# Tagged Questions

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### Why is the Earth so fat?

I made a naive calculation of the height of Earth's equatorial bulge and found that it should be about 10km. The true height is about 20km. My question is: why is there this discrepancy? The ...
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### Why does a system try to minimize its total energy?

Why does a system like to minimize its total energy? For example, the total energy of a $H_2$ molecule is smaller than the that of two two isolated hydrogen atoms and that is why two $H$ atoms tries ...
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### Prove that negative absolute temperatures are actually hotter than positive absolute temperatures

Could someone provide me with a mathematical proof of why, a system with an absolute negative Kelvin temperature (such that of a spin system) is hotter than any system with a positive temperature (in ...
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### Why do most office chairs have 5 wheels?

(Inspired by Why are four legged chairs so common?) I've been wondering for a while... Why do most wheeled office chairs have 5 wheels? My guess would be that while stability vs. simplicity ...
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### Why does the nature always prefer low energy and maximum entropy?

Why does the nature always prefer low energy and maximum entropy? I've just learned electrostatics and I still have no idea why like charges repel each other. http://in.answers.yahoo.com/question/...
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### How quickly should a fluid come to hydrostatic equilibrium?

Let's say I'm holding a one-liter water bottle, full of water, which I then drop. Before dropping the water bottle, the equilibrium is for there to be a pressure gradient in the water canceling the ...
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### Why doesn't an object, despite having a non-zero potential energy stored in it, fall by itself from the elevation?

When an object is at a certain height, it has some energy stored in it as we have done some work on it to get it to that height. So when it already has energy, then why doesn't it fall off from the ...
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### Why is (von Neumann) entropy maximized for an ensemble in thermal equilibrium?

Consider a quantum system in thermal equilibrium with a heat bath. In determining the density operator of the system, the usual procedure is to maximize the von Neumann entropy subject to the ...
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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 non-...
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### Spikes on ferrofluid?

What exactly are the spikes, or peaks and valleys, caused by in pictures such as these Wikipedia states that "From the point of view of magnetic energy, peaks and valleys are energetically ...
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### If the Earth is in constant motion then why do we say that an object is in a state of rest?

I got this question as my physics class homework for tomorrow. Anyone please help me out. If Earth constantly rotates and revolves, then how can we call an object in a state of rest?
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### Airplane with banner in a windy day

Will the banner of this airplane be always in the proper direction if the airplane flies in any direction on a windy day?
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### Is “equilibrium state” equivalent to “well-defined state variables”?

Follow up to Intuitively, why is a reversible process one in which the system is always at equilibrium? and How slow is a reversible adiabatic expansion of an ideal gas? Suppose you have a ...
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### Applying the Maxwell–Boltzmann statistics to astrophysical objects

Quoting Wikipedia: In statistical mechanics, Maxwell–Boltzmann statistics describes the statistical distribution of material particles over various energy states in thermal equilibrium, when the ...
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### Do airlocks in space decompress violently as they do in movies?

The standard scene follows: The good guys have finally captured the enemies and have them in an airlock chamber on a spaceship. A button is pressed and out they go, violently propelled by the blast of ...
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### How do we perceive hotness or coldness of an object?

Some objects, especially metallic ones, feel cold on touching and others like wood, etc. feel warm on touching. Both are exposed to same environment and are in their stable state, so some kind of ...
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### Do the stars in a galaxy have a thermal kinetic energy distribution?

I think, there is practically everything given to that: many point-like masses, able to exchange energy pseudo-randomly, and far long enough time to reach a thermodynamical equilibrium. Of course, ...
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### Adiabatic piston: why is Callen's argument flawed?

This question is related to this one. I would ask you to read that question and my answer to the question itself before answering this one. The problem is the following. In his book Thermodynamics, ...
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### What determines the (minimum) angle at which a domino falls over?

Dominoes, when placed upright, remain that way. Sometimes, even if you tip them a little bit, they will go back to their upright position. However, if you tip them too far, they will fall over. ...
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### Will Cone standing on its tip, without any other force other than gravity topple?

A cone standing on its tip is considered to be in unstable equilibrium as a slightest force could topple it. So, if the cone is stood on its tip with no other force other than gravity (and the ...
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### Fluids in thermodynamic equlibrium

I am reading about the Euler equations of fluid dynamics from Leveque's Numerical Methods for Conservation Laws (Amazon link). After introducing the mass, momentum and energy equations, some ...
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### Is it true that spring has more force acting on it at its positive maximum amplitude than than at the negative one?

Am I missing something? It seems obvious to me that at $+A$ and $-A$, the spring has restorative forces equal in magnitude but opposite in direction. But since gravity is always pulling it down, ...
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### Difference between steady state and equilibrium?

