# All Questions

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### Virtual work (generalized forces) for rotation with Euler angles

Here is what we know from virtual work: $$\delta W=\sum_{i=1}^N{\vec F_i\cdot\delta\vec r_{i}}$$ Where $N$ is the number of bodies in the system. I am considering a ...
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### Pressure change with a pipe diameter change

I have a question regarding a water line from the street to a house. What happens to the pressure in a water line if it goes from a 1" diameter pipe (at street) and increases to a 1.5" diameter pipe ...
6k views

### Why does a free-falling body experience no force despite accelerating?

Note: For the purposes of my question, when I refer to free fall assume it takes place in a vacuum. From my (admittedly weak) understanding of the equivalence principle, falling in a gravitational ...
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### Area under v-t and a-t graphs

I know that the magnitude ( i.e. considering all area as positive) of area under v-t graph gives distance travelled by the particle in the given time. But I've been told that magnitude of area under ...
89 views

### Travelling near the speed of light, will radar give me sufficient warning of approaching debris?

If I’m travelling in space near the speed of light, I would like to use radar to give me advance warning of approaching hazards. However, will I have enough time to be able to react? From a ...
58 views

### Is Ronald Ace's “solar trap” patent plausible?

This newspaper article and a few others from last year discussed a patent by independent US inventor Ronald Ace. It's about a kind of absorber for solar thermal energy systems, and it's supposed to ...
23 views

### What causes a eutectic point?

What causes the melting point depression known as a eutectic point? The temperature depression of a eutectic point can be calculated from the enthaplies and entropies of fusion of the two ...
14 views

### Counting modes Rayleigh-Jeans

In the derivation of the Rayleigh-Jeans Law, we count the number of EM modes in a square cavity. After calculating the number of allowed modes due to boundary conditions, we multiply it by a factor of ...
186 views

### Boltzmann Distribution of Electrons in Confining Potential

I have a particle simulation wherein many non-interacting electrons are trapped in a electric potential well. I would expect, and therefore I initialize according to this, that the electrons would ...
70k views

### A mirror flips left and right, but not up and down

Why is it that when you look in the mirror left and right directions appear flipped, but not the up and down?
56 views

### What makes us twist in a somersault?

In a backwards straight somersault you can decide whether you twist early or late. Twisting early means, that you induce the twisting movement before you rotatation hits 180° and twisting late means, ...
73 views

### What is high energy physics?

Is high energy physics the same as particle physics? Does research in high-energy physics include things like quantum gravity, string theory and quantum field theory? Is unifying the four ...
36 views

### Producing energy with entanglement

If we entangle two electrons for example and move one of the electron to Mars for instance. Are we able to somehow transfer the kinetic force of Mars from its movement (spinning and orbitting) to ...
2k views

### Is the speed of light in vacuum always the same value?

The escape velocity of different planets and stars vary. If they vary, the velocities of bodies escaping from the respective stars or planets should also vary. Like, if I want a ball to reach 10 ...
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A long cylinder of radius $R$ is made from two different material. Its radius $r<r_0$ $(r_0<R)$ part is a material with superconducting transition temperature $T_1$, and its $r_0<r<R$ ...
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### Question about intensity of EM waves

For electromagnetic wave if it's reflected from a perfect conductor standing wave can be form. I wonder why Poynting vector can be used to describe the intensity of standing EM wave. (see p.19 of ...
23 views

### One Pion Exchange Potential properties for a two-nucleon system

I'm going through my Nuclear Physics book, and has come across a section called "Properties of OPEP for the two-nucleon system". It start out by considering the n-p system in a singlet spin state ...
42 views

### Number density of CMB photons?

When we talk about the present-day number density of photons ($n_\gamma\approx 10^8$ $m^{-3}$) in the universe do we mean the number density of CMB photons? I mean there are other sources of ...
26 views

### What causes this IR emission in UV 365nm LED?

I've just got access to spectrometer and measuring spectrum of everything. But when I took a look at 365nm UV led from chinese UV flashlight I got something really strange - it had some IR at exactly ...
39 views

### What maximum theoretical speed can reach spaceship with EM Drive in space? [on hold]

There are several studies (by Institute of Aerospace Engineering, Technische Universität Dresden and by NASA) that are concluding that the EM Drive produces thrust. So I am wondering: if the EM Drive ...
58 views

### When we ride a bicycle, why do we tend to move left when we turn it right?

When we ride a bicycle, we tend to move our body left if we are taking a right turn and vice versa. Why is it like that?
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### Visible wavelengths of fluorescence of constituents of air at sea level for school demo laser telephone

I'm seeking to find out about flourescence in visible wavelengths (390-700nm) of any of the main constitutents of air ( \$N_2, O_2, CO_2, H_2O, CO, etc. \$, ) that can be excited with a low-cost? ...
257 views

### Virial theorem and the energy in a gas

I clearly am interpreting the Virial Theorem incorrectly, but I don't know how. In dipole gases, the molecules can exhibit five kinetic modes, while they can only experience 2 potential modes. Doesn't ...
1k views

### Calculate speed from accelerometer

I am trying to use my accelerometer on my mobile device (smart watch to be specific) to calculate a persons arm swing speed. The data returned from the accelerometer is in $m/s^2$. Since the ...
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Do free massless particles have a Schwarzschild radius? I'm curious about the mass in the equation for the Schwarzschild radius. I know that you can calculate a Schwarzschild radius for any massive ...
29 views

