All Questions
209,658
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Anomalous calculation about stellar collision
Suppose two bodies of mass 'm',one of them is fixed and the another is free to move,they are separated by a distance 'l' initially and the system is at rest.What would be the speed of the free body if ...
2
votes
1
answer
125
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Pauli-Lubanski vector for Maxwell's equation
In the book quantum field theory by Itzykson and Zuber, page 53, the authors prove that Dirac's equation has spin 1/2 by showing that if $\psi$ is a solution to Dirac's equation, then compute that $\...
- 2,508
2
votes
1
answer
121
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Zero mass Kerr metric
When mass in Kerr metric is put to zero we have $$ds^{2}=-dt^{2}+\frac{r^{2}+a^{2}\cos^{2}\theta}{r^{2}+a^{2}}dr^{2}+\left(r^{2}+a^{2}\cos^{2}\theta\right)d\theta^{2}+\left(r^{2}+a^{2}\right)\sin^{2}\...
- 2,508
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22
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First order and second order phase transition in signal recovery
Warning: I'm a computer scientist and my understanding about physics is very limited
Recently I came across the term first-order phase transition in a paper where we try to recover a signal $x$ from $...
- 2,508
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1
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9
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Kinematics constant acceleration problem
A stone is being thrown vertically upward with a speed of 12.5m/s from the edge of a cliff 75.0m high, what’s the total distance did it travel?
It is a constant acceleration kinematics problem, I ...
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6
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Finding the Radial Velocity from the Orbital Velocity
Is it possible to find the radial velocity of a satellite orbiting earth from its oribtal velocity only? if so how? if not what other parameters are needed?
0
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2
answers
25
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Phases of vectors
This maybe because I am missing a whole concept itself, but how can vectors have a phase?
I have been studying forced oscillations and I read that when multiplying a vector with i (sqrt(-1)) , the ...
- 5,210
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answers
7
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State excitation table using T flipflop of a sequence detector circuit which gives an output when it detects the sequence 0101? [closed]
State excitation table using T flipflop of a sequence detector circuit which gives an output when it detects the sequence 0101? Overlapping sequence are accepted
- 172k
1
vote
1
answer
32
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What does divergence of scalar times vector vector field physically mean?
We know that:
$\nabla \cdot (f \vec{A}) = f \nabla \cdot \vec{A} + \vec{A}\cdot(\nabla f)$
Now divergence of any vector field can be understood in terms of whether the concerning flux is outgoing ($\...
- 172k
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votes
1
answer
170
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Thermal diffusion equation with heat per unit length?
Let us say we have a metal rod. Along that rod there is a rate of heat generation of H per unit length. If we assume we are in the steady state then I would expect us (from the thermal diffusion ...
CommunityBot
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12
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What is the dependence of fusion power of the particle energy at fixed pressure?
The fusion reaction power depends on the reaction rate. The reaction rate depends on the concentration and the cross section.The cross section depends on the energy of the particles or temperature. ...
- 172k
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1
answer
33
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Electronics transitions of a molecule
I have some doubts on electronics transitions of molecules in Franck-Condon principle:
1. The principle states that if a molecule absorbs a photon of the appropriate energy, we could have both ...
- 1,776
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0
answers
8
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Young's modulus for a string that is being applied by two unequal forces at the both ends
In space for example if we apply two forces 10,6 respectively at the end of an elastic string (10 at the right end and 6 at the other) then obviously that body will expand and accelerate. But how ...
2
votes
2
answers
53
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Phase change boundary near Lifshitz Point
I am following the text of Chaikin and Lubensky.
They breifly discuss the results of the following equations but I find myself confused by some of the details:
$$
F = \frac12 \int \text{d} \vec{x} (r\...
- 285
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5
answers
911
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If rest mass does not change with $v$ then why is infinite energy required to accelerate an object to the speed of light?
I know that as the velocity increases, the mass of the object also increases so it becomes tougher and tougher to move the object which ultimately leads to a requirement of infinite energy to ...
- 1,379
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3
answers
37
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Why is the complex time not considered in the solution?
In the solution, why is the the complex time not considered and real and positive time is considered? Is it because complex time does not exist or there is some other reason?
- 26.7k
2
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1
answer
174
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Invariance of magnetic moment in slowly-changing magnetic fields (plasma physics)
I am trying to understand how the magnetic moment is invariant in slowly changing magnetic fields. There is a proof in the textbook I am using, but I'm stuck on how $-e\int\frac{\partial B}{\partial t}...