In semiconductor physics, what is the difference between steady state and equilibrium. How analysis of devices varies in these processes?
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### Thermodynamic equilibrium or thermal equilibrium and equipartition theorem

In all derivations of the equipartition theorem I can find a thermodynamic equilibrium distribution is used to show it's validity. But more vague sources (physics.stackexchange answer by Luboš Motl, ...
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### Understanding the Jacobian Matrix

Taking the example of a two dimensional system, desribred by the following ODE's: \begin{align} \frac{dx_1}{dt}&=f_1(x_1,x_2)\\ \frac{dx_2}{dt}&=f_2(x_1,x_2) \end{align} The Jacobian Matrix ...
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### Understanding moments as forces?

I was watching this lecture on analysis of stress for mechanics of materials. At time 7:20, the lecturer says that in equilibrium, the sum of forces and "moments" in each direction (x,y,z) must be ...
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### Does the entropy of a system vary in the equilibrium?

I am not physicist and this question may seem trivial. But I understand that in the equilibrium the magnitudes such as temperature or volumen do not vary. Is the same for entropy? My logical says that ...
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### The “stationary potential energy” condition for static equilibrium in mechanical systems

I've often read that, for a mechanical system which can be described by $n$ generalized coordinates $q_1,...,q_n$, a point $\mathbf{Q}=(Q_1,...,Q_n)$ is a point of equilibrium if and only if the ...
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### Why a system should be at its lowest energy state for its stability?

Every possible reaction in chemistry is to attain stability. In physics, the alignment of an electric dipole in an external electric field and in all other physical systems (at least those I study in ...
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### Dissolving oxygen into water

I was wondering how one would actually calculate how much oxygen would dissolve into water given the necessary initial conditions, and what those initial conditions would need to be. I assume they ...
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### Boltzman distribution for chemical potentials

I read that if we have a system with two co-existing phases with chemical potentials respectively $\mu_1$ and $\mu_2$ then, at the equilibrium, the concentrations $X_1$ and $X_2$ are related by the ...
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### When a ball is tossed straight up, does it experience momentary equilibrium at top of its path?

This question has been asked many times all over the Internet and answers can be found on places such as yahoo and ask.com, but I'm not satisfied with those answers and I don't trust the validity of ...
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### Metal sphere and charged ring

I think this is an interesting question, to which I don't really know the answer to. (Also, not a homework question.) Say you have an uncharged metal sphere constrained to move in the z-axis. There ...
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### What are some creative illustrations of the nature of dissipative forces?

I'm teaching a conceptual introduction to physics for American 13-15 year old students this summer. One of the main ideas I want to hit on is the relationship between energy conservation, equilibrium,...
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### Average temperature of the universe

Is it possible to define some average temperature of the universe? If yes, what fixes this temperature and how t estimate today's temperature? Is it different from the temperature of the black-body ...
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### What's the point with equilibrium in Thermodynamics?

All the Thermodynamics books I saw until now state that in Thermodynamics we are mainly concearned with equilibrium states (I know there's "non-equilibrium Thermodynamics", but I'm interested on the ...
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### Why is $\omega = \sqrt{K/m}$ valid for a quantum oscillator?

I'm working in the 3rd edition of Modern Physics by Serway, Moses, and Moyer. In 6.6, it talks about a quantum oscillator. I don't fully understand how the definition of frequency works. Now, we ...
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### Lagrangian formulation of the problem: small oscillations around an equilibrium

I'm having trouble understanding how some conclusions are made in my book. I'm studying from a coursebook based on Goldstein's "Classical Mechanics", here's what's written in my book, with my problems ...
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### When do thermal and chemical equilibrium not coincide?

What is an example for a system, which is in chemical equilibrium, but not in thermodynamical equilibrium? And what about the other way around? It seems to me, that as long as Parameters like ...
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### Difference between state variable and state function in thermodynamic?

I think these two terms are sometimes used as synonyms and other times as different concepts. Can some explain what is the difference? An example would be good. Also, can state variable be the state ...
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### What is the equivalent of a eutectic point for a (two-liquid)+solids system?

I deal with complex phase equilibria involving solid and liquid phases at high temperature. Below is a simplified temperature-XH2O diagram for a multicomponent system at constant pressure. (Note: ...
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### Pumping water from a closed column. Will the column get empty or just circualte?

As a picture is worth a thousand words, here is my problem: It is a closed water column with a bit of air in the top section. If I run the pump and make the water flow from IN to OUT, Will the ...
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### Conservation of Energy of two charged blocks connected by light spring

Two identical blocks resting on a frictionless, horizontal surface are connected by a light spring having a spring constant $k = 100 N/m$ and an unstretched length $L_i=0.400 m$. A charge $Q$ is ...