# All Questions

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### Kinetic energy during 1 minute - wrong?

I've come across this IEEE paper whilst researching for a project I've undertaken regarding a bicycle generator. This paper concerns an exercise bike powered electric generator. Essentially they have ...
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### Can i extend the non-linear realization of the chiral group to $U$ with complex pions?

In chiral perturbation theory we build a Lagrangian invariant under $SU(2)_L\times{}SU(2)_R$ which acts on the matrix $U$ that accommodates the pion degrees of freedom in the following way ...
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### What is the general relativity explanation for why objects at the center of the Earth are weightless?

The idea that as you move through the earth you get a symmetric cancelling of gravitational acceleration which approaches zero acceleration due to gravity at the center of the earth makes a lot of ...
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### experiments/ theory of digitizers mixed with wave mechanics

classical waves arise in a wide variety of situations and are often/ usually/ typically measured with analog measurements. however, combining/ use of a digitizer can complicate the analysis. ...
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### Is there a time operator in quantum mechanics?

The question in the title has been asked many times on this site before, of course. Here's what I found: Time as a Hermitian operator in QM? in 2011. Answer states time is a parameter. Is there an ...
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### Systems evolving towards equilibrium

I believe that in quantum transport the states in contacts are forced to be in equilibrium (can someone comment on it? the boundary conditions used (ansatz) essentially suggest that states in contacts ...
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### Determine load that can be lifted with a screw jack

I've been scratching my head on this one now for a while so would appreciate pointers in the right direction! Determine the load that can be lifted with a screw jack if an effort of 75N is ...
339 views

### Why is the focal length of a convex mirror negative?

According to the Cartesian sign convention, the focal length should be positive since the convex mirror forms a virtual image to the right of mirror (positive direction) when light comes from left. ...
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### Calculating flow rate of a syringe

I wanted to know how would I go about calculating the flow rate of a syringe with a metal tip that is dispensing water using a pressurized air. I have had a look on internet about this and on this ...
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### Prove that the transformation is canonical

Does the existence of a generating function $F$ prove that a given transformations is canonical? How?
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### Electromagnetism boundary condition

The dielectric slab surrounded by air as shown in diagram has permittivity $\epsilon$ different from air and permeability almost same as air. Electric field everywhere within the slab is given to be ...
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### Why is the gravity in a mine shaft independent of depth if the local density is 2/3 of the average density?

While searching for something, I have come across the fact that the gravitational acceleration in a mine shaft is independent of depth if the local density is $2/3$ of the average density (of the ...
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### Cooling of Atomic Clocks

I have read of cooling atoms to one-quarter of one-thousandth of a degree above absolutely zero. According to Physicist William Phillips, this improves the measurement of the ticking frequency. I have ...
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### What are some ways to justify the Einstein field equations?

Since they are a postulate of general relativity, it is not really possible to "derive" the Einstein field equations $$R_{ab} + \left(\Lambda - \frac{1}{2}R\right)g_{ab} = -8\pi T_{ab}$$ in any very ...
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### magnetic field and permeability

Doubt on magnetism. The the problem with Magnetic Flux Density(B) and Magnetic field intensity(H). I can Imagine magnetic flux(phi) lines and flux density(B) Physically. But magnetic field intensity ...
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### Lasing in a 2-Level system?

What exactly is the difference between 2-Level, 3-Level and 4-Level systems? Why can we not achieve stimulated emission in a two-level system using optical pumping?
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### What motion does this experience?

I have two Earths of $x$ distance away from each other. I place a spacecraft perpendicular to the centre of mass and the distance between the two Earths. If the Earths are fixed, then the spacecraft ...
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### Impact force in joules

I have to calculate an arc length so that when a $50\ \mathrm{kg}$ ball hits a wall it produces a force of $60\ \mathrm{J}$. Here is the method from bsi. Suspend the $50\ \mathrm{ kg}$ ball on a wire ...
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### General scattering theory

I have studied a bit of scattering/diffusion theory in an introductory course in quantum mechanics (the kind where the scattering potential is delta, box, and similar easy functions). But when the ...
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### Comparing rates of cooling

There are 2 metallic spheres A and B. Mass of A is thrice the mass of B. Both the spheres are heated to the same high temperature from same initial temperature. The spheres are thermally insulated ...
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### How to construct an unaccelerated frame of reference?

While reading law of inertia I found this problem that how one can make a body completely isolated from all interactions that is to construct a galilean frame of reference.. I found that a well ...
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### Does positive charge flow from low to high potential?

In this question How does a wire carry alternating current?, the author says this: Now if a DC current source is attached across A and B, say terminal B is at +5 potential and terminal A is at 0 ...
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### Angular velocity from orientational displacement

A 3-dimensional object is rotating around an unknown 3-dimensional axis through the object's center of mass. Its orientation is described by two unit vectors. I know an initial orientation and a final ...
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### what is wavelength of electric field generated in a wire?

When a wire and a resistor are connected across a battery then what is the wavelength and frequency of electric field generated in the wire ?
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### What happens to the resistance, voltage and current when DC passes through saltwater?

I have a circuit of a battery(1.5V or 9V) connected to a multimeter, a connected to 2 metal pieces in water(distilled)(see picture, in place of the LED is a multimeter). With the addition of salt 1. ...
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### Angular velocity relative to different frames

In Goldstein it is said "It is intuitively obvious that the rotation angle of a rigid body displacement, as also the instantaneous angular velocity vector, is independent of the choice of origin of ...
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### Why does Higgs break the symmetry

Usually classical analogies are given of the form: A ball sits on top of a mexican hat potential, rolls down spontaneously and will come to rest at at minimum. But classical systems of balls on hills ...
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### Time Symmetry in two body problem

I was solving Two-body problem in two dimension numerically using second order Taylor expansion. In order to check the time-symmetry of the energy, I considered the following equations. \begin{align} ...
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### why don't we consider atmospheric pressure or weight of liquid while deriving the equation of thrust in a fluid?

More specifically , when we try and prove that up thrust experienced by an liquid column inside the liquid is equal to the weight of the liquid column , I have seen it being derived like this: thrust ...
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### What is the relationship between rolling resistance and velocity?

I'm a games programmer, trying to write a simple car physics simulation. I'm aware that a car travelling in a straight line will exert a traction force that drives it forwards (by turning the wheels ...
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### Can I control distance of surfaces with a magnet?

I need to make a model of a finger expanding without anything elastic. I want to use two metal square surfaces, one is fixed and the other movable. Is there a way to control the distance between the ...
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I was reading about Supplee's paradox, which is about whether a relativistic projectile, subject to uniform gravitational acceleration, would float or sink underwater. However the solution of the ...
976 views

### Why are turns banked?

I have seen everywhere that where on road, on railway track when there is a turn then one side of that road or railway track is below the surface or some what down then actual surface. Any one ...
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### Plasma frequency for ultra-relativistic maxwellian electron plasma [on hold]

How to find the expression for the plasma frequency and cyclotron frequecy for the ultra-relativistic maxwellian electron plasma. $\omega_{p}={4\pi n e^{2} c^{2}}/3 T$ and $\omega_{c}={eB_{0}}/{T}$ ...
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### Generallized Canonical Ensemble - Isobaric Ensemble

I am trying to understand the way generalized canonical ensembles like the pressure ensemble are derived from the standard canonical ensemble. In the derivation for the standard form, one defines a ...
I got a very vague question. A thermal state is defined by $\rho=\frac{e^{(-\beta H)}}{Z_\beta},$ where $Z$ is the partition function. I want do now calculations with "non-thermal states", but I'm ...