# All Questions

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### Advantage of doing research in theoretical high energy over other fields?

I am undecided about the field I want to do my PhD in, in graduate school. I am asking because the applications that I am filling ask me to write the intended field of study. I found the people who ...
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### Calculation of the maximum load to the bar

Looking for a way of calculating the maximum weight (W) to the rod with the given length (L) where the rod did not break and that only bend for (b) mm. Need only approximative solution (read: ...
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### Problem based on Rotational Motion #2 [closed]

One fourth length of a Uniform Rod of mass $m$ and length $x$ is placed on rough horizontal surface and it is held stationary by means of a light thread as shown in the figure. The thread is brunt ...
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### Are black hole event horizons filled with black holes? [closed]

An observer hovering close to an event horizon will observe huge energies, like blue shifted radiation falling in, or Hawking radiation going out. So does the observer observe that black holes are ...
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### How do bikes stay upright? [duplicate]

Possible Duplicate: Why doesn't the bike fall if going with a high speed? How does a bike stay upright? I know there are various theories involving castor effects, and gyroscopic motion ...
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### What is the difference between a Rodin coil and a Rodin starship?

I've seen various designs for Rodin coil and a 'Rodin starship'? Are these just regular electromagnets? Or something different? How do they differ from regular electromagnets?
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### How to concile flat spacetime and big bang?

After reading How do we resolve a flat spacetime and the cosmological principle? I still remain perplex. Please excuse my ignorance and try explaining to me : I thought that basically, when we ...
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### Finding The Resultant Vector [closed]

A plane is flying southeast at 215 km/h. Suddenly there is a wind from the north at 75 km/h. What is the plane's new velocity with respect to the ground in standard position?
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### How to calculate the coefficient of restitution for 2 bodies?

I have 2 rigid bodies (from different materials) in a collision. As you know I should have the coefficient of restitution value to get the velocities after collision. What is the information/values ...
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### Expansion of Helmholtz energy

To get an expansion of Helmholtz energy of a) an ideal gas b) a Van der waals gas we must integrate $\left ( \frac{\delta A }{\delta V} \right )_{T}=-P$ I saw the solution is : Can you ...
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### Can the expansion space time reverse itself and contract the same way?

If there's a mechanism for space-time expanding faster than the speed of light, is there an example of It contracting in the same manner? If whatever mechanism is causing it to expand, can the ...
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### Can a photon see ghosts?

Does it make sense to introduce Faddeev–Popov ghost fields for abelian gauge field theories? Wikipedia says the coupling term in the Lagrangian "doesn't have any effect", but I don't really know ...
332 views

### Is there a readily available (or at least not theoretical) material that reflects only infrared light?

By way of context, please picture those little motion capture lights that are put over an actors body. I am interested in finding out if there is some kind of material that can do the same thing but ...
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### Is there a valid Lagrangian formulation for all classical systems?

Can one use the Lagrangian formalism for all classical systems, i.e. systems with a set of trajectories $\vec{x}_i(t)$ describing paths? On the wikipedia page of Lagrangian mechanics, there is an ...
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### random matrix ensembles from BMN model

My friends working on Thermalization of Black Holes explained solutions to their matrix-valued differential equations (from numerical implementation of the Berenstein-Maldacena-Nastase matrix model) ...
27k views

### How fast do electrons travel in an atomic orbital?

I am wondering how fast electrons travel inside of atomic electron orbitals. Surely there is a range of speeds? Is there a minimum speed? I am not asking about electron movement through a conductor.
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### Why does maximal entropy imply equilibrium?

From a purely thermodynamical point of view, why does that entropy have to be a maximum at equilibrium? Say there is equilibrium, i.e. no net heat flow, why can the entropy not be sitting at a ...
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### How can I measure the conductivity of a copper rod?

I would like to perform an experiment to measure the conductivity of a copper rod. What device can I use to perform to experiment? is there such a thing as a conductivity meter? All i found was an ...
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### How can constants… change?

I recently noticed that the value of Planck's constant has changed on Wikipedia. The value that I had in mind (from Google), $6.626068\cdot{}10^{-34}\text{J.s}$, has changed to ...
281 views

### Why do control moment gyroscopes exhibit “torque amplification”?

There are a number of articles that describe the benefits of using control moment gyroscopes (CMGs) over reaction wheels in inertial navigation applications. One of the primary benefits of using a CMG ...
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### How to describe the inner curve of a crescent?

What is the equation which describes the inner circle of the crescent that a celestial body displays when view at an angle from its light source, as a function of the crescent-cycle period? For ...
1k views

### What is the shear stress of a fluid?

One book defines the shear stress $\tau$ of a (Newtonian) fluid as $$\tau = \eta \frac{\partial v}{\partial r}$$ where $\eta$ is the viscosity. There is not much context, so I've made some guesses. ...
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### How to calculate the electric field at a point in space

Let's say I have a uniformly-charged wire bent into a semi-circle around the origin. How can I find the electric field (magnitude and direction) I'm not even sure if I should use Coulomb's or Gauss' ...
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### Why does electron-positron annihilation prefer to emit photons?

