All Questions
219,609
questions
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Does a point mass have constant mass?
Expanding on the title, I am hoping for a little bit of insight into the philosophy of Newton's mechanics as a physical theory. My question, essentially, is does a point mass have constant mass by ...
- 683
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1
answer
17
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Does an excitation of any quantum field extend forever through space?
For visualization purposes, let’s say I slam my hand into sheet metal and it vibrates. Will that vibration extend beyond the metal sheet into the space surrounding it, and would that energy wave ...
1
vote
0
answers
12
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Is weak hypercharge carried by Z boson?
In the PBS spacetime video about charge (https://m.youtube.com/watch?v=esayi49OAk4&feature=youtu.be), at 10:55 he said that weak hypercharge is carried by the Z boson. Is this accurate?
- 29
-5
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answers
22
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How would power the baby Yoda (Grogu) Pod? [closed]
Lets go wild! Throw in your theories and ideas how you would power such a floating device? Lets have some fun wild theories!
- 1
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0
answers
15
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If two equal and opposite charges are not taken apart why they are considered as neutral .and what is neutral
If two equal and opposite charges are not taken apart why they are considered as neutral and what is neutral, charge or field
-6
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51
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Have I found evidence that a megaparsec is a fundamental distance and not an arbitrary one? [closed]
If I take the expression for the radius excess of the Earth from general relativity and swap the gravitational constant G for the coulomb constant K and use the total electric charge of the protons in ...
- 89
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votes
1
answer
16
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Why the Hubble parameter that is proportional to dark energy is squared in the Friedmann's equation?
I'm studying Alexander Friedmann's equation about the Hubble parameter and, thus, the time dependence of the cosmic scale factor varies as the matter density, ρ, and as the dark energy, Λ as shown in ...
- 69
1
vote
1
answer
25
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Hand-wavy method to ampere's law for a single coil of wire
Based on this question
Ampere's law and Biot-Savart law gives different terms for magnetic field in middle of a current running in a loop
I made the following square Amperian loop:
where I have ...
- 909
2
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0
answers
16
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Period of small amplitude oscillations [closed]
So I am trying to do a question where I have two objects that are rolling without slipping. We have found the potential function for this setup, have found the H, we are given y and we're also give ...
- 21
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0
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4
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Is there an analogous method of finding moment for inelastic bending like in the case of inelastic torsion?
When calculating for Torque in torsion or for Moment in the case of bending, the concept is usually first introduced with the simplification of the assumption that the stresses and strains are in the ...
- 43
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1
answer
20
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Aperture/image circle in a DIY digital microscope?
I have just taken apart a broken USB microscope, and recognized that there is actually no magic going on inside.
So I thought how about building one of my own. The idea is to buy a regular (possibly ...
- 7,230
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answers
39
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Can FTL be done in a way that it dosen't break causality? [closed]
So I was wondering if we could travel faster than the speed of light without creating time paradoxes?could we achieve FTL in a certain way that it dosen't break causality?or does FTL always break ...
1
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0
answers
34
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What are non-propagating degrees of freedom? [closed]
I have read at different places that in 3 spacetime dimensions, there are NO propagating gravitational degrees of freedom. This seems to imply that we have only "non-propagating" degrees of ...
- 229
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votes
1
answer
32
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Doubt in expansion of integral
While solving for the chemical potential of a fermi gas at $T\ne 0$, we first take
$$n = \int_0^{\mu}g(\epsilon)d\epsilon+(\pi^2/6)(k_BT^2)g'(\mu)+O(T^4)$$
Which is obtained by the Sommerfeld ...
- 193
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answers
14
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The problem of the density of the induced charge on a dielectric cylinder. Why is my solution wrong? [closed]
Why is my solution wrong?
Task:
The dielectric cylinder is in an external uniform electric field E, which is parallel to the axis of the cylinder. Find the surface density of the charges induced on ...
- 1
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votes
1
answer
24
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When two neutron stars collide to form a black hole, for how long will last the emitted signal from the very vicinity of the new formed event horizon?
When two neutron stars collide they may form a black hole which is not a supermassive black hole but it should provoke that outside its newly formed event horizon photons are emitted as a consequence ...
- 2,173
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votes
2
answers
46
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Energy conservation in a driven harmonic oscillator
The ODE for a driven harmonic oscillator is given by
$$
\ddot{x}+2\gamma \dot{x}+\omega_0^2 x = \frac{F}{m}\cos(\omega_dt)
$$
By assuming balance of forces, i.e. energy conservation, one can solve for ...
