# All Questions

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### Interesting approach to Kepler problem

I won't go into the explanation of this idea, because it is explained in this blog post. In this paper, which was featured on John Baez's blog, $\frac {dt}{d\lambda}$ is given as $\frac r V$, where ...
218 views

### Why is the orbital angular momentum of a pi electron along the axis of two atoms' molecule one?

I'm reading quantum chemistry. The book says that the orbital angular momentum of a $\pi$ electron along the symmetry axis of a molecule made up of two atoms is $\pm 1$. I think this is a primary ...
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### Units of eigenvectors

Consider for example a mass matrix $M$, $\lambda$ one eigenvalue and $X$ a corresponding eigenvector. Then $[M]=\text{mass}$ (the brackets indicate the "unit operator"), and $MX=\lambda X$ so ...
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### Lorentz-invariance of step function

I was reading about the Lorentz invariant integration measure $\int \frac{d^3k}{2E_K}$, and ways to prove that this was Lorentz invariant. Many of the proofs I have read use the step function (or ...
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### Induction Oscillation

A bar (m=100g, l=10cm) is moving in a constant magnetic field $B$ as shown above. The resistor $R$ is $1\Omega$. The bar is attached to a spring with spring constant $10N/m$, can you derive ...
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### A sketch of various combinations of numbers of space and time dimensions

I came across this picture/sketch on the internet, however there is no any explanation about it: What is "UNPREDICTIBLE"? "UNSTABLE"? "TOO SIMPLE"? "elliptic", "ultrahyperbolic"?? Some related ...
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### What determines the forward voltage drop for a diode?

I have always had the idea that the forward voltage drop in a semiconductor diode was related in a simple way to the bandgap energies in the semiconductor. However this is apparently not the case: ...
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+50

### Do different plasmas from different elements have different properties?

If so, what are some differences? Like between iron and gold? EDIT: Sorry, I need to clarify: By 'difference' I mean... do they retain their chemical properties from more normal temperatures? Like is ...
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### Hydrogen-boron fusion [on hold]

aRecently I found out that Hydrogen-1 and Boron-11 together are considered a viable nuclear fuel. Fusion of Hydrogen-1 and Boron-11 produces three highly energetic Helium-4 nuclei. Basically the ...
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### Relation between black hole mass and size applied to observable universe [duplicate]

If I apply the relations given in http://xaonon.dyndns.org/hawking/hrcalc.js and http://www0.arch.cuhk.edu.hk/~hall/ag/sw/SpinCalc/SpinCalc.htm to the values for the mass of the observable universe ...
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+100

### Analogies between Fraunhofer diffraction and Josephson junctions

There are several analogies between diffraction patterns and Josephson junctions, especially between a double slit experiment and two Josephson junctions in a superconducting ring (like this): Both ...
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### Why the non-analyticity of free energy function implies phase transition? And what's its connection with other 'higher level' free energies?

I have seen 'free energy' arising from several contexts in very different forms, and each contains different amount of information. For example free energy is defined as the logarithm of the ...
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### Timelike curves in Special Relativity

I have a question that probably might sound silly to most of you. We know that a natural Lorentz-invariant parametrization of a timelike curve is provided by: $$\tau$$ the Lorentz-invariant proper ...
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### space contraction near lightspeed

I read a lot of the Time-Life Science books when I was a kid (well, mostly the pictures and captions). One of the things that always stuck in my mind was an explanation of the twin paradox, using ...
104 views

### Lattice geometry and dispersion relation

Is there a general theorem which gives some information about which influence have the lattice geometry (for example sub-lattice structure, square lattice, honeycomb lattice, lattice symmetries, ...) ...
11 views

### Propagation of incertitude - specific example

I am setting up an experiment, where I know the force and the incertitude (delta) of that force. However, I need to graph the logarithm of that force with the incertitude of the log. Can someone help ...
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### Why do rings form on microwaved CD/DVDs?

Here's a microwaved CD. As you can see, the disc has been destroyed, but there are very clear circular rings bridged with lines. Why are rings formed when microwaving discs?
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### Why do electrons, according to my textbook, exist forever?

Does that mean that electrons are infinitely stable? The neutrinos of the three leptons are also listed as having a mean lifespan of infinity.
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### Polarization vector of dielectric electrostatics

I know two laws for Polarization vector of a dielectric material $$\vec P = N q \vec{\Delta L}$$ Where $N$ is the number of dipoles per meter cube, $\vec{\Delta L}$ is the vector that represents the ...
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### Is the planet Mesklin as described in Hal Clement's Mission of Gravity correct?

The book Mission of Gravity describes a planet with a mass of 16 times Jupiter spinning at a rate of one revolution every 17.75 minutes. This causes it to stretch out into an oblate spheroid and has ...
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### Why a timelike geodesic maximizes path length?

I'm studying some GR and my book says that in Pseudo-Riemannian manifolds geodesics may even maximize the path locally. That's what happen to the timelike geodesics, for example. My first question: Is ...
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### Why the direction of dipole moment is from negative charge to positive charge?

An electric dipole moment is defined as $p = q\times 2d$. How to understand it physically? Why the direction of the electric dipole moment is from negative charge to positive charge?
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### Is there any disadvantage to sending rockets straight up?

When we send satellite into space using a rocket, logically we consider the shortest path which is a straight line perpendicular to Earth's surface. My question is do rocket takes the shortest path to ...
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### Why is the rotation not zero and the divergence zero in the figures below?

Why is the rotation not zero and the divergence zero in figure 1 and figure 2 below? Figure 1 Figure 2
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### Why do we need complex representations in Grand Unified Theories?

