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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111 votes
6 answers
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Are Newton's "laws" of motion laws or definitions of force and mass?

If you consider them as laws, then there must be independent definitions of force and mass but I don't think there's such definitions. If you consider them as definitions, then why are they still ...
user5402's user avatar
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115 votes
11 answers
49k views

What Is Energy? Where did it come from?

The simplistic undergrad explanation aside, I've never really understood what energy really is. I've been told that it's something when converted from one kind of something to another kind, or does ...
Anna's user avatar
  • 1,697
73 votes
12 answers
24k views

What is a tensor?

I have a pretty good knowledge of physics, but couldn't deeply understand what a tensor is and why it is so fundamental.
0x90's user avatar
  • 3,316
60 votes
3 answers
27k views

What is the difference between implicit, explicit, and total time dependence, e.g. $\frac{\partial \rho}{\partial t}$ and $\frac{d \rho} {dt}$?

What is the difference between implicit, explicit, and total time dependence, e.g. $\frac{\partial \rho}{\partial t}$ and $\frac{d \rho} {dt}$? I know one is a partial derivative and the other is a ...
CuriousAutomotiveEngineer's user avatar
52 votes
4 answers
36k views

What's the difference between the five masses: inertial mass, gravitational mass, rest mass, invariant mass and relativistic mass?

I have learned in my physics classes about five different types of masses and I am confused about the differences between them. What's the difference between the five masses: inertial mass, ...
QEntanglement's user avatar
11 votes
4 answers
11k views

Define Pressure at A point. Why is it a Scalar?

I have a final exam tomorrow for fluid mechanics and I was just looking over the practice exam questions. They do not provide solutions. But pretty much I have to define pressure at a point and also ...
Greg Harrington's user avatar
25 votes
11 answers
541k views

What is the difference between weight and mass?

What is the difference between the weight of an object and the mass of an object?
Hobbs's user avatar
  • 675
21 votes
1 answer
5k views

What is the definition of how to count degrees of freedom?

This question resulted, rather as by-product, the discussion on how to count degrees of freedom (DOF). I extend that question here: Are necessary1 derivatives such as velocities counted as individual ...
Tobias Kienzler's user avatar
92 votes
9 answers
50k views

Are matrices and second rank tensors the same thing?

Tensors are mathematical objects that are needed in physics to define certain quantities. I have a couple of questions regarding them that need to be clarified: Are matrices and second rank tensors ...
Revo's user avatar
  • 17k
48 votes
14 answers
27k views

What is entropy really?

On this site, change in entropy is defined as the amount of energy dispersed divided by the absolute temperature. But I want to know: What is the definition of entropy? Here, entropy is defined as ...
user avatar
31 votes
2 answers
6k views

What is the Wilsonian definition of renormalizability?

In chapter 23.6, Schwartz's quantum field theory book defines renormalizability as follows, paraphrasing a bit for brevity: Consider a given subset $S$ of the operators and its complement $\bar{S}$....
knzhou's user avatar
  • 102k
120 votes
3 answers
26k views

Differentiating Propagator, Green's function, Correlation function, etc

For the following quantities respectively, could someone write down the common definitions, their meaning, the field of study in which one would typically find these under their actual name, and most ...
Nikolaj-K's user avatar
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34 votes
4 answers
139k views

What is the difference between center of mass and center of gravity?

What is the difference between center of mass and center of gravity? These terms seem to be used interchangeably. Is there a difference between them for non-moving object on Earth, or moving objects ...
DarkLightA's user avatar
  • 1,422
34 votes
4 answers
21k views

What exactly is a virtual displacement in classical mechanics?

I'm reading Goldstein's Classical Mechanics and he says the following: A virtual (infinitesimal) displacement of a system refers to a change in the configuration of the system as the result of any ...
Gold's user avatar
  • 35.9k
33 votes
11 answers
35k views

What is the difference between electric potential, electrostatic potential, potential difference (PD), voltage and electromotive force (EMF)?

This is a confused part ever since I started learning electricity. What is the difference between electric potential, electrostatic potential, potential difference (PD), voltage and electromotive ...
new her's user avatar
  • 439
12 votes
2 answers
15k views

Understanding terms Twist and Wrench

In kinematics, physics and especially robotics, we often encounter the terms Twist and Wrench. Twist is (LinearVelocity, AngularVelocity) and Wrench is (Force, Torque). The reason I'm confused is I ...
Shital Shah's user avatar
7 votes
2 answers
3k views

What is charge actually? How to define it? [closed]

Is charge of something for (e.g.) an electron related to electromagnetic space if it exists due to energy, due to which it may have mass? I don't know about quantum mechanics or advanced particle ...
Vishvajeet Patil's user avatar
22 votes
2 answers
3k views

What's the most fundamental definition of temperature?

