# 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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### 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 ...
2answers
2k 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?
2answers
68 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 ...
3answers
239 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 ...
2answers
170 views

3answers
167 views

### Linear and non-linear systems

When I read about the superposition principle, it says that it works only on linear systems, my problem is that I cannot really understand the difference between a linear and a non-linear system. I ...
1answer
53 views

### Is a displacement a vector, a line segment, or something else?

It probably seems ridiculously naive of me to ask such a basic question, but I have a need to use accurate language. Typically, I think of a displacement as a directed line segment whose end-points ...
1answer
528 views

### What is difference between rolling, kinetic and static friction? [duplicate]

If there is a toy train on the floor moving linearly then would the friction acting be rolling or static?
1answer
646 views

### Stellar Isochrones, what are they?

So I have been reading and I am trying to understand what stellar isochrones are and what relationship they have to the Hertzsprung-Russell diagram. My understanding at the moment is that, the ...
1answer
63 views

### Is mass a property or a quantity?

From definition mass is the amount of matter that an object has. So why we call it a property? What is the difference between a property and a quantity in the dictionary of physics? Wiki says physical ...
0answers
29 views

### What is the difference between a defining equation and a word definition? [closed]

For example, can we define kinetic energy as the product of 1/2mu^2 and also as the energy that an object has due to motion ? are these definitions equivalent ?
2answers
97 views

### What is the definition of force? [closed]

Wikipedia says a force is any interaction that, when unopposed, will change the motion of an object. But in a site i find this , force is a "quantitative" description of an interaction that causes a ...
1answer
74 views

### What is the definition (not on the basis of energy level) of scattering state?

A question is given in Griffiths. Consider the "step" potential V(x) which rises to Vo (at x=0) which is 0 for x<0. For the scattering states and E less than Vo, the reflection coefficient comes ...
1answer
72 views

### What is the difference between dual CT and spectral CT?

I'm reading about different types of CT scans and I'm frequency seeing the words "Dual x-ray CT" and "Spectral x-ray CT" being used to describe different things but I can't find a single explanation ...
2answers
859 views

### Why can't we define fundamental unit of mass? [duplicate]

In my physics textbook of class $11^{th}$ The kilogram was defined as :- mass of the platinum-iridium standard cylinder kept at Sevre's France But this isn't a proper and scientific definition ...
1answer
502 views

### What is the difference between thermodynamic free energies and the Landau free energy?

How and why is the Landau free energy any different from thermodynamic free energies? It is written on page 140 of Nigel Goldenfeld's book Lectures on Phase Transitions and The Renormalization Group ...
2answers
240 views

### Difference between physicist's vector and mathematician's vector

Mathematically a vector is defined as an element of vector space which obeys certain properties. While reading about the special theory of relativity, I came to know about another definition of ...
2answers
357 views

### What is the precise relationship between enthalpy and heat?

I am struggling to resolve the definitions of heat and enthalpy of reaction. This is based on research I have been doing around the production of course materials. I have found the following ...
1answer
81 views

### Cross product of vectors

I am unable to comprehend the following lines given in page 657 of Shankar's Principles of Quantum mechanics: One tricky point: The cross product is defined to be orthogonal to the vectors in the ...
1answer
161 views

### Adjoint of Weyl Spinor

Given a (Dirac), spinor in the Weyl basis, $\psi = \begin{pmatrix} \psi_{L}\\ \psi_{R} \end{pmatrix}$ , where $\psi_{L}$ and $\psi_{R}$ are Weyl spinors we define the adjoint of the Dirac spinor as; ...
0answers
54 views

### Clarifying the physical significance of the attenuation coefficient

I have seen several sources define the attenuation coefficient as the fraction of a beam's intensity which is attenuated per unit distance e.g. "the fraction of attenuated incident photons in a ...
3answers
303 views

### What is the “displacement” of the object in the definition of work?

Work in physics is mathematically defined as force $F$ applied on an object multiplied by the displacement $d$ it covers in the direction of the force. In a system where, a restrictive force exists ...
3answers
125 views

### Why are electric and magnetic susceptibilities defined in such an unintuitive way?

When I studied electromagnetism the $\mathbf{B}$ and $\mathbf{E}$ fields were introduced as fundamental quantities to me, and the $\mathbf{H}$ and $\mathbf{D}$ fields were introduced as something of ...
2answers
70 views

### Confused with heat as a form of energy [duplicate]

I have quite a simple question. Energy can be defined as capacity to do work. But I have read When energy is exchanged between thermodynamic systems by thermal interaction, the transfer of energy ...
2answers
36 views

### Definition of a system in flow processes

In classical thermodynamics the state of a system (pressure, temperature etc.) can only be specified if it is in equilibrium. My understanding of equilibrium leads me to believe that the state is ...
4answers
499 views

### Difference between pure quantum states and coherent quantum states

In the post What is coherence in quantum mechanics? and the answer by udrv in this post it seems to imply that a pure quantum state and coherent quantum state are the same thing since any pure state ...
2answers
485 views

### Definition of physical quantities

Physical quantities are often defined in textbooks as measurable quantities. I find this definition confusing. For example, if you think about it, the number of clothes in a cupboard is also a ...
0answers
386 views

### Local and Non-Local Potentials

Can anybody explain the difference and concepts between local and non-local potentials in light of quantum mechanics?