If gravitons are massless, and neutrinos nearly so, why aren't pairs of either of them normally expected products of electron-positron annihilations? Are they possible but simply unlikely, or is there ...
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### Why has the ether been disregarded as a valid medium through which light can propagate? [duplicate]

Possible Duplicate: Ether theory acceptance Although the Einstein's theory of relativity seemed to make the concept of an ether obsolete, did it necessarily invalidate it? Are there any ...
2k views

### Physics book for 15 year old boy [duplicate]

Possible Duplicate: List of good classical physics books my name is Bruno Alano. As stated in the title, I'm 15 years old (I'll do 16 on 7 of Feb) and much love Computer Science (C, C++), ...
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### About constructing potential energy functions

There are many classical systems with different potential functions. My problem is that I do not understand how one can construct a certain potential function for a certain system. Are there any ...
387 views

### How to perform a scale (invariance) transformation?

According to this wikipedia article in the $\phi^4$ section, the equation $$\frac{1}{c^2}\frac{∂^2}{∂t^2}\phi(x,t)-\sum_i\frac{∂^2}{∂x_i^2}\phi(x,t)+g\ \phi(x,t)^3=0,$$ in 4 dimensions is invariant ...
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### What un-ionizes an electron after the photo-electric effect?

In an x-ray tube, the atoms in the anode are ionized by incident electrons "knocking" their K-shell electrons out of orbit. Following this, an outer-shell electron decays to fill the vacancy. Now: ...
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### Find the Unit Vector of a Three Dimensional Vector

How can I find the unit vector of a three dimensional vector? For example, I have a problem that I am working on that tells me that I have a vector $\hat{r}$ that is a unit vector, and I am told to ...
339 views

### What is the process called that produces characteristic X-rays?

X-rays mostly consist of bremsstrahlung and characteristic X-rays. What is the process called that gives rise to the latter (but not the former)? I'm trying to write a sentence like "X-rays are ...
271 views

### What would be the optimal weight of a ball if I want to throw it as far as possibile?

Assume the force behind my throw to be X. Assume the point of release is 2 meters above ground. Asume the ball is made of shiny steel. I'm not sure if the material matters, I'm just thinking that ...
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### How can I stabilize an unstable telescope?

I have an 80 mm refractor telescope on a tripod, but it shakes on every touch. It's very hard to see via 6 mm (x120) ocular. Even a little wind causes the image to become too unsteady. How ...
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### Where does the energy come from when a current heats a wire (resistor)?

I'm trying to figure out an example from a textbook (Demtröder -- Experimentalphysik 2, pg. 198) where the energy transport caused by a current is depicted: Assume you have a wire (with some ...
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### What telescope would I need to observe Mars's polar caps?

What telescope refractor aperture size would I need to observe Mars's polar caps? I have a Levenuhk Strike NG 80mm, and I can see red disk of planet but cannot see any details on the surface.
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### Does a charging capacitor emit an electromagnetic wave?

Assume you charge a (parallel plate) capacitor. This establishes an electric field (the $\mathbf E$ vector points from one plate to the other) and a circular magnetic field (the $\mathbf B$ vector ...
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Is there any (proposed) relation between the B-band isophotal radius of a spiral galaxy and its virial radius (r_200)? If you know of such a relation, please post a reference paper.
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### Why are refrigerators 4 degrees and freezers -18 degrees?

I assume that the refrigerator's temperature of 4 degrees has something to do with the fact that water is densest at that temperature. Does that inhibit microbe growth? But what about the freezer, ...
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### What is the mass of individual components in a gravitationally bound system?

When material of rest mass M falls from infinity onto a black hole accretion disk, it gets heated and then emits so much light that the energy radiated away can measure up to about 30% or so of M c^2. ...
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### why is it hard to push the empty upside down mug inside the water

It is hard to push the empty upside down mug inside the water as it has air present inside its cavity which prevents the water from entering inside the mug.But when we tilt that mug which is still ...
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### Purple doesn't occur in rainbow - or does it?

Usually, when asked whether the purple color exists rainbows, an answer similar to this is given: The purple color is perceived by human eyes via the activation of both red-sensitive and ...
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### Trinary System Arrangment?

I'm writing a science fiction story and wondered if someone could help me with the setup of a trinary star system I have in mind. Basically I just wanted to know if: a) a system with a central, ...
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### Electric field due to nonconducting plastic sheets [closed]

Two very large, nonconducting plastic sheets, each 10.0 cm thick, carry uniform charge densities $\sigma_1, \sigma_2, \sigma_3$ and $\sigma_4$ on their surfaces (the four surfaces are in the ...
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### A different type of Gaussian Grassmann Integral

Let $A$ be an antisymmetric matrix. Usually, one proves that for a Grassmann integral of the type, $$\int d\psi d\theta \exp( \psi^T A \theta) = \det(A)$$ where $\psi$ and $\theta$ are vectors of ...
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### Can we use only the observables of Fermion fields?

There are legion ways to consider fermionic Dirac spinor fields, but is it possible to consider the asymptotic free field only in terms of observables, which in the case of the Dirac spinor field must ...