CommunityBot
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0
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14
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If our planet had more gravitational force would rain clouds still form? If so, would they form at the same elevation, higher or lower?
If the acceleration of gravity on earth were greater would rain clouds still form? If so, would they form at a lower, higher or the same elevation? My question actually is related to a curiosity about ...
- 172k
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0
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14
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Rindler's criterion and definition of linearity of the Lorentz and Poincaré transformations
In Wolfgang Rindler's "Introduction to Special Relativity", 2.ed. 1991, on p.11 his derivation of the special Lorentz transformation $ x'=f(x) $ starts with the proposition that this ...
- 189
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7
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Water waves transverse
When waves arise from a point in water, they form circles around the point and it is referred to as a transverse wave. Is this because the x and y components are trigonometric graphs and so a graph of ...
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2
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1
answer
248
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Finding constraints on acceleration if two objects' positions are related non-linearly
I'm taking a a first-quarter mechanics class at a university and we're currently covering Newton's laws of motion. The problems themselves are pretty similar to what I did in high school, but it seems ...
CommunityBot
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vote
0
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22
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How shift the system Hamiltonian change the interaction term?
I'm reading this paper about a model of a qubit coupled to an Ising spin bath. The interaction between the system qubit and the bath is described by the Ising Hamiltonian:
$$H_{I}^{\prime}=\alpha \...
- 101
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answers
20
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What does the term "physical concept" mean?
I've been learning the topic of dimensions for a while now. My book stated that dimensions were used to denote the nature of a physical quantity. I tried to search more about the topic online and came ...
- 172k
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0
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20
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A question involving Torque
I have a few questions which have all seemingly originated from the following thought:
Question 1
It is known that torque is given by τ=r × F. If we use the formulae F=mdv/dt and v=ω × r, we arrive at:...
- 172k
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votes
1
answer
33
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Charge in crossed E and B-fields
This post describes the motion of a charge in crossed electric and magnetic fields as:
$$
x = r \cos ωt + \dfrac EB t\\
y = r \sin ωt\\
$$
Can you please explain how these equations are derived. Why ...
- 172
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1
answer
72
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An object falling from very far would have varying acceleration, how can that be accounted for?
I want to have a function that describes where a falling object is.
Like this one:
h(t) = -g*t²/2
But this one is for the usual close to the surface case, where there is no variation of gravity due to ...
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4
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0
answers
48
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Vorticity: what is the difference between $\nabla \times f$ and $\nabla \wedge f$?
I have always thought that we may obtain information about the vorticity of a vector field $f$ by considering the cross product $\nabla \times f$. In higher level texts on fluid mechanics I see that ...
- 17.7k
1
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1
answer
20
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Independence and ambiguity of holonomic constraints
I've got a couple of questions concerning constraint equations:
Suppose I've got $n$ holonomic constraint equations for a particle, how can I be sure those are all the ones there are and I didn't ...
- 172k
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1
answer
147
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Why is the magnitude of the electric field dependent only on $x$ and $y$ coordinates?
So I am reading a book on electromagnetism and in it they are assuming that the magnitude of the electric field is dependent only on $x$ and $y$ coordinates in a waveguide and I wanted to know why ...
CommunityBot
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0
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13
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Rayleigh's Dissipative Function in Lagrangian Mechanics
What exactly is the derivation of Rayleigh's dissipative function? How does one know what to assume the function to be while dealing with a problem in Lagrangian mechanics? These two questions have ...
- 172k
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votes
1
answer
27
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UV-absorption capability of air
I am looking for an absorption chart for UV light with a wavelength of around 250 nm in air (ambient, no specific preparation). How much of a beam of this light would be absorbed per meter of path ...
- 172k
3
votes
1
answer
4k
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Most rigid materials
Ordinary web searches are not turning up lists of the most rigid materials for me.
I am interesting in finding out the relative rigidity of commonly available materials and how to measure that ...
- 75k
4
votes
0
answers
43
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Reference for list of the non-zero, unique components of the Riemann tensor for the Kerr metric
Does there exist textbook or webpage which has a list of the unique components of $R^{\mu}_{\nu \alpha \beta}$ for the Kerr metric?
Such a reference is easy enough to find for Schwarzchild (including ...
- 172k
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0
answers
17
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Reference for anomalies
I'm reading the preface to the Dover edition of Itzykson and Zuber's "Quantum Field Theory". On page xix, Zuber says
"... Perhaps the most dramatic obsolescence in the book is the ...
