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Questions tagged [definition]

The definition tag is used in situations where the question is either about how some term or concept is defined or where the validity of an answer depends on a subtle definition of some term or concept used in the question.

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1answer
18 views

What is the physical meaning of static and velocity pressure?

According to Bernouli's equation, the static pressure inside a pipe with streamline and steady flow decreases with fluid's velocity. One explanation to rationalise this phenomenon is because velocity ...
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0answers
10 views

Quantum synchronization description used in paper

In the paper "Steady-state spin synchronization through the collective motion of trapped ions" it states the following: "Steady-state synchronization of atomic dipoles forms the foundation for ultra-...
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2answers
57 views

Definition of the spherical harmonics do not agree

I Griffiths' Introduction to quantum mechanics, the spherical harmonics are defined as $$Y_l^m(\theta,\phi) = \epsilon\sqrt{\frac{2l+1}{4\pi} \frac{(l-|m|)!}{(l+|m|)!}}e^{im\phi} P_l^m(\cos \theta)$$ ...
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1answer
45 views

How might I write an operational definition for Vertical and/or Horizontal? [on hold]

WARNING: PLEASE DO NOT GIVE ME A DEFINITION, JUST TELL ME HOW I CAN DO THIS. I really don't want to have to rely on a forum website to tell me how I can solve my physics homework, but I honestly have ...
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0answers
36 views

What is a fuzzy space?

Can someone give a down-to-earth explanation of what is a fuzzy space? (As known from M-theory and noncommutative geometry)
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1answer
41 views

How do we quantify this kind of information in Quantum Mechanics?

Let a quantum system be given with Hilbert space $\mathscr{H}$. Suppose it is described by the quantum state $\rho$, i.e., a density matrix. We can define the Von-Neumman entropy as $$S(\rho)=-\...
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3answers
57 views

Meaning and significance of the Levi-Civita symbol

I am recently reading Sean Carroll's Spacetime and Geometry: An introduction to General relativity. I am much of a beginner but am really curios to learn about GR. In the first chapter, after ...
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0answers
14 views

How to find inertial frame of reference? [duplicate]

How do you find an inertial frame of reference or what you can regard as it? How do you evaluate the difference between a given frame and an inertial frame?
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0answers
27 views

What exactly is a nonequilibrium quantum state?

How do we define a quantum equilibrium and nonequilibrium state? And how are these two different from each other?
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2answers
56 views

Definition of velocity in classical mechanics

Let $(r_1,r_2,r_3)$ be the coordinates of a particle $r$ in the coordinate system $\phi$. Let $\{\hat{e_1},\hat{e_2},\hat{e_3}\}$ be the coordinate basis of $\phi$. Why do we define the velocity $v$ ...
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5answers
720 views

Paradox in the definition of work [duplicate]

I'm trying to understand the Oberth effect and came across this paragraph that seems crazy to me: when the rocket moves, its thrust acts through the distance it moves. Force multiplied by distance ...
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1answer
66 views

Is there a way to quantify “porousness”? [closed]

With respect to modeling acoustics, I'm interested in a system that quantifies how porous a material is. Does such data exist in physics? The intent is to estimate the cutoff frequency of sound waves ...
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1answer
294 views

Generalized divergence of tensor in GR

Although I've forgotten the proof (and cannot find it in, say, Carroll's book), the following formula holds for the covariant divergence in general relativity: $$\nabla_{\mu} A^{\mu} = \frac{1}{\sqrt{...
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0answers
33 views

Difference between ideal gas and perfect gas?

I tried seaching it in the internet but I didn't understand anything. I know the definition of ideal gas. But I don't understand what perfect gas is. Can you help me, please?
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0answers
19 views

What is the precise definition of phase? [duplicate]

the phase of matter can be understood by landau argument and/or with topology. how ever, I cant find anywhere a precise definition of what phase is?
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4answers
78 views

Velocity definition in quantum mechanics?