- 1
1
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0
answers
32
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What is the boundary from N to S called in magnetism? [closed]
I am doing some research on asymmetric magnetic fields. I have simplified my problem to an isosceles triangle mangnetized with the N side on the tip with the smallest angle and the S side opposite of ...
-1
votes
0
answers
37
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Is the free Hamiltonian unique (as defined in Weinberg vol 1 chap 3)? [closed]
In chapter 3 of Weinberg, he defines asymptotic states by first splitting up the full interacting hamiltonian as
$$
H=H_0+V
$$
where $H_0$ is the free Hamiltonian. He makes a point of saying that
...
- 1,775
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votes
2
answers
19
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Will the ideal gas equation change if we consider non equillibrium condition and then use kinetic approach and thermodynamic approach?
My teacher said that in equilibrium no matter what approach we use we arrive to same result i.e. in this case ideal gas equation. So if we were to use the two approaches (thermodynamics and kinetic ...
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answers
34
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Why doesn't the sun appear to move backwards when we move forward? [duplicate]
I have observed that while travelling (in a train, for example), as the train moves forward, all the objects in the surrounding environment seem to move backwards (like the trees). However, if we ...
- 446
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answers
17
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The combined gas law and the ideal gas law
This might sound a bit weird, but I do not understand what I am missing!
We have the combined gas law:
$$PV/T$$
And the ideal gas law:
$$PV/nT$$
My first question is that why didn't French physicist ...
1
vote
2
answers
40
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Calculating change in Momentum
A 100g ball with a speed of 5m/s hits a wall at an angle of 45 degrees. The ball then bounces off the wall at a speed of 5m/s at an angle of 45 degrees. What is the change in the momentum of the ball?
...
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38
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Quantum physics book good explanation with minimal maths [duplicate]
Kindly suggest quantum physics book good explanation with minimal maths. I read once beginning of such a book with good explanation and minimum maths but am forgetting book name. Maximum suggestions ...
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0
answers
25
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How to create a more accurate gravity simulation? [duplicate]
Goal:
I am interested creating a simple gravity simulation and was wondering if it was possible to describe two objects motions through time with percision. All other parameters are known (...
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0
answers
21
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Solution to Laplace Equation [closed]
Consider two dimensional Laplace equation in cylindrical coordinates
$$\frac{\partial^2 \phi}{\partial^2 r} + \frac{1}{r}\frac{\partial \phi}{\partial r} + \frac{1}{r^2}\frac{\partial^2 \phi}{\partial^...
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1
answer
27
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Units of the derivative of an exponential function with respect to a quantity with units in the argument? [closed]
I am trying to understand how to track the "units" (or dimensions) of the following derivative, which seems to depend on the degree to which the expression for the derivative is simplified ...
- 111
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votes
1
answer
38
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Does Relativity Theory Allow Cyclic Time Travel? [closed]
In the H.G. Wells novel "The Time Machine" and original 1960 movie we saw that the protagonist traveled from his present time to the future and back again to the present just within a few ...
- 3,390
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1
answer
46
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What does it mean to 'combine' spin 1/2 particles
I'm trying to understand a phenomenon covered in several resources that I think I'm struggling with due to my lack of understanding of its meaning.
The idea is that 2 spin 1/2 particles can combine to ...
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1
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62
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How can I relate group theory with 2 spin-1/2 electrons coupling?
I've learned some basic ideas of group theory like the basis for a group representation.
In 2 spin-1/2 electrons coupling case,
it is said $$2\otimes2=1\oplus3.$$
The direct product can be reduced to ...
- 3
-2
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0
answers
48
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Is Schrodinger equation postulate in quantum mechanics? [duplicate]
In some books (Shankar, Griffiths) Schrodinger equations is treated as postulate of QM but in books like Weinberg, Sakurai they have not used Schrodinger equation as postulate.
Can some one explain ...
1
vote
1
answer
56
views
QFT symmetry breaking and CP violation
Since the weak interaction violates CP symmetry but electromagnetism doesnt,does it mean that the electroweak fields also violates CP symmetry?Can a field which violates some symmetry be the product ...
- 120
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answers
23
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$R_\xi$ gauge and degrees of freedom counting
In the standard classical Maxwell theory, we use the following arguments to claim that there are only two propagating degrees of freedom
$A_\mu$ has 4 components
$A_0$ is non-dynamical (-1)
$\...