What's the most fundamental definition of temperature? Is it the definition concern about average energy, number of micro states, or what? By "fundamental", I mean "to be applied" in such general ...
anonymous67's user avatar
  • 1,493
45 votes
2 answers
2k views

Identification of particles and anti-particles

The identification of an electron as a particle and the positron as an antiparticle is a matter of convention. We see lots of electrons around us so they become the normal particle and the rare and ...
John Rennie's user avatar
35 votes
2 answers
9k views

What is the definition of a timelike and spacelike singularity?

What is the definition of a timelike and spacelike singularity? Trying to find, but haven't yet, what the definitions are.
user23071's user avatar
  • 353
19 votes
7 answers
8k views

Definitions and usage of Covariant, Form-invariant & Invariant?

Just wondering about the definitions and usage of these three terms. To my understanding so far, "covariant" and "form-invariant" are used when referring to physical laws, and these words are ...
Josh's user avatar
  • 542
14 votes
2 answers
5k views

How do you define the resonance frequency of a forced damped oscillator?

Consider a forced, damped harmonic oscillator $$\ddot{\phi} + 2\beta \dot{\phi} + \omega_0^2 \phi = j(t) \, .\tag{1}$$ If I pick a sinusoidal driving force $j(t) = A \cos(\Omega t)$, I find $$\phi(t) =...
DanielSank's user avatar
  • 24.5k
44 votes
3 answers
8k views

History of interpretation of Newton's first law

Nowadays it seems to be popular among physics educators to present Newton's first law as a definition of inertial frames and/or a statement that such frames exist. This is clearly a modern overlay. ...
user avatar
52 votes
4 answers
12k views

What's the real fundamental definition of energy?

Some physical quantities like position, velocity, momentum and force, have precise definition even on basic textbooks, however energy is a little confusing for me. My point here is: using our ...
Gold's user avatar
  • 35.9k
37 votes
8 answers
8k views

Is the Big Bang defined as before or after Inflation?

Is the Big Bang defined as before or after Inflation? Seems like a simple enough question to answer right? And if just yesterday I were to encounter this, I'd have given a definite answer. But I've ...
Jim's user avatar
  • 24.5k
29 votes
1 answer
5k views

Operator Ordering Ambiguities

I have been told that $$[\hat x^2,\hat p^2]=2i\hbar (\hat x\hat p+\hat p\hat x)$$ illustrates operator ordering ambiguity. What does that mean? I tried googling but to no avail.
Greta's user avatar
  • 471
24 votes
6 answers
73k views

Why is the speed of light defined as 299792458 m/s?

Why is the speed of light defined as $299792458$ $m/s$? Why did they choose that number and no other number? Or phrased differently: Why is a metre $1/299792458$ of the distance light travels in a ...
Xplane's user avatar
  • 421
7 votes
1 answer
3k views

What is meant by a local Lagrangian density?

What is meant by a local Lagrangian density? How will a non-local Lagrangian look like? What is the problem that we do not consider such Lagrangian densities?
SRS's user avatar
  • 26.4k
10 votes
2 answers
4k views

In layman's terms, what is a quantum fluctuation?

What causes it and how does it occur? If you do post some mathematics, please explain what each term means too please.
Matthew's user avatar
  • 101
3 votes
6 answers
2k views

Mathematical Reasoning for Fluid Pressure as a Scalar

This question from a while ago and answers/comments to this question from earlier today both make heavy mention of the fact that fluid pressure is a scalar. Although this information was surprising ...
Sean's user avatar
  • 2,963
33 votes
11 answers
9k 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. ...
Dominic Roy-Stang's user avatar
32 votes
15 answers
5k views

What is the fundamental definition of force?

As I pick up more physics I see that the definitions of force commonly provided in books and classrooms are misleading. "A force is a push or pull." This seems to be a "correct" ...
Ethan Dandelion's user avatar
9 votes
3 answers
4k views

What is quantum entanglement? [closed]

What is quantum entanglement? Please be pedagogical. Edit: I have updated my background under my profile.
Amir Rezaei's user avatar
5 votes
2 answers
3k views

Transpose of (1,1) tensor

When we transpose a (1,1) tensor, shall we simply switch the two indices while keeping their upper/lower positions or switch them and also switch their upper/lower positions? In general, would the ...
Shadumu's user avatar
  • 1,211
4 votes
4 answers
1k views

Equivalence of two definitions of proper time in special relativity

According to Wikipedia, proper time along a timelike world line is defined as the time as measured by a clock following that line. This makes sense to me, but my book defines proper time in the ...
user avatar
65 votes
5 answers
23k views

What does it mean for a Hamiltonian or system to be gapped or gapless?