What is the definition of velocity in quantum mechanics? is it an operator?
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1answer
37 views

What is the (quantum) definition of thermal equilibrium?

What is the necessary condition for a quantum system to be in thermal equilibrium? The quantum systems I have in mind are a bunch of cold atoms, of photons in a cavity. Does a system need to obey a ...
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1answer
88 views

What does existence mean in physics? [closed]

I would like to understand what we mean when we say, within a conversation about physics, that some structure or some phenomenon exists. I tried searching a couple of online physics dictionaries ...
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1answer
64 views

What is the real definition of density? [closed]

What does density really mean? Everyone just provides me with the equation of finding density whenever I ask this question.
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1answer
68 views

Is there any moon without its planet?

Is there any planet without its star? Is there any moon or any planet wandering in outer space without a definite orbit? (The name moon or planet used here serves only for size and spherical shape ...
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1answer
78 views

Definitions of $\vec{B}$ and $\vec{H}$

From here, I have got the definition of $\vec{H}$. However even in wikipedia and other sites, I cannot find a definition for $\vec{B}$ which shows its similarity with $\vec{H}$. Similarity: I know ...
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9answers
5k views

Why does work depend on distance?

So the formula for work is$$ \left[\text{work}\right] ~=~ \left[\text{force}\right] \, \times \, \left[\text{distance}\right] \,. $$ I'm trying to get an understanding of how this represents energy. ...
2
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0answers
76 views

Why there are these two angular coordinates in a general spacetime?

In the paper "Gravitational Waves in General Relativity. VIII. Waves in Assymptotically Flat Space-Time" by R. Sachs, the author constructs a system of coordinates in section 2 for a general spacetime....
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2answers
67 views

What is a 'spacelike surface' in relativity?

I am studying Noether's theorem in field theory and I am not understanding what spacelike-surfaces mean. I will reproduce the bit of the argument below that contains the term "spacelike-sufaces" in ...
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2answers
127 views

If an object moves at constant speed, does it necessarily have constant velocity? [closed]

If an object moves at constant speed, does it necessarily have constant velocity?
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2answers
51 views

What exactly is meant by magnetic poles?

If I have a magnet, which of the following is really called magnetic pole and why?: $(1)$ Half of the volume of the magnet - north pole, other half volume - south pole. $(2)$ Area at one end of the ...
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3answers
102 views

Exergy, entropy and energy quality

I'm teaching a class on the subject of "energy quality". I'm supposed to tell them that energy quality is a measure of how easily (effective I guess) the type of energy can be converted to different ...
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2answers
56 views

What are chromoelectric and chromomagnetic fields?

Are they the normal elctric and magnetic fields from Maxwell fields? Or are they just the corresponding components from $G_{\mu\nu}^a$ (the gluon fields), say chromoelectric fields are simply $G_{0i}^...
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1answer
38 views

Pauli Matrix's eigenvector formalism

In the Wikipedia page for the Pauli matrices, there is a list of the eigenvectors of the Pauli matrices. Notice at $\sigma_y$, it's eigenvectors are $\begin{pmatrix} 1 \\ \pm i \end{pmatrix}$ but not ...
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2answers
130 views

Definition of magnetic field

Can we define magnetic field at a point as: Force on a point magnetic north pole at that point divided by its pole strength. Anything wrong in this definition? (The concept of point magnetic pole ...
2
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1answer
21 views

Experimental physics sensitivity vs observation

what does it mean when experimental physicists refer to an experiments sensitivity? What in general is it compared to (e.g. observation?) Is there a formal definition or does it widely depend on the ...
3
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3answers
454 views

All central forces are conservative forces, but are all conservative forces central forces?

I have just been introduced to the concept of central forces, and to the fact that they are per definition conservative forces. I have looked up several examples of central forces (gravity, electric, ...
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2answers
103 views

Meaning of the word 'canonical' in physics

I often encounter the term canonical in my study of physics. What does it mean? There is canonical momentum, canonical transformations and I have even heard the phrase 'proving something more ...
1
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1answer
36 views

What is the definition of the moment of inertia tensor?