- 172k
2
votes
1
answer
4k
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Why do my glasses make the ground look tilted?
I just started wearing glasses to correct my myopia today. I have -0.25 spherical and cylindrical power in my right eye and -0.5 spherical and -0.25 cylindrical in the left eye. Everytime I look at ...
- 75k
3
votes
1
answer
515
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Momentum conservation in FRW spacetime
The spatially flat FRW metric in Cartesian co-ordinates is given by:
$$ds^2 = -dt^2 + a^2(t)(dx^2 + dy^2 + dz^2)$$
As I understand it, since the metric does not depend on the spatial co-moving co-...
CommunityBot
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1
answer
32
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Electromagnetic tensor in a FRW metric
In some papers [like https://arxiv.org/pdf/2204.06883.pdf, eq. (31) ], I see that the Electromagnetic tensor field, for a FRW metric (written in a conformal way)
\begin{equation}
ds^{2} = a^{2}(\tau) \...
- 12.9k
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0
answers
35
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Understanding where equation $2.3$ comes from?
I'm trying to understand this paper (equation $2.3$ specifically):
Bini, D., Carini, P., & Jantzen, R. T. ($1995$). Relative observer kinematics in general relativity. Classical and Quantum ...
- 1,063
1
vote
1
answer
196
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How creation operator pops out while expanding field operator?
While doing QFT when we try to canonically quantize the Klein Gordon equation $\Box \phi =0$ we promote the $\phi $ to an operator field and impose the commutation rule $[\phi(x,t),\pi (y,t)]=i\hbar\...
- 115
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0
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12
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Speed of light problem [duplicate]
This question totally hypothetical and I'm aware we can't travel at speed of light but IF ....
If you were to travel at the speed of light according to Einsteins theory of relativity you would gain ...
- 172k
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0
answers
15
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If liquids are often assumed to be incompressible (i.e constant density), how is buoyancy explained?
My understanding of buoyancy is that it is caused by a difference in the density of the fluid at the bottom of an object vs the top. however, if a fluid was assumed incompressible, then there would be ...
- 1
19
votes
4
answers
3k
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Schrödinger wavefunctional quantum-field eigenstates
The reason that I have this problem is that I'm trying to solve problem
14.4 of Schwartz's QFT book, which've confused me for a long time.
The problem is to construct the eigenstates of a quantum ...
- 12.3k
1
vote
2
answers
29
views
Does equations of motion work for distance and speed?
In some books,when solving to find the distance and speed of the object having motion in straight line,the three equation of motion are used.so my question is whether these equation of motion (i.e $v=...
- 7,720
5
votes
1
answer
940
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Intrinsic parity
When we apply parity on a field two times, we demand that we should get back the same field. This gives us, $P^{2} =1$, which implies, $ P \psi = e^{i \theta} \psi$ . This extra phase factor is ...
CommunityBot
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1
vote
0
answers
12
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Octopus Ride how much +G force in upward Jerk?
This is a version of the Octopus Ride here:
https://www.youtube.com/watch?v=8flmhIAKTvQ
They call it Hip Hop but really it is what you call it in other places, I felt my left Butt Cheek or Thigh cramp ...
- 172k
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votes
0
answers
7
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Does the direction of angular momentum of a table fan different for two people standing behind the fan and in front of the fan?
Does angular momentum of a table fan depend on place of observation?
What is its direction of angular momentum of a fan when I stand behind a table fan? In front of a table fan? And along the same ...
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-2
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2
answers
245
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Does a form of energy exist without any medium?
Does the Sun contain fire? Or some pure form of energy?
If space has no air how can fire exist there? It may exist cause a burning piece of magnesium keeps on burning in vacuum but, to start fire, ...
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0
answers
21
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How does this cyclist accelerate without expending energy?
https://youtu.be/2QBcc87yoc8
I came across this cyclist video on YouTube. Cyclist here is seen accelerating without peddling or expending energy into the system. How is it possible? Assuming air ...
- 133
1
vote
1
answer
156
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How can we create suction, without the use of a vacuum?
How is it possible to create suction without the use of a vacuum?
For example: As I understand, positive displacement pumps transport fluid by creating a vacuum at their inlet to draw in a distinct ...
CommunityBot
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0
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22
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What is the information content of a single-particle system?
I'm trying to better understand how information is quantified in the case of a closed system containing a single (fundamental) particle located at position $\vec{r}$ and traveling with a velocity $\...
- 1