# All Questions

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Yesterday i got a MCQ in exam that a body in projectile motion with initial velocity 20m/s creating an angle of 60 degree with horizontal. What is its velocity at max. Height (A)10m/s Or (B)0m/s ...
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### “Anti-classical” limit of large Planck's constant

A central theme in quantum physics is the classical limit where $\hbar \rightarrow 0$, and there is lots of interesting structure to this limit (i.e. classical mechanics). Is there anything ...
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### Type of motion when a long wire carrying current is inserted between two wires carrying current is moved normal to its length

Two long rigid wires separated by a distance d and placed parallel, carry current I in the same direction. Determine the force developed . Now a third wire is placed in between the rigid wires keeping ...
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### Meaning of non diagonal terms in decoherence

It is my understanding that the non-diagonal terms in the density matrix of a macroscopic system that it is initially in an entangled state go exponentially fast to zero as the system interacts with ...
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### Does 'real' photon electromagnetically interact with matter?

Consider the standard description of Compton scattering - radiation is constituted of stream of photons (these are supposed to 'real' as contrasted to 'virutal' photons of the QED). One of these ...
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### Is the time ordering in Dyson series either 1 or -1?

Because I think to make it a unitary operator, the norm of the unitary operator should be one. But I did not see any claim about the value of time ordering in Dyson series.
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### How do I determine the information needed to design a solenoid electromagnet that can ionize a gas?

I have been trying to answer this question for a long time. I watched a video where a person used a solenoid to heat up aluminum till it became a liquid. I want to apply this, to a gas. I want take ...
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### Equation of distance for two racing cars?

Two cars were racing along a straight track namely CAR A and CAR B. They both started from same starting point with initial velocities $v_1$ and $v_2$ and constant acceleration $a_1$ and $a_2$ ...
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### How does a blue laser pointer beam diverge?

When I read the excellent Physics Stack Exchange article, [Why does a laser beam diverge? I was really impressed with the first answer written by @Ruslan shown below. Due to Heisenberg uncertainty ...
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### Basic concepts of electrostatics

How to deal with a grounded conducting sphere (I.e finding charge on it) in uniform electric field ? And if any two charged conducting spheres are in each other's electric field then will ones field ...
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### Confusion in Maxwell's derivation of Ampere's Force Law - Part II

I am reading Maxwell's "a treatise on electricity and magnetism, Volume 2, page 156" about "Ampere's Force Law". I have some confusion in the following pages: My question is of two parts: 1. ...
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### How can quantum tunnelling lead to spontaneous decay?

I have never understood what measuring process (if any) is supposed to be continuously polling the quantum state of an unstable bound system subjected to decay via quantum tunnelling. The reason I ...
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### Does a degassed magnet weigh less than one that is not?

I learned matter with an electric current weighs more than matter with no current. Does that apply to magnetic material in the same way but with it's magnetic fields instead of electric current? A ...
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### What's the interpretation of Feynman's picture proof of Noether's Theorem?

On pp 103 - 105 of The Character of Physical Law, Feynman draws this diagram to demonstrate that invariance under spatial translation leads to conservation of momentum: To paraphrase Feynman's ...
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Suppose we have a lever $AB$ of length $1m$ and that the fixed point is $O$ and $AO=0,3m$. If in $B$ I have a down vertical force of $F_1=5N$, find the force $F_2$ applied in A that guarantees the ...
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### Electric field in a hollow object

I am currently visiting a course about electrodynamics. In my last lecture it was said that if a hollow sphere is inside of a bigger sphere, but only in the bigger sphere there are spherically ...
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### Why do High Pressure Gases produce a Continuous Spectrum?

I am aware that low pressure/density gases produce an emission spectrum as there are specific energy transitions that the electrons can make, emitting certain frequencies of EM waves. However high ...
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### Thought Experiment: Force on magnets in a Stern Gerlach Experiment

Background: In the SG experiment, an inhomogenous magnetic field affects a force on particles passing between two magnets. "Measurement" takes place when a screen is placed on one end, blocking one ...
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### Total Angular Momentum of Hadrons

Hadrons are made of 2 or 3 quarks. Quarks have spin 1/2, so Mesons can have spin S = 1 or spin S = 0, and Baryons can have spin S = 3/2 or spin S = 1/2. Is there orbital angular momentum of the quarks ...
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### Terminal velocity after vertical launch

If an object is launched directly up at it's terminal velocity, will it have enough time/energy to reach it's terminal velocity again before it hits the ground, or will drag prevent this? In this ...
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### Matching Gravity with Centripetal Force

I was wondering if I'm correct in thinking that to determine the tangential velocity necessary to create the same gravitational force on a spaceship as on Earth you would use the formula ...
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### Radial and angular coordinates of a route?

Certain bug has the route described by the radial coordinate $r(t)=\frac{at²e^{-bt}}{1+t²}$, where $t$ is in minutes, and the angular coordinate $\theta(t)=b+ct²$ in radians. Consider a,b, and c real ...
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### Mechanics Laws of motion

Two Blocks A and B are pushed against the smooth wall with force F. Surfaces of A and B are rough. Which of the following is true for the system of blocks? A) F should be equal to weight of A and B ...
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### why in such a fundamental theory as QFT we learn only two computations?! (cross sections, decay rates)

when i learned Newtonian mechanics i found a wast variety of computations that i could do and that was so interesting. and it was so when i learned Maxwell theory. when i started learning QFT i hoped ...
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### Definition of pole of a spherical mirror

what can be a suitable definition to a pole of the spherical mirror. It cant be the geometrical center of a spherical mirror as if a mirror is cut then its pole does not change. moreover if a ...
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### Potential Energy for dipole in Electric Field

Potential Energy is the work done by the external agent in turning the angle of the dipole from the U=0 position to another position against the influence of the electric field applied right? So if ...
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### Finding the resultant electric field intensity?

