The definition tag is used in situations where the question is either about how some term or concept is define or where the validity of an answer depends on a subtle definition of some term or concept used in the question.
0
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2answers
39 views
Viewing a reference frame from inside of it?
Can one observe one's own frame of reference without moving out side the frame?
For example, if you are on a train and you throw ball in the air up and down.
When you are on the train (frame 1) you ...
12
votes
2answers
403 views
What accounts for the discrepancies in my calculations of year lengths?
A common exercise in many introductory astronomy texts is to use the lengths of various kinds days to calculate the approximate length of the corresponding year.
For example, ratio $k$ of the length ...
-1
votes
0answers
142 views
What's the definition of Heat Flux Vector? [closed]
I am not content with the statement that heat flux is the quantity per second per area. I deem that there are many mathematics details omitted here. I want somebody help to express it clearly.
Fist ...
2
votes
0answers
35 views
Regular initial data
I have a very basic question. What exactly is meant by "regular" initial data in general relativity? Does it mean smooth? at least $C^{2}$?
All literature on the subject just uses this term without ...
0
votes
1answer
51 views
What is the difference between Feshbach resonance and Fano resonance?
What is the interpretation of "resonance" in Feshbach resonance and Fano resonance?
What is the difference of Feshbach resonance and Fano resonance?
1
vote
1answer
56 views
Difference between Poynting vector and energy flux density?
Are those two terms the same, or...?
My book says that the Poynting vector is an energy flux density given by:
$$\mathbf{S} = \frac{1}{\mu_{0}}(\mathbf{E} \times \mathbf{B})$$
So that alone should ...
4
votes
2answers
80 views
What is the gamma five matrix $\gamma_5$?
This Wikipedia page explains that for each of the four main gamma matrices $\gamma^{\mu}$, you can find the covariant matrices $\gamma_{\mu}$ with the equation $\gamma_{\mu} = ...
1
vote
1answer
23 views
What is a “lateral fringe displacement”?
I encountered the term "lateral fridge displacement" in my optics homework for a problem about inserting a thin plate of glass over one of Young's double splits.
So what does "lateral fringe ...
8
votes
2answers
139 views
Why are they called “cyclic” coordinates?
In Lagrangian formalism, when $\frac{\partial L}{\partial q} = 0$, the coordinate $q$ is called cyclic and a corresponding conserved quantity exists. But why is it called cyclic?
-3
votes
0answers
72 views
What do $\hat{h}$ and $\hat{g}$ mean in the Ising model?
I'm an undergraduate student learning the Ising model, but can't understand what the unit vectors $\hat{h}$ and $\hat{g}$ are, in the general interaction between two spins.
...
2
votes
1answer
84 views
Question on Radiance equation
The radiance equation is
$$
L = \frac{d}{dA} \frac{2(\phi)}{dW cos(\theta)} (watt/srm^2)
$$
where $\phi$ is the flux.
I am thinking, should not be the cosine term on the numerator instead of the ...
0
votes
3answers
82 views
How would you determine whether an object is at equilibrium? [closed]
How would you determine whether an object is at equilibrium or not?
What is the definition of equilibrium?
-3
votes
2answers
69 views
Do physics laws really apply to anything? [closed]
My professor said that a law was stated and announced as a law because it happens in our everyday life. He gave us an example of Newton's 3 laws. He said that walking possess 3 laws of Newton's. Is ...
2
votes
3answers
288 views
What does physics study? [closed]
Wikipedia definition: Physics (from Ancient Greek: φύσις physis "nature") is a natural science that involves the study of matter[1] and its motion through spacetime, along with related concepts such ...
1
vote
3answers
83 views
What is “first order“ and “second order” in time?
What is the meaning of the text quoted below?
In the physical world, if a system is described by an equation that is
first order in time, the system is general dissipative (has energy
loss). ...
1
vote
1answer
178 views
Definition of energy
What is the definition of energy $E$ given a dispersion relation $\omega=\omega(k)$ where $k=|\vec k|$ and $\omega$ is not necessarily linearly proportional to $k$? What about momentum $\vec p$?
This ...
2
votes
1answer
73 views
Hamiltonians, density of state, BECs
When working with Bose-Einstein condensates trapped in potentials, how can one tell what the density of state of a system of identical bosons given the Hamiltonian, $H$? (I have been told that it is ...
1
vote
1answer
77 views
A physical sense of an Inertial frame
Definition clarification needed, please: I am hoping to get physical sense of an "inertial frame".
Do inertial reference frames all have zero curvature for their spacetime?
So is an inertial frame ...
9
votes
0answers
144 views
Unitary quantum field theory
What do physicists mean when they refer to a quantum field theory being unitary? Does this mean that all the symmetry groups of the theory act via unitary representations? I would appreciate if one ...
3
votes
2answers
577 views
Why is a degree Celsius exactly the same as a Kelvin?
How on earth is it possible that the difference between two temperatures in Celsius and Kelvin is exactly the same. Given the historical definition of Celsius, I find it hard to believe that this is ...
4
votes
5answers
200 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 ...
2
votes
1answer
109 views
Doubts about the definition of mass
I'm having some problems understanding what are the possible definitons of mass and how they are related to each other.
In Classical Mechanics, we can distinguish between inertial and gravitational ...
2
votes
3answers
139 views
What is the general statistical definition of temperature?
Temperature in an isolated system is defined as:
$$\frac{1}{T} = -\frac{\partial{S(E,V,N)}}{\partial{E}} $$
But I wonder how one can generalize this to a random system.
Or for instance to a point in ...
0
votes
3answers
2k views
Difference b/w Kinetics & Kinematics w/concrete example
(I know whether I understand this or not doesn't matter much to my work & study but am just curious.)