- 659
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1
answer
63
views
How is a black hole compatible with quantum theory? [duplicate]
A black hole has a radius of $R = \frac {2Gm}{c^2}$, in this context, if we take a single proton and neutron as a black hole, its Schwarzschild radius will be near about $4.8 \times 10^{-52} \mathrm{m}...
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3
votes
3
answers
460
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Do black holes have a size?
I'm wondering if one can say that a black hole is an object "made of matter" that has a size (as a size, I'm not talking about the size of the event horizon).
I would like to know if one can ...
- 131
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votes
6
answers
654
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Is Earth really flattened at the poles because of centrifugal force?
My question is pretty much all in the title. I was always told that our planet is flattened at its poles due to the centrifugal force generated by its own rotation. However I don’t see how centrifugal ...
- 227
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vote
2
answers
57
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Integrating both sides of an equation w.r.t. two different variables ($dz$ and $dv$)
so I'm trying to derive an equation for the maximum height an object reaches after being vertically thrown upwards, where the two forces acting on it are gravity and quadratic air resistance, so that
$...
- 11
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0
answers
34
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Stress tensor with indices in two-dimensions
I am interested in the stress tensor of a CFT_2 living on a conformally flat background, say,
\begin{equation}
ds^2 = \frac{1}{x^2}(-dt^2 + dx^2).
\end{equation}
What if I want to know the stress ...
- 63
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votes
1
answer
34
views
Breaking product of three vectors into symmetric and anti-symmetric vectors
Let's consider we have three arbitrary vectors A, B and C. We have the quantity $A_{\mu}B_{\nu}C_{\rho}$. Is it possible to break the above quantity into sum of symmetric and anti-symmetric vectors in ...
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0
answers
20
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Apparent weight of body due to rotation of earth [duplicate]
This question has been asked several times on this site. However, most of the answers were
qualitative in nature.
Suppose a body is placed at a point on earth's surface at latitude $\theta$ from the ...
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vote
2
answers
35
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Why must we follow the formula of potential $V$ if we want to calculate the charge,instead of following the formula of electric field [closed]
There are two spherical conductor A and B,and their radius are $r_a$ and $r_b $,and their charge are $q_1$ and $q_2$,and $q_1+q_2=Q$.
Now they are connected with each other with a wire, assume the ...
- 23
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1
answer
29
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Radiation back reaction in classical electrodynamics with point and extended sources [closed]
I've been doing some research on the topic of radiation reaction force/self force in classical electrodynamics and although there are some discussions on the internet I would like direct answers to ...
- 1,301
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votes
1
answer
50
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Fourier Transformation if we only have relative coordinates
Let’s say we want to do a Fourier Transformation (FT) of a function $f(t-t‘,r-r‘)$ i.e. the function to be Fourier transformed only depends on relative coordinates. This is for example the case if we ...
- 377
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1
answer
39
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Numerical model of 1D equations for compressible flow
The equations for 1D compressible flow are
$$\frac{\partial u(x,t)}{\partial t}-A\frac{\partial v(x,t)}{\partial x}=0,$$
$$\frac{\partial v(x,t)}{\partial t}-B\frac{\partial u(x,t)}{\partial x}=0,$$
...
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2
votes
1
answer
28
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Tensionless string in Nambu-Goto action
I am studying string theory from the book "String theory and M-theory", written by Becker, Becker and Schwartz. My question is:
We are taught that one of the advantages of introducing a ...
- 2,815
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1
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48
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Where does it's gravitational force come from to maintain its constant somewhat spherical shape? [duplicate]
How dose a Tidally locked gas giant have a balanced gravitational force to keep it's spherical shape when one side is constantly facing the sun? Where does it's gravitational force come from to ...
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answers
18
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Proper acceleration along perpendicular directions
Let's take an object (all times mentioned in this problem are proper times). There is a constant proper acceleration
"$a$" on this object in the x-direction for time $t, 0\leq t \leq \tau$. ...
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0
answers
20
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How does the increased binding energy per nucleon help in stability?
I have some read answers on this forum, some books like Halliday Resnick and my course books. I have understood that binding energy is the energy we need to supply to break nucleus into individual ...
- 1
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0
answers
24
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One homework about berry phase [closed]
I have a homework problem that I don't even know where to start. Could anyone give me at least some hint to start? I can now use the ansatz given in the problem, and then write $|\Phi(T)\rangle=e^{i\...
- 11
2
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1
answer
42
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Nambu-Goto action and the World-Sheet Area
I am studying string theory from the book "String theory and M-theory", written by Becker, Becker and Schwartz. My question is:
We are told that the Nambu-Goto action is simply the one that ...
- 2,815