# All Questions

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### What is the equation of the ball? [on hold]

I'm a student in literature and I'm curious about physics. I have a ball who leaves at a speed of $10 \mathrm{km/h}$ , at an angle of $30^\circ$ according to the ground. Ball weight$=2\mathrm{kg}$ ...
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### Force on a test charge placed at the center of a 13-sided regular polygon?

Assume a test charge with charge $Q$ is placed at the center of a 13-sided regular polygon, the vertices of which are charges with equal charge $q$. What is the electrical force on the center charge? ...
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### Why doesn't diamond glow when hot?

In an answer to this SE question, the respondent explains that heating a perfect diamond will not cause it to glow with thermal blackbody radiation. I don't quite follow his explanation. I think it ...
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### What is the theoretical wattage output of a Tokamak fusion reactor?

By that I mean the complete radiative wattage of any type of energetic radioactivity or electromagnetic wave or even particle if that's what they output. My purpose is to compare this to the suns ...
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### Wave equation in the case of non-monochromatic light

monochromatic light refers to electromagnetic radiation of a single frequency, $\omega$ The wave equation in this case turns into the well known helmholtz equation.. How about for non-monochromatic ...
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### Definition of Ampere

On Wikipedia it says: This force is used in the formal definition of the ampere, which states that it is "the constant current that will produce an attractive force of $2 × 10–7$ newton per metre ...
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### Have I calculated the IPU mass right [on hold]

I have a question. Okay I am trying to calculate what the brown dwarf threshold should be given that 500 IPU = .75 solar masses. I divided .75 by 500 to get the solar masses per unit of mass in the ...
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### Gamma Matrices in Dimensional Regularization

Prove that $tr\left(\gamma_\mu\gamma_\nu\gamma_\rho\gamma_\sigma\gamma_5\right)=0$ when the spacetime dimension is not 4. What I have tried: We know that $\gamma_\alpha\gamma^\alpha=d\mathbb{1}$, so ...
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Faraday's Law seems more like an observation than an explanation. Sure, a changing magnetic current causes emf, but why? How does a changing magnetic field cause electrons to move in the direction of ...
7 views

### Why is radiation for an ultrarelativistic charge zero on axis?

I attribute it to the fact that for an ultrarelativistic charge the field is contracted and essentially there are only fields in the transverse direction and nothing longitudinally (wrt the charges ...
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### Where I could find values for rare earth metalls levels diagramm (Dieke diagram)?

The Dieke diagrams like this: could be easily found in internet. But it is impossible to determine from where they get values initially, everyone is making cross-links. I'm especially interested ...
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### Are there any systems in physics which can only be formulated as an integral equation?

My question is are there any systems in physics that can only be formulated as an integral equation? Or do all integral equations have an equivalent differential equation?
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### Physical difference between the derivative of the Hubble parameter and the second derivative of the scale factor?

In Carroll's GR book, he discusses the difference between $\dot H$ and $\ddot a$, stating that they are the answers to two different questions, pg. 349. He seems to imply that if one set up two ...
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### Where does 1/Gamma characteristic angle come from in EM Radiation?

Very curious as to where this angle comes from? It describes the peak of radiation for almost all radiation regimes, but I am having a difficult time seeing where it comes from. Also, the physical ...
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### Solving actual weight lifted

I was wondering if anyone knew the formula to calculate how much force is required to lift a tire or any object by the edge. For example if a tire weighs 500 lbs (or has a mass of 226.8 kg). I would ...
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### Analytic solution for propagation of an planar acoustic wave through weakly heterogeneous halfspace over time

I am attempting to validate an acoustic model. The simulation has a planar, sinusoidal, acoustic wave propagating from one fluid to another (weakly heterogeneous fluid halfspace). As part of this ...
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### Rotation of angular momentum eigenfunctions?

I am struggling to understand this apparently obvious example in my group theory notes: Where do the $e^{i\phi}$ and $e^{-i\phi}$ factors come from? I know that the $m_l$ = -1,0 and +1 angular ...
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### Addition according to significant digits

I have been studying about significant digits and addition rules and I can't quite digest the rules of addition completely. It states that in the answer number of decimal places will be equal to the ...
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### Significant figures vs. absolute error

On NIST Avogadro's Number, $N_A = 6.022\;141\;29 \times 10^{23} \text{ mol}^{-1}$ has 9 significant figures and a standard uncertainty of $0.000\;000\;27 \times 10^{23} \text{ mol}^{-1}$. First, can ...
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### Angular acceleration and revolutions [on hold]

The angular speed of a motor wheel is increased from 1200 rpm to 3120 rpm in 16 seconds. (i) What is its angular acceleration, assuming acceleration as uniform? (ii) How many ...
18 views

### Differentiating between mass number (A) and activity (A) in a nomenclature/glossary

I'm not sure if this is the best place to ask this question, but as it's related to the terminology of nuclear physics I thought it would probably be a logical place to start. I'm currently writing ...
65 views

### Speed of gravity in cosmological codes and ephemeris generation

There are few questions in Phys.SE concerning the speed of gravity, and the answers are traditionally that the speed of gravity equals to the speed of light. But in that case I have three more ...
69 views

### The best way to cool the teapot

My professor today in the class made us a question: "Lets say we have a teapot with water in it.The water is hot.Now we want to cool the water. Will it cool faster if we put an ice cube above the ...
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### Why does a remote car key work when held to your head/body?