I still can't differentiate in my head kinetics and kinematics (similar thread is found but ...
2
votes
0answers
101 views
Holonomy twisting
There is Witten's topological twist of standard SUSY QFTs with enough SUSY into Witten-type TQFTs. What is a holonomy twist?
0
votes
2answers
73 views
Definition of Static Electricity
The result of an imbalance of electrons between objects is called static electricity. It is called "static" because the displaced electrons tend to remain stationary after being moved from one ...
0
votes
2answers
144 views
Definition of electric charge and proper explanation
Is there a definition of electric charge and proper explanation of it?
It is said "Electric charge is the physical property of matter that causes it to experience a force when close to other ...
1
vote
2answers
65 views
Clarification regarding Newton's Third Law of Motion and why movement is possible [duplicate]
Newton's third law states that to every action, there is an equal and opposite reaction.
If that's the case, then how do things move at all? Shouldn't all applied forces be canceled by the equal and ...
3
votes
4answers
2k 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 ...
1
vote
0answers
65 views
Motivation For Definitions [closed]
I noticed in my physics textbook that we define certain relationships to be true. I can see how this is considerably helpful in deriving other relationships from these definitions; for instance, take ...
0
votes
6answers
1k views
What distinguishes between physics and chemistry? [closed]
What are the major differences between physics and chemistry? I know that they both study atoms, electrons and molecules, but what makes some topics part of one and some part of another?
1
vote
2answers
61 views
Electrostatic Potential Definition
In the book, Introduction to electrodynamics by David J. Griffiths, he introduces potential separately as a function and potential energy through that function.
How can potential be defined before ...
9
votes
4answers
238 views
Two planets in same orbit - not planets?
Let us pretend for a moment that there are two identical planets that are exactly opposite their star from each other and are the same distance from said star. (This would make them, at all times, ...
10
votes
5answers
2k views
What is an asterism compared to a constellation?
I'm doing an astronomy exam tomorrow and in the practice paper it asks for the difference between constellation and asterism. It seems asterism is a group of recognizable stars; however I thought that ...
3
votes
1answer
79 views
For how long must a molecule remain stable to be considered “stable”?
In the Star Trek: Voyager episode The Omega Directive, Seven of Nine says that the Borg synthesized a molecule which was "kept [] stable for one trillionth of a nanosecond before it destabilized". ...
1
vote
1answer
57 views
A sphere, a simple object?
In this video, the woman says that a sphere is a pretty simple object. What intrigues me is the use of a sphere for such a calculation. First of all, the sphere wouldn't be perfect as a perfect sphere ...
2
votes
2answers
3k views
What is the exact definition of center of gravity?
I've come across many definitions. Is it
1) The point from which the weight of the body acts, i.e., the point at which if the entire mass of the body is assumed to be concentrated, the gravitational ...
1
vote
1answer
86 views
How do you actually define an orbit?
How do you actually define an orbit?
I believe, Newtonian Mechanics describes an orbit as one object in free fall around another where projectile paths become elliptical. I think, Einstein describes ...
2
votes
2answers
3k views
Are all metals good conductor of electricity?
I am writing an article for kids, which is on conductors and insulators of electricity.
If I make a statement that "All metals are electrical conductors and all non-metals are electrical insulators" ...
1
vote
3answers
308 views
What is meant by potential energy for a particle in a field?
Potential energy is usually defined using a field and a particle that experiences the field force, as the work down in moving a unit particle from infinity to a position in that field.
But some ...
0
votes
0answers
81 views
Curvilinear abscissa confusion
How's exactly defined the curvilinear abscissa in kinematics? Surfing on the net I found different definitions:
a) Fixed a point $O$ and a direction, the curvilinear abscissa $s$ at a point $P$ is ...
0
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0answers
202 views
How is the term “Born level” usually defined?
How is the term "Born level" usually defined, e.g. in talking about the $pp\to Z/\gamma^*\to e^+e^-$ cross section at Born level?
4
votes
3answers
528 views
Why is 1 AU the distance between the Sun and the Earth?
Why 1 AU is defined as the distance between the Sun and the Earth? (approximately if you like to be precise)
An astronomical unit (abbreviated as AU, au, a.u., or ua) is a unit of length ...
3
votes
1answer
51 views
Does a reference or classification standard for altitude classifications of geocentric orbits exist?
I'm looking for a primary reference of the altitude classifications of geocentric orbits (LEO, MEO, GEO, HEO), but I was not able to find something so far.
I noticed that there is very different ...
5
votes
1answer
7k views
Is the moon a planet?
Can our moon qualify as a planet? With regard or without regard to the exact definition of the planet, can the moon be considered as planet as Mercury, Venus and Earth etc. not as the satellite of the ...
8
votes
6answers
888 views
What is a tensor?
I have a pretty good knowledge of physics but couldn't understand what a tensor is. I just couldn't understand it, and the wiki page is very hard to understand as well. Can someone refer me to a good ...
4
votes
1answer
99 views
Does the Kelvin have a rigorous definition?
From Wikipedia:
The kelvin is defined as the fraction 1⁄273.16 of the thermodynamic temperature of the triple point of water.
That presupposes that we can take a fraction of temperature.
Now, ...
0
votes
0answers
20 views
Rice Allnatt distribution function
Can anyone give me an article of which explains Rice Allnatt distribution function or can you explain the function here?
4
votes
1answer
45 views
Vesta dwarf planet status
Now that we have close-up photos of Vesta, which the IAU had previously said was a candidate dwarf planet, when is the IAU going to decide the issue? Personally, Vesta doesn't look round enough to me.
...
7
votes
4answers
3k views
What is sound and how is it produced?
I've been using the term "sound" all my life, but I really have no clue as to what sound exactly is or how it is created. What is sound? How is it produced? Can it be measured?
