into the shielding of the device. Get an earthed plug and it will disappear. I've experienced the issue in dozens of shielded but unearthed electrical appliances, not just laptops. It feels exactly like a vibration (when you move the finger over the surface) but ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2824",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "24",
"answer_count": 5,
"answer_id": 1
} |
Impedance formula for an edge coupled differential stripline in an asymmetrical stackup? I've been searching and have not been able to find a formula for calculating the impedance of differential lines on inner layers where the dielectric above and below it is not symmetrical. I've seen plenty of examples of symmetrica... | Of course it doesn't matter what voltage the voltage planes are held at so assume both are at ground. Now, take advantage of symmetry: assume that the voltages on the two strips are opposite. Use the same logic as with the presence of a charge near a conducting plane implies the presence (for calculation purposes) of a... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2891",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
Accuracy of the Boltzmann equation I have had this question for some time now. Hopefully someone can answer it.
I know that the Boltzmann equation is widely regarded as a cornerstone of statistical mechanics and many applications have been explored with a linearized version.
I also know that it's extremely hard to obt... | I provided a response to question in the same context here:
Experiments that measure the time a gas takes to reach equilibrium
However, specifically to your question, this is a good quick overview:
http://homepage.univie.ac.at/franz.vesely/sp_english/sp/node7.html
Where you find out that with certain simplifying assump... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/2933",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 4,
"answer_id": 2
} |
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