In almost every document about GUTs it is claimed that we need complex representations (=chiral representations) in order to be able to reproduce the standard model. Unfortunately almost everyone ...
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### Are neutrinos weekly interacting particles? [on hold]

Occasionally, I read about weekly interacting neutrinos, and although I know this is a typo, I wonder if it happens to be true. Do average neutrinos, as they fly through the universe, really interact ...
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### How do you calculate the angular momentum of a dumbbell after a collision? [on hold]

In outer space, far from other objects, a small block of mass $m$ traveling at speed $v_1$ approaches an object initially at rest that consists of two small spheres each of mass $M$ connected by a ...
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### Geophysics: Dynamic form factor and Equipotential surfaces [on hold]

I've got the following problem which I need help with. I'm used to calculating coefficients when the problem is about ellipsoids and first order approximations. But when it comes to spheres and ...
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### Net force acting on the block along the inclined path can be mgcos(theta)? [on hold]

If we take the angled marked with red curve then the net force along the x axis or inclined path comes out mgcos(theta) is is correct?
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### Relationship of the volume of a plumbing system to pressure

How does the volume of water a plumbing system holds vary with water pressure? I know the ratio would include the modulus of elasticity of the plumbing material, the total surface area of the plumbing ...
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### Prove $e^Âe^{-Â}=Ê$

I want to prove that the Operatorexponential times the Inverse Operatorinverseexponential equals the Normoperator. I dont really know how to advance here, as I am still really new to the whole ...
21 views

### Weightless person in a cart on a rail

In a problem I'm trying to solve I've having a conflict between my understanding and the proposed solution of the problem. A cart ($m=35kg$) is at rest in the start of a friction-less ("covered with ...
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### Moment of Inertia Calculation [on hold]

A uniform disc has centre O, radius a and mass 2m. It is free to rotate in a vertical plane about a horizontal axis through O. A particle P of mass m is placed on the highest point of the rough edge ...
16 views

### How to derive velocity pressure in English units?

I know that velocity pressure can be calculated from dynamic pressure according to the potential energy of pressure (from Bernoulli's equation): $$P = \frac{1}{2} \rho \overline{V}^2$$ where P = ...
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### Relative wind velocity explanation - understanding Irodov problem 1.6

I am having trouble understanding the reasoning behind the solution in this Irodov General Physics problem. The problem is 1.6: 1.6. A ship moves along the equator to the east with velocity vo = ...
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### Dropping a weight onto a spring scale

Say I drop a 5kg weight from a height of 1 meters onto a spring scale like many people have in their bathrooms. On impact the scale will show a higher weight than 5kg. Question: Which quantities ...
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### A question about genus one string amplitude

In BCOV's paper http://arxiv.org/abs/hep-th/9309140 the genus one string amplitude of a Calabi-Yau 3-fold was explained in the B-model as the Ray-Singer torsion (there is a similar discussion in the ...
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### Why is the heat transferring best from within the tube within a tube-heat exchanger

If you've got gas and cooler fluid, you'd run the gas through the tubes, because the gas has a lower h-value. Inside a tube heat-exchanger it is said that the best heat transfer is from within the ...
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+100

### Where does the smell of electrostatic charge come from?

Everybody knows you can produce electrostatic charge rubbing two different materials together. But have you ever smelt e.g. at the plastic after charging it? There actually is a distinct ...
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### Point charges in a sphere

If we put N point charges (with charge +q) in a sphere with radius R and the charges can move freely in the sphere but can't get out of it, where will the charges go to and what will be the ...
41 views

### Electrostatic field in a vacuum

Can an electrostatic field, the direction of which is constant in space, but the magnitude varies in space, exist in a vacuum?
26 views

### Can this RC boat work in thoery? [duplicate]

We're a bunch of friends building an RC boat, this may sound stupid but I had an argument with a friend where he insisted that his design will work. Basically, he's saying that by putting a ...
26 views

### What mechanism creates bubbles in water that stays awhile

If I fill glass jar with tap water and let it stay, small bubbles will appear on the side of jar after a while. I am wondering what processes create those bubbles and what conditions require to ...
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### Determining straining force and reaction forces on a beam

A homogenous beam $OA$ with the length $4a$ and the mass $m$ can rotate in the vertical plane along a horizontal axis through $O$ and is kept in equilibrium by a wire running from the points A and C ...
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### Amplitude $\langle0|e^{-iHT}|0\rangle$ in A. Zee's QFT In A Nutshell

In his Quantum Field Theory In a Nutshell, in page 12, (Second Ed), A Zee says that conventionally, the amplitude $\langle0|e^{-iHT}|0\rangle$ is denoted by $Z$. In the next paragraph, he considers ...
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### If Earth and Jupiter are orbiting around Sun in same orbit, will they have same time period?

Let us suppose that Earth and Jupiter are orbiting around the Sun in same orbit, If we talk about at any particular point. (Which is not possible but for the sake of question) I was wondering that ...
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### Is the flux through A the same as the flux through B?

In the figure below, the amount of field lines through A is the same as the amount of field lines through B, but can you say the flux through A is the same as the flux through B as well?
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### Can you ever exert more downwards force than your weight?

So, because I'm a hardcore person, I risked all this afternoon by going out in the wind, the rain and the cold to construct a willow den. Yes, it seems a menial task, but it was actually quite ...
There is a floor that friction is proportional to its velocity (like $F=-kv$) and there is a box with its width as $l$ and its height as $h$. (you may assume that $l$ is longer than $h$). It is on the ...
In elementary treatments of quantum mechanics, we are taught that the wavefunction of a single particle is complex valued ($\Psi : \mathbb{R}^3 \to \mathbb{C}$). In particular, the wavefunction has a ...