# All Questions

7 views

### Kinematic problem (initial acceleration)

"A ball is rolling over a soccer field with constant velocity Vb (> 0) under an angle of 45° with respect to the goal line. It is starting at the corner of the field (distance d beside the middle of ...
16 views

### How are Earth and Venus actually rotating? [on hold]

As we know something moving in circle or ellipse path cannot move forward in space. Is earth moving on a helix path with also having its own rotation in helix shape? and is venus rotating in same ...
36 views

### What does it mean for an integral's differential to be raised to a power, like e.g. $d^3r$ or $d^3u$?

I was reading Zwelfel's book on reactor physics and saw the following notation: Notice that the differential of the integral is raised to the third power. What is the meaning of this notation? ...
20 views

### Particle attached in equilibrium to two elastic strings

The question I'm trying to answer is this: This is the solution: The only part of the solution I have a problem with is $x_{ps} + x_{qs}=l$, where $x_{ps}$ is the extension of the string from $P$ ...
35 views

### How does the distribution of mass affect gravitational attraction?

For example, if a planet is considered as a point mass, then an object inside the planets' gravitational field experiences an equal attraction from all of the planets' mass. However if the planet is ...
18 views

### Are Axinos an attractive candidate for Dark Matter and why?

Out of the SUSY DM candidates Gravitinos and Neutralinos are considered more likely than Axinos however I cannot find much on these. NB: Note the difference between axions and axinos.
31 views

### Is the magnetic field zero inside an ideal conductor carrying a current?

By an ideal conductor, I mean one with zero resistance. The question is not about a superconductor. As I know superconductor and conductor with zero resistance are not the same thing. The question ...
20 views

### Why would be axions be a candidate for warm/cold dark matter?

Axions are the current main favoured candidate for the identity of dark matter. While their masses are sub-$\mathrm{eV}$, why is it that they would be cold or possibly warm dark matter candidates ...
36 views

### Does electron absorb energy?

According to my teacher, an electron is point sized and it does not absorb or release energy. Moreover, my teacher says their orbital absorbs energy rather than the electron. If that is the case, then ...
12 views

### How would shape noise be distinguished from weak gravitational lensing?

Weak gravitational lensing distorts the image of galaxies by making them elliptical and non elliptical, but this we see in every day effects. So how do you determine what is just the shape noise of ...
19 views

### What is the physical meaning of relative equilibrium in nonholonomic systems?

I know the physical intuition of a nonlinear system equilibrium point, but I now want to know the physical meaning of relative equilibrium in non-holonomic systems. I have read some mathematical ...
20 views

### Calculation of stone dropping [on hold]

A stone dropped from a building of height $h$ reaches the ground after $t$ seconds in the air. From the building top, if two stones are thrown, one upwards, the other downwards, with same velocity $u$...
19 views

### Converting (time domain) Fluctuations to Power Spectral Density (PSD)

I have a Voltage signal, $V$, with fluctuations that go as: $$\langle\delta V(\tau)^2\rangle=\ln f_a\tau$$ Here $f_a$ is an attempt frequency from an Ahrennius Law. According to many papers, "...
19 views

### Existence of effective Hamiltonian for variational ansatz

I recently saw on Wikipedia that in perturbation theory one can define in a systematic way effective Hamiltonian $\hat{H}_{\rm eff}$ that produces the same kind of states and energies as if one ...
24 views

20 views

### Coin frequency distribution [on hold]

When considering the coin frequency distributions for coins of different masses, does the surface on which the coins are dropped influence the power spectral analysis of the sound waves thus produced?
20 views

### A fundamental theory of superunification [on hold]

So, an year ago I was searching over the Internet about anti gravity (just out of curiosity).. And I stumbled upon this article: http://theoryofsuperunification-leonov.blogspot.bg/2015/01/russia-...
15 views

### Photoelectric effect - elastic collision

In an elastic collision, since the electron leaves with the same amount of energy as it originally had, does that mean the photon receives no energy? Does this even result in a photon emission, if so, ...
20 views

### How to calculate position of object that have linear and angular acceleration at the same time?

Let's imagine that we have a boat with two engines: main and rotary engine. Main engine gives boat acceleration $a$ m/s² up to constant linear speed $V$ m/s. When engine turns off, boat slows down ...
21 views

### How to calculate the heating power of a metal plate in nature?

I like to heat up a part of a metal plate to a given temperature. For that I want to use an electric heater. The metal plate is located outside in nature. For that I need to take in account the wind ...
26 views

47 views

### Lagrangian of a coupled pendulum

I am trying to find the Lagrangian for a coupled pendulum: the two pendulums have the same characteristics (length $l$ and mass $m$) and are attached to the same roof at a distance $d$. In addition, ...
15 views

1k views

### Green dot on photos when taking photo facing the sun

When taking a photo towards the sun, I found there is a green dot on the photo. I'm pretty sure this is caused by the sun, the camera has no problem. Question: what is the cause of this green dot? why ...
16 views

### Explanation of a match between shadows and wet floor at Niagara Falls

The question is about the cause of a wet-floor phenomenon. My vacation trip started at Niagara Falls (Canadian side, horseshoe fall on the upper left corner). It was around 10-11 AM, not too sunny yet....
29 views

### Do all smoothly curved pieces of transparent material have a focal point?

I understand that a lens by definition has a focal point but do all pieces of glass with smoothly curved surfaces have a single point at which all parallel rays of light passing through it in a ...
24 views

### Applying Ampere's Law for Two Parallel Wires

This seems a simple question but I'm not able to understand it properly. Say we have two parallel wires carrying same current I in different directions. We need to calculate the magnetic field at any ...
17 views

### Find velocity of ambulance from the observed frequencies?

I have a problem where an ambulance with a siren is traveling past me (a stationary observer) at a constant velocity. When the ambulance is approaching, I measured the frequency at 1000 Hz and as it ...
60 views

### Skeptical of the “Cannon Shot Rule” [on hold]

What I consider a highly suspect figure was imparted to me today in a lecture and it very immediately did not sit well with me. To wit, this cannon shot rule (see here for a reproduction of the ...
56 views

### Can the Copenhagen interpretation and the many-worlds theory be somehow reconciled? [on hold]

Is it possible that, at the moment of observation, a timeline is created where an object is in a state which contrasts its state in the current timeline? Using Schrodinger's cat as an example, let's ...
26 views

### Experimental tests of electric and magnetic fields in a gravity field

Has anyone experimentally verified that the measured values of the electric and magnetic fields do not change due to gravity, that there is no post-Newtonian effect on the strength of either $E$ or $B$...