# All Questions

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### What causes destruction in car crash?

Suppose a car crashes at a speed $v$ against a wall and comes to a stop. Now if the car crashes at $2v$, does that mean it suffers twice as much destruction, if that can be objectively measured? If ...
632 views

### How are the comoving coordinates NOT a prefered reference frame?

Physics me this: The equivalence principle has rigorous physical definitions that say, for one, that the laws of physics are the same in all inertial reference frames. This is to say that the ...
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### Will a warm body naturally slowdown?

Suppose a warm body moving in an empty space with high speed. The body emits radiation based on its temperature. The protons emitted forwards of the body will have higher energy due to Doppler shift ...
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### Why are most astronomy things spherical in the shape (like, the Sun, the Moon, the Earth, and other planets)?

What is the reason for all the astronomy things being spherical in the shape (like, the Sun, the Moon, the Earth, and other planets)?
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### Question Concerning Position Of A Particle At Any Given Time

After years of procrastinating i've decided not to "move ahead" with physics without getting this ridiculously trivial question clear!*I know i had asked a similar question as silly and stupid as this ...
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### Causes of surface tension between two fluids

Suppose that we have two fluids $A$ and $B$ in a container $\Omega$, and we notice that $A,B$ do not mix. Can you pleas explain to me what is the cause of this property? What properties of the two ...
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### Is there an optimum length for a swing for a childs mass?

If you want to maximize the maximum velocity a child could go, what would be the optimum height? If you wanted to maximize the efficiency of a child "pumping" their legs to gain velocity, what would ...
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### Sunspots formula

I used the package 'EUREQA', version Formulize, to analyse the monthly smoothed sunspot timeseries from 1750 till 2010. It gives me a simple formula, with 8 coefficients, that match data with a ...
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### Magnetic Susceptibility- Permeability

What is the permeability of magnesium sulphate and therefore the magnetic susceptibility?
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### Joining the definitions of entropy

$\int \frac{Q_{rev}}{T} = \Delta(k_B\ln\Omega)=\Delta S$ Could anyone give some definite proof for this? I was able to prove that the two definitions of change in entropy are equivalent for an ...
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I'm looking for the appropriate term to use for what gets "used up" as potential energy is converted to heat and work. For example, some of the the energy in solar radiation is converted by ...
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### Nonablianity of Wilczek Zee Potential

Consider a hamiltonian with degeneracies of energy in parameter space $(R)$.Now the Geometric phase(Wilczek Zee Potential) will acquire a non abelian nature. To prove the non abelian nature of Wilczek ...
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### What is a Free Body Diagram?

I have this problem: A 10 kg box is resting on a frictionless table and someone drags it by a string at a 30 degree angle to the horizontal with a force of 40 Newtons. Find the acceleration of the ...
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### Could a Dyson sphere destroy a star?

Freeman Dyson proposed that the power needs of an advanced civilisation would eventually require the entire energy output of a star to be collected, so that the star would end up surrounded by a ...
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### Is there zero energy cost to generate a gravity field?

It seems that all objects with defineable mass emit a gravity field, which interacts with other objects within said field. My understanding on this may be very limited, but how is the field ...
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### Working with atomic (?) units in solid state physics

I'm having some troubles understanding the units used in solid state physics paper. In the paper I read $\Lambda a \sim 1$ where $\Lambda$ is a momentum cutoff and $a$ is the lattice spacing of a ...
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### What is the Hilbert space of a single electron?

Is it the same as the space of all possible descriptions of a single electron? If not, how do they differ? Please give the mathematical name or specification of this space or these spaces.
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### Why must the field equations be differential?

In Landau–Lifshitz's Course of Theoretical Physics, Vol. 2 (‘Classical Fields Theory’), Ch. IV, § 27, there is an explanation why the field equations should be linear differential equations. It goes ...
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### Why, intuitively, must a solution in physics be unique?

When solving Laplace's equation or Poisson's equation say, we require that the solution must be unique, which can be shown. In general, what is the physics behind seeking a unique solution? Are ...
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### Potential of a spherical conductor next to a point charge

Griffiths, when he's talking about the method of images, shows how to calculate the potential distribution when given a point charge that is outside of a spherical conductor which is held at a fixed ...
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### As ISA Practical - Resistors in Parallel [duplicate]

Possible Duplicate: Current against the inverse of resistance graph, I = V/R +c How would you set up a circuit with a fixed resistor in parallel with a variable one. We are told to measure ...
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### Are objects in a gravitational well shortened?

Bob is in a gravitational potential well, he moves a long vertical stick up and down a distance of 1 meters. Alice observes the upper end of the stick, at upper location. There is the phenomenon of ...
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### What is the velocity area method for estimating the flow of water?

Can anyone explain to me what the Velocity Area method for measuring river or water flow is? My guess is that the product of the cross sectional area and the velocity of water flowing in a pipe is ...
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### Pulsar beam radiant intensity distribution

I'm curious about the radiant intensity distribution of pulsars: what's the general dependence of intensity on angle, and what are typical angular beam widths? How much does the beam width vary ...
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### What causes millisecond pulsars to speed up?