### Which centripetal force equation should I use for centripetal motion in uniform magnetic field, and why?

So far I learn two equations, doing the same thing: $$F_c = m\frac{v^2}{r} = qvB$$ It leaves me undecided when a question asks "by what factor is the force increased when velocity is double". The ...
23 views

### Calculate constant acceleration for a length of track with start at rest and final velocity known [on hold]

My maths and physics are rusty... I'm a bit ashamed, but I have this problem: Let's have a 1000m linear accelerator (mass driver) propel a payload at constant acceleration, with a final velocity of ...
92 views

### How many of the Standard Model free parameters are mutually independent: (all of them)?

My knowledge of the standard model is very limited so please let me spell out my assumptions first (and please let me know when I have mangled concepts, terminology or I am plainly just out of my ...
216 views

### Free parameters in the Standard Model

From my understanding of the standard model, I understand that there are 19 or 20 free parameters that we need to put in by hand as, and I'm guessing here, there is as yet no theoretical basis for ...
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### Calculating the drag force provided by a laminar flow [duplicate]

If I drop a ball into viscous fluid (e.g. oil), by calculation I know that the fluid flow is laminar. So which equation do I need to calculate the drag force provided by fluid? The drag equation, ...
102 views

### what does “an average over noise” mean in Zwanzig's book

This is a very specific question about Robert Zwanzig's book Nonequilibrium Statistical Mechanics. Specifically, what is he talking about in equation 1.25 on page 10 that he calls "an average over ...
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### What is the largest possible mass for a stable planet made of iron?

Consider a large spherical planet made of pure iron. Think of something similar to Mercury or Earth without its mantle, only much bigger, though those planets have elements other than iron mixed in ...
240 views

### Does tunneling transmission probability depend on the density of states or velocity?

In some quantum text books [1], the tunneling transmission formula depends only on the density of states of 2 regions (DOS) involved in tunneling. ($T(E) = C \times DOS_1(E) \times DOS_2(E)$, where C ...
45 views

### What is the perfect Ice Cube? [on hold]

I have a freezer with -25°C temperature and Soda with +30°C. What is the ice-cube-sized material I can put in the freezer that can bring my glas of soda to the lowest temperature? Are e.g. saltwater ...
18 views

### variation of electrostatic potential on moving radially outwards from the nucleus of an atom

I was wondering how would the electrostatic potential change on moving radially outwards from the nucleus in an atom, considering the effect of the electron clouds around it.
87 views

### I am confused about the idea that the speed of light is independent of the speed of the source of light

Imagine that a mast of a sailboat is 186,000 miles long, and the sailboat is moving to the right at a constant 10 miles per second. If you drop a ball from the top of the mast, the ball will land ...
29 views

### Why does temperature affect the toxicity of aluminium? [on hold]

They say that it is OK to cook (tomato sauce) in aluminium pans, but that it is not advisable to store food in them at room temperature or in the fridge/ freezer. Can someone explain the physics ...
57 views

### Why do cylinders roll easily and not square or rectangular slabs? [duplicate]

Why do cylinders roll easily and not square or rectangular slabs?? Let us say both have same materiel and vertical dimensions? What is the reason for the slab always tends to slide?
432 views

### Semiconductor problem: how much Boron is required to establish a certain amount of charge carrier density

The problem description is as follows: Boron is used to dope 1 kg of germanium (Ge). How much boron (B) is required to establish a charge carrier density of 3.091 x 10^17 / cm^3. One mole of ...
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### Setting up superlattices in VASP based on two POSCARS

Does anyone know how to setup a superlattice in VASP/QE using the atomic simulation environment based on two materials/POSCARS?
87 views

### Why can the probability function for a particle in an infinite square well be larger than 1?

For a particle in a one dimensional infinite potential well of width $L$ the probability function is: $$P_n(x)=\left(\frac{2}{L}\right)\sin^2\left(\frac{n\pi x}{L}\right)$$ for $0\leq x\leq L$. The ...
38 views

### Classical mechanics explanation of Liouville's theorem [on hold]

What is the classical mechanical explanation of Liouville's theorem when density explicitly depends upon time and density implicitly depends upon time?
22 views

### Double groups in Crystallography

I'm currently studying double point groups and their applications in condensed matter physics. Let me start by giving you the definition of the double group that is used in my textbook: Let $G$ be a ...
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### Is it worthwhile letting the Hubble Space Telescope stare at one sky point for a year before it dies?

The Hubble Space Telescope will end it's useful life, probably, around 2018. The question I have concerns it's possible uses after that time. The pictures of the Pillars of Creation shown below were ...
31 views

### Diagonal part of the configuration space of two indistinguishable quantum particles

Why is the configuration space of two indistinguishable particles given by $\frac{M^n-\Delta}{S_n}$? My question is about the $\Delta$. (Notation: $M$ is the configuration space of 1 particle. $M^n$ ...
99 views

### What equation describes virtual photons becoming real photons?

From my understanding, virtual particles are actually "disturbances" between two elementary particle interactions and appear during cross sections of these interactions. What equation(s) describe this ...
35 views

### Charge of $W$-bosons in Feynman diagrams

When looking at Feynman diagrams of particle decays, how would I be able to find out the charges of the $W$-bosons involved in the decay?
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### Explicit Matrix Representation for 54 Higgs rerpesentation for $SO(10)\rightarrow$ Pati-Salam Breaking

Which Higgs field or linear combination of Higgs fields gets a vev if we want to break $$SO(10) \rightarrow SU(4) \times SU(2) \times SU(2) ?$$ Formulated a bit differently, I'm looking for an ...