I've read some papers recently that talk about gapped Hamiltonians or gapless systems, but what does it mean? Edit: Is an XX spin chain in a magnetic field gapped? Why or why not?
Jordan's user avatar
  • 1,583
11 votes
4 answers
2k views

Nomenclature: Yang-Mills theory vs Gauge theory

If you're writing about a theory with Yang-Mills/Gauge fields for an arbitrary reductive gauge group coupled to arbitrary matter fields in some representation, is it best to call it a Yang-Mills ...
Simon's user avatar
  • 3,513
9 votes
2 answers
2k views

Why do we still not have an exact (constants-based) definition for a kilogram?

I read that there is an effort to define a kilogram in terms that can exactly be reproduced in a lab. Why has it taken so long to get this done? It would seem this should be fairly important. Edit: ...
Ted Ersek's user avatar
  • 531
5 votes
1 answer
1k views

Definition of Local Function

Now a days I am studying Srednicki's QFT book. In its third chapter it is written that Any local function of φ(x) is a Lorentz scalar, [...] . Now my question is: What is a local function?
rainman's user avatar
  • 2,993
60 votes
10 answers
9k 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 ...
Jasmeru's user avatar
  • 1,138
36 votes
5 answers
82k views

What's the difference between Fermi Energy and Fermi Level?

I'm a bit confused about the difference between these two concepts. According to Wikipedia the Fermi energy and Fermi level are closely related concepts. From my understanding, the Fermi energy is the ...
Eddy's user avatar
  • 517
17 votes
3 answers
14k views

Definition of the $Q$ factor?

According to Wikipedia, the $Q$ factor is defined as: $$Q=2\pi\frac{\mathrm{energy \, \, stored}}{\mathrm{energy \, \,dissipated \, \, per \, \, cycle}}.$$ Here are my questions: Does the energy ...
user avatar
3 votes
4 answers
10k views

Which one true in First law of thermodynamics: $Q = \Delta U \pm W = \Delta U \pm p\Delta V$ or $\Delta U= \Delta Q + \Delta W $?

Which one true in First law of thermodynamics: $Q = \Delta U \pm W = \Delta U \pm p\Delta V$? (where $\Delta U$ is change of internal energy, $W$ work made by system and $Q=cm\Delta T $ heat made by ...
alvoutila's user avatar
  • 735
1 vote
1 answer
856 views

How to know if something is a primitive concept, a law, a definition or a theorem

Some basic Physics books are often misguiding in the sense that they don't make clear whether something is a primitive concept, a law, a definition or a theorem. This is often a little confusing. I've ...
Gold's user avatar
  • 35.9k
35 votes
7 answers
253k views

What is the difference between diffraction and interference of light?

I know these two phenomena but I want to know a little deep explanation. What type of fringes are obtained in these phenomena?
Abdul Wajid Lakhani's user avatar
6 votes
4 answers
745 views

Potential energy definition

I am too much confused about the definition of potential energy. I am giving two different definitions below: The negative of work done by conservative force to bring a mass from infinity to the ...
user avatar
44 votes
12 answers
87k views

What do people actually mean by "rolling without slipping"?

I have never understood what's the meaning of the sentence "rolling without slipping". Let me explain. I'll give an example. Yesterday my mechanics professor introduced some concepts of rotational ...
pppqqq's user avatar
  • 4,594
41 votes
5 answers
8k views

Hilbert space vs. Projective Hilbert space

Hilbert space and rays: In a very general sense, we say that quantum states of a quantum mechanical system correspond to rays in the Hilbert space $\mathcal{H}$, such that for any $c∈ℂ$ the state $\...
user929304's user avatar
  • 4,645
33 votes
5 answers
6k views

What is the definition of temperature, once and for all? [duplicate]

Can someone please explain to me what the formal definition of temperature is? Neither my textbook, nor my professor, nor any of the online sources I've checked are able to give me a proper ...
math_lover's user avatar
  • 4,536
24 votes
4 answers
2k views

Definition of mass

In high school physics, I was taught that mass was just how much "stuff" or matter there is in an object. However, now that I am learning physics again in college, I am taught that mass of an object (...
baker's user avatar
  • 517

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