Millisecond pulsars are supposed to be old neutron stars. However, they are spinning even more rapidly than newly formed pulsars. Since pulsars slow down as they age, something must have caused ...
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### How do we explain the phase change between high and low star formation rates?

During a recent talk I was at, someone, who models galaxy characteristics from dust amounts and spectral energy distributions (SEDs), quoted a fairly prompt change from 'high' to 'low' stellar ...
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### Pushing, inertia of a cart system

I have a cart with another cart on top which gets pulled down by another cart on the side. There is no friction. The question is: How strongly do I have to push with $F$ to keep the cart $m_1$ ...
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For separable potential, say $x^4+y^4$, its symmetry are degenerate. Is that a generic case to every separable potential? I will explain my question: The potential $x^4+y^4$ has $A_1, B_1, A_2, B_2, ... 1answer 495 views ### BCS theory, Richardson model and Superconductivity I'm studying Richardson Model in second quantization. There are many initial points that I don't understand: We supposed that an attractive force between 2 electrons exists, due to electron-phonon ... 3answers 788 views ### Is there a valid Lagrangian formulation for all classical systems? Can one use the Lagrangian formalism for all classical systems, i.e. systems with a set of trajectories$\vec{x}_i(t)$describing paths? On the wikipedia page of Lagrangian mechanics, there is an ... 1answer 395 views ### What are the theoretical explanations behind the “no-boundary condition” in cosmology? In his book "The Grand Design" on page 167-173 Stephen Hawking explains how one can get rid of the so called "time zero issue", meaning by this the very special initial state needed for inflation to ... 1answer 333 views ### What is Electromagnetic Induced Transparency? Please can anyone explain the concepts of electromagnetic induced transparency? I am having problem with the technicality of the explanation on wikipedia. Please I am an engineer with a physics ... 4answers 2k views ### Should I heat my room when I'm not here, energy-efficiently speaking? I was wondering as it's getting cold : is it better for my electricity bill to shut down completely my (electric) heater during day, and to turn it on again when I come home (then it will have to heat ... 1answer 459 views ### Physically what happens during Avalanche breakdown to the pn junction? What does breakdown mean physically? I saw this in wikipedia: The avalanche process occurs when the carriers in the transition region are accelerated by the electric field to energies ... 1answer 456 views ### Physics of the Internet? [closed] Is there a way to describe the Internet in terms of a physics theory, like how the atom is described by quantum mechanic? If there is, how is it described by this theory. 1answer 263 views ### Angular velocity$\omega$by$v$We have two girls, with mass (M). They become close to each other in speed of V. The distance between them is 3L. I was asked to calclute the Angular velocity (w) of the two girls. So I set the ... 4answers 565 views ### What is the physical definition of causality? Maxwell's equations give a physical relationship between the electric and magnetic fields$\vec E$,$\vec B$at the same time, which some interpret as changes in one causes changes in the other etc. I ... 1answer 411 views ### Vibrational motion of linear diatomic molecule This question concerns the following exercise from an old exam: The vibrational motion of a linear diatomic molecule can be approximated as simple harmonic motion. A CO molecule has a bond ... 2answers 826 views ### Testing conservation laws experimentally How conservation laws are tested experimentally independently from each other? what do I mean by that question? It seems that to test one conservation law experimentally, such as conservation of ... 1answer 486 views ### Finding coefficients of volumetric expandtion from a know coefficient of linear expandtion Starting from a homework problem: An aluminum cup of$100 cm^3$capacity is completely filled with glycerin at$22°C. How much glycerin, if any, will spill out of the cup if the temperature ... 2answers 275 views ### Orientation of Magnetic Dipoles Does a magnetic dipole (in a permanent magnet) tend to align with the B-field or with the H-field? The current loop (Ampère) model of the magnetic dipole suggests the former, while the ... 1answer 109 views ### Introduction to neutron star physics I enjoy thinking about theoretical astrophysics because I want to understand black holes. Given that no one understands black holes, I like to ponder the nearest thing to a black hole: a neutron star! ... 2answers 689 views ### How are magnetic fields transmitted? A common analogy for gravity is the ball-on-a-rubber-sheet model. In this model, mass distorts spacetime and creates a 'valley' into which other mass can fall. Is this same principal valid for ... 0answers 264 views ### Exam review question on thermodynamics [closed] A cube has a side length of20\text{ cm}$. An atom in the gas moves around the cube as shown. It continually bounces off the four lateral walls of the cube. The atom has a mass of$6.6\times ...
Having a discussion at work about the $H$-field around a big parabolic antenna. All of the safety tests done around the antenna only mention $E$-Field. They state in the radiating near-field the $E$ ...
I just finished reading Feynman's Lectures on Physics vol.I, §34-9: "The momentum of light". The author explains that there is a relation between the wave 4-vector $k^{\mu}$ and the energy-momentum ...