I was trying to unlock my car, but I was out of range. A friend of mine said that I have to hold the transmitter next to my head. It worked, so I tried the following later that day: Walked away from ...
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### How to calculate the general moment formula? [migrated]

I am doing the calculation: $$F=\int_Sx^{i-1}y^{j-1}z^{k-1}ds$$ where $S$ is a polygon in $\mathbb{R}^3$ (every point are in the same plane), and $I,J,K$ are integers. Could anyone please help me to ...
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### Problem evaluating $C^{-1}M^\dagger C$

How can I show the following? $$\overline{\psi_L}M^\dagger (\psi_L)^c=\overline{\psi_L}CM^\dagger\overline{\psi_L}^T$$ where $\psi^c=C\overline{\psi}^T$ and ...
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### Lagrangian approach to spinning thread reel

I am trying to better understand Lagrangian dynamics and am struggling to complete the following question: A reel of thread of mass $m$ and radius $r$ is allowed to unwind under gravity, the upper ...
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### Optics - Faraday Rotator using waveplates

I'm trying to replicate the effect of a 45° Faraday rotator using a series of waveplates instead. I've encountered some difficulties using the jones matrix notation, the main book I'm using is: ...
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I am reading Wen's book, QFT of many-body systems ( @Xiao-Gang Wen ). I am a little confused about the orthogonal catastrophe introduced in Chap.5. Below Eq.(5.1.6), it is stated that the influence ...
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### Path Integral back in time

For the non relativistic path integral we have to consider all contributions of all paths that connect two space-time coordinates, form $(\mathbf{x}_0,t_0)$ to $(\mathbf{x}_1,t_1)$. Are there also the ...
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### Why does sound travel faster in iron than mercury even though mercury has a higher density?

The speed of sound depends on the density of the medium in which it is travelling and increases when the density increases. For example, in solids sound travels faster than in liquid and even more ...
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### Horizontal force on elevator?

I would like to ask the following: If an observer travels up or down in an elevator on earth, does he experience a horizontal force due to earth's rotation?
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### How to make the pod?How a positively charged pod can be created? [on hold]

It is said that to control/stop the vehicles of criminal on highways statically positively charged pod can stop the vehicles firing. So as a curiosity I would like to know what is pod? How it is ...
57 views

### Would there be fringe pattern in this arrangement?

Figure shows a standard two slit arrangement with slits S1, S2. P1, P2 are the two minima points on either side of P. At P2 on the screen, there is a hole and behind P2 is a second 2- slit arrangement ...
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### Does the photon-phonon interaction always rotates the photon polarization of 90°?

I'm reading about the acousto-optic effect and on the Acousto-Optical Tunable Filters on particular and wanted to understand the physics under its working. I found this paper ...
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### multiple frequency trapped power signal

is there any possibility to generate a waveform that consists of multiple power signals with different frequencies such that these signals travel together like they constitute [to form a ...
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### Does putting color filters make the sources incoherent?

In Young's double slit experiment a single source is used to illuminate two slits which then acts as two coherent sources to produce interference pattern. But, what if I put color filters on the two ...
38 views

### Capacitance of Parallel Plates

For Parallel Plate Capacitors, I was wondering why, in the derivation, they set charge to $Q$, when there is a net charge of zero? I know it wouldn't be very helpful to use zero, but if the definition ...
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### What are circles on broth (eating soup) surface?

Think about broth in the soup, usually it has circles on its surface. What are their properties? Why there are many of them (not a few big blobs)? Are they depended on liquid's temperature? What needs ...
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### What do I get when measuring on top of metal/insulator/metal by kelvin probe?

I am using a macroscopic Kelvin Probe for work function measurement. There's one confusing condition. If I measure a trilayer thin film stack with a structure of ITO(bottom)/PMMA(100-200nm)/Al(top), ...
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### efficiency of electric cars vs gas? [on hold]

I have been listening to my friends brag about electric cars and I can't seem to convince them that electricity is not some magic force that comes out of the wall outlet. It must be produced ...
For my system I can write down the Hamiltonian in this form:  H = \begin{pmatrix} \epsilon_{1\downarrow}-\mu_{B}B & 0 & 0 & 0 \\ 0 & \epsilon_{2\uparrow}+\mu_{B}B & 0 & 0 \\ ...