Q: Two charges Q1 = 4×10^-7 and Q2 = -8×10^-7 are placed at the two corners A and B of an equilateral triangle ABP of side 20 cm. Find the resultant intensity at P? I calculated the electric field ...
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### How is a potential difference created between two points?

I would like to know how potential difference is created between two points?Is potential difference created by adding more electrons at one side?If that's the case how to add more electrons at one ...
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### Is it true that the Schrödinger equation only applies to spin-1/2 particles?

I recently came across a claim that the Schrödinger equation only describes spin-1/2 particles. Is this true? I realize that the question may be ill-posed as some would consider the general ...
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### Conditions for heat exchange and evaporation

I wonder if there is any law for heat exchange. I have learnt that heat exchange between systems occurs when the temperature of the systems differ , but in case of evaporation, the process creates a ...
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### Dirac Delta Potential and bound/scattered states

Why does the attractive Dirac Delta distribution (function) potential $V = \alpha\delta$(x) (for negative $\alpha$) yield both bound AND scattered states? Is this due to the definition of the Dirac ...
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### Density of the universe

Why is the universe of the same uniform density at all distance and time scales? Why haven't cosmologists realised this simple fact disproves the big bang? Why do they keep pushing theories as ...
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### CP-Symmetry of the axion field and decay constant

In the $U(1)$-Peccei-Quinn-Symmetry an axion field $a$ is introduced in order to solve the strong CP-Problem. It is said, that below a certain scale $f_{a}$ this symmetry is broken and you are able to ...
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### Can a magnetic field speed molecules up?

I heard somewhere that If you add a magnetic field to atoms in a closed space you would speed them up. Similarly to heating them up. Is this true is so what the atom consists of for this to work?
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### How does crystal lattice explain electrical conductance?

From http://education.jlab.org In a metal, the atoms are arranged in a crystal-like configuration. ... Now, in a metal, the valence band is relatively close to the conduction band - ...
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### How come Wifi signals can go through walls, and bodies, by kitchen-microwaves only penetrate a few centimeters through absorbing surfaces?

The only difference that I know of between Kitchen Microwaves and Wifi signals is how much power is pumped through them. Why is it that Wifi signals, being 1000 times weaker can travel so much ...
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### Classical dynamics of a matrix

For a system of interacting particles, we can formulate Hamiltonian dynamics in terms of a vector of position coordinates $q$ and a vector of momentum coordinates $p$. Then the Hamiltonian takes the ...
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### Correct Definition of Angular Momentum of a Charged Particle in an Electromagnetic Field? (Classical Mechanics) [duplicate]

What is the more correct definition of angular momentum $\vec{\mathbf{M}}$ in three dimensions? (I.e. classically/Lagrangian/Hamiltonian, not necessarily quantum or relativistic) ...
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### Newton's Third Law As Discussed in Den Hartog's Mechanics

A question, but some preliminaries in order to avoid answers involving concepts not yet introduced in the text I'm reading--Den Hartog's Mechanics. Near the very beginning of his book, Den Hartog ...
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### What does a speedometer measure?

When speaking of the velocity of a particle, we must specify a reference point. The same holds when we talk about the speed of a particle (is this correct?). If so, a speedometer measures instaneous ...
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### Which timestep should I use for a $N$-body simulation of the Solar system?

I am trying to implement a $N$-body simulation of the Solar system and I am stuck on the issue of the simulation timestep size. At first I would like to use a simple Euler stepping scheme. Knowing ...
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### Find the relation between length of pulley and strings

A pulley is pulled with external force $F$. $x$ and $y$ denote the displacement of two ends of strings of the pulley and $z$ is the displacement of the pulley. Prove That z = ...
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### Travel time in combined driving/walking trip [on hold]

A car is sent to take a student from coaching every day. The car arrives at the coaching 15min before the time. One day coaching gets closed one hour before the usual time. Then the student without ...
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### Is there a way to make atom move faster without heating them?

The more heat you add the faster the atom will move. This is something that is common knowledge. My question is it possible to make the atoms in let's say a gas move faster without adding heat of a ...
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### If there are two conserved quantities that do not commute, why is there typically another?

I have come to the following conjecture: Consider two observables $A,B$ that do not commute and represent conserved quantities, there is typically a third conserved quantity. E.g. for some ...
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### Wind turbine impact on cars fuel consumption

I have question that I still can't resolve. How much cars fuel consumption rises(l/100km) if I attach 1 kW wind generator to cars body? How can I calculate that and what do I need to know? ...
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### On the expansion of space on small distances

After reading this post :Why does space expansion not expand matter? and after posting some comments to Peter Diehr, I was invited to make my argument to a question. The main things one needs from the ...
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### Chapter 6 Relativity [on hold]

I am having trouble visualizing the last sentences in the chapter. "However, the most careful observations have never revealed such anisotropic properties in terrestrial physical space, i.e.a physical ...
This question arises from the classical example: imagine a laboratory frame, and a space shuttle frame, the space shuttle moving in the laboratory frame with a constant velocity, let's say in the $x$ ...