I can find volume integrals for the moment of inertia in 2D and 3D, but is there a definition that works in an arbitrary number of (spatial!) dimensions?
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0answers
58 views

Phase space meaning [duplicate]

In the field of medical physics, specifically in monte carlo simulation of radiation beams produced by electron accelerators, people call ‘phase space’ to a file that contains the data of a large ...
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3answers
97 views

Minkowski spacetime conformal infinity: why not allow the full range of $R$?

Let $(\mathbb{R}^4,\eta)$ be Minkowski spacetime. We want to describe infinity as a place in some bigger manifold containing Minkowski spacetime. The idea is to work with null incoming/outgoing ...
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10answers
5k views

Quantum made easy: so what *is* quantum mechanics all about? [closed]

Being a physics grad student, I got used to the weird concepts behind quantum mechanics (used to doesn't mean I fully understand it though). What I mean is that I'm not surprised anymore by the fact ...
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1answer
66 views

Tracing over a Fock space?

Suppose you have a bosonic Fock space with a vacuum $|0\rangle$. A particular state is labeled by the parameter $N \in \mathbb{Z}$. You can construct states like $$ | n_{N} \rangle = \frac{ \left( \...
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1answer
24 views

Are angles for vectors always measured from the horizontal?

I have a vector which has been stated to have a force of: 96.0 N at $51.3^\circ$. I had a different answer because I was measuring my angles from the north. By default if an angle is given by itself ...
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3answers
1k views

What does the 'displacement' refer to in the definition of work?

The definition of work given in books is The work is said to be done by a force on a body, when the body is moved by the force through some 'displacement'. Now let a body of mass $m$ at rest. When a ...
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2answers
127 views

Distinction between spacetime and vacuum

In GR, is spacetime just a mathematical abstraction, and in reality, it's the vacuum - whatever that is!!- is what curves, bends, and warps? In other words, is the distinction between spacetime and ...
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0answers
28 views

Definition of Galilean structure in Arnold's book?

I am reading Arnold's Mathematical Methods of Classical Mechanics. He quickly introduces the notion of Galilean structure. The universe is defined as the affine space $A^4$ and time is defined as a ...
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0answers
46 views

Is Force a fundamental concept just like position and time? [duplicate]

The doubt arises from my understanding of Newton's second law. First I though it was just the definition of force but it actually states the relation between force, acceleration and mass (which can be ...
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2answers
531 views

What does “degrees of freedom ” mean in classical mechanics?

The definition I come up with is 3M - N ...where N is the number of constraints. I assume M is the number of distinct points. In what context is it used ? According to Wiki it says "an ...
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3answers
2k views

Definition of an operator in quantum mechanics

In J.J. Sakurai's Modern Quantum Mechanics, the same operator $X$ acts on both, elements of the ket space and the bra space to produce elements of the ket and bra space, respectively. Mathematically, ...
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3answers
140 views

In general, how are representations used in physics?

I want is a basic overview, if there is one, of the meaning (and purpose) of the word representation in general terms. I have looked up sources such as Particle Physics and Representation Theory, but ...
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2answers
31 views

Distinction between “assumption”, and “build in definitions” - What is considered an assumption in a physical model?

This question I want to deal with the basics of modelling a physical theory: Let's say we start with observing in the world (be it little bubbles in the water, a particle moving, a pattern in the ...
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1answer
50 views

Inertial and gravitational Mass

Why definition of mass is not stated as " the property of object to change radius of curvature of space time fabric is called mass"
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2answers
48 views

Work Energy misunderstanding

Suppose I have to displace a body to a height $h$ and I applied force more than its weight. So, there will be an acceleration on the body. Then the body will reach the height $h$. Then I let the ...
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5answers
136 views

Why is voltage described as potential energy per charge?

Voltage is often called an electromotive force since it causes a flow of charge. However, it is described in terms of Joules per Coulomb or Potential Energy per Charge. Question: How does the ...