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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3answers
90 views

Defining center of mass in the presence of curvature [on hold]

The definition of center of mass in most text books will look something like $\frac{\sum m_k r_k}{\sum m_k}$ or in the continuous case, $\frac{1}{\int \rho(r) dr}\int \rho(r) r dr$. However, this ...
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1answer
47 views

When is a flow a shear flow?

Let us say I have a velocity profile of a flow: $$\vec u= \begin{pmatrix} v_x \\ v_y \\ v_z \end{pmatrix}$$ Under what circumstances would this be called a shear flow? And can we say anything for an ...
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2answers
73 views

SI Base Unit definition of mass - obsolete?

According to the formal definition of the SI Base unit of mass, the kilogram, it is stated that "The kilogram is the unit of mass; it is equal to the mass of the international prototype of the ...
0
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1answer
65 views

Definition of a meson? [closed]

I am looking for a definition of a meson that does not include the quark model. After some research I have come across this definition: A meson is a particle that is (1) believed to be ...
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0answers
27 views

Symmetries in quantum systems - Wigner theorem and commutation with Hamiltonian

I was reading about symmetries in quantum systems and found (at least) two different definitions. According to Wigner's theorem symmetry transformation (of a quantum system) is a bijective map between ...
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0answers
20 views

LHCb Peaking Background

I was reading a paper on the estimation of the branching fraction of B+ to K pi pi gamma. I was wondering whether you could clarify the meaning of peaking background. Thanks!
4
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3answers
580 views

Is a theory the same as a hypothesis? [duplicate]

“Any physical theory is always provisional, in the sense that it is only a hypothesis: you can never prove it.” Excerpt from Stephen Hawking, A Brief History of Time, iBooks. So does that ...
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1answer
67 views

Why don't we call string, inflation theories just hypotheses?

These theories predict multiverse and multiverse is not falsifiable So Is it more accurate to call them just hypotheses?
3
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0answers
69 views

What's the difference between hypothesis, theory and law? [closed]

What's the difference between hypothesis, theory and law? I think I'm confused now because I exactly learned the misconception one as explained below in high school. According to this university's ...
0
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1answer
84 views

Why is mass times position not used in physics? [closed]

The momentum $\vec{p}=m \vec{v}$ plays a very prominent role in physics. Why is the same not true for the quantity $\vec{q}=m \vec{r}$? E.g. one can write angular momentum as $\vec{L} = \vec{r} \...
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0answers
26 views

Is there a standard in the manner in which significant figures are used?

I have always understood significant figures to be those figures which we know with certainty. Wikipedia (https://en.wikipedia.org/wiki/Significant_figures) provides a related but less rigid ...
1
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0answers
62 views

Definition of heat that does not produce a cyclic argument?

If we are going to define (empirical) temperature from the zeroth law, we need a definition of heat that does not depend on the concept of temperature, else this would produce a cyclic argument. ...
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2answers
53 views

Difference between space of reference and system of coordinates

In the book "The meaning of the relativity" by A. Einstein, it is referring to two different concepts: space of reference and system of coordinates. What it is the difference? It says: "we ...
1
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0answers
38 views

Definition for a trajectory in phase-space

When we say "a trajectory in phase space", when the parameter is time, do we mean the set of points in phase-space corresponding to a continuous segment in time? Does it have to be continuous? Does it ...
2
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2answers
90 views

Question about inertial mass and gravitational mass

I know that inertial mass $m_i$ is the quantity that appears in Newton's second law: $F=m_ia$ and that gravitational mass $m_{g_1}$ is the quantity that appears in Newton's gravitational law: $F_g=Gm_{...
1
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1answer
61 views

What is the difference between a tensor, vector, and a matrix? [duplicate]

I'm currently going through notes on a physics course and I'm having trouble understanding the difference between a tensor, a vector, and a matrix. I know that a vector is a kind of tensor and that a ...
0
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3answers
84 views

Is this definition of work right?

My thermodynamics textbook defines work as follows. Work is motion against an opposing force. But this definition of work doesn't imply that work is done in accelerating a body does it? So is ...
30
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3answers
2k views

Is there any truth to interpreting definition of a second as corresponding to oscillations?

As far as I understand the definition of a second, the Cs-133 atom has two hyperfine ground states (which I don't really understand what they are but it's not really important), with a specific energy ...
0
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2answers
43 views

Gravitation and gravity

Are gravity and gravitation the same thing? Actually I have 2 teachers at my school. One of the said that gravitation is the force of attraction between any two bodies in the universe due to their ...
1
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2answers
86 views

How do we define what is “External” force or “Internal” force in the context of momentum conservation?

I know that without presence of any "External" force momentum is always conserved. But how do we distinguish between "External" force and "Internal" force where all are "Force"?
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1answer
38 views

CFT conformal weight vs. scaling dimension

I was wondering if anybody could clarify what the difference between the conformal scaling dimension $\Delta$ and the conformal weight $h$ is? Is it correctly understood that $\Delta$ is related to ...
3
votes
1answer
74 views

Will centre of gravity coincide with centre of mass if density of object is non uniform?

I read that for bodies of very large dimensions, but having non-uniform density, the centre of gravity does not coincide with centre of mass. I can understand that with large dimensions the strength ...
1
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1answer
60 views

Conditions for free fall

What are the conditions at which free fall occurs? If a body is falling freely under the effect of gravity only without external resistance, the motion is called free fall. This definition gives me ...
0
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1answer
43 views

What is the exact mathematical definition of oscillation/vibration?

My question is basically is what criteria need to be fulfilled to decide wether a motion is osciliiation/vibration or not. I found two definitions, def1: "moving around an equilibrum", def2: "...
3
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1answer
53 views

Difference between Fermi and Riemann normal coordinates

What is the difference between Fermi normal coordinates and Riemann normal coordinates? Which one of them is related to the vanishing of the Christoffel symbols?
0
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1answer
54 views

What is the difference between the potential difference and potential energy of an electron?

What is the difference between the potential difference and potential energy of an electron? Let's take an example the potential difference (PD) across a resistor. if there's a current flowing, the ...
0
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1answer
53 views

Angular momentum definition? [closed]

The definition of linear momentum is this: Momentum is a vector quantity defined as the product of an object's mass, $m$, and its velocity, $\vec v$. So According to that definition,The definition ...
3
votes
1answer
317 views

What is the meaning of “moment”?

What is the meaning of moment? I'm little confused about the word as there are some terms like moment of momentum, moment of mass, moment of force, etc. I want to know what exactly is meant by the ...
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0answers
36 views

What is the general definition of a quench?

I've seen the term "quench" used in many different contexts. It's easy to understand the meaning when the context has a simple physical analogue, such as lowering the temperature of a system to cause ...
0
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1answer
31 views

Earth, Sun and beginner's reference frames

In the post-Newton era, where "absolute space" is not absolute, how is the reference frame in which "the Earth moves round the Sun" accurately defined?
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2answers
58 views

How can you tell if the work done by a force is negative?

This is kind of confusing to me. I'm guessing that it's specific to the problem. Is the work done by friction always negative? Is the work done by gravity always negative? Spring as well? It seems ...
0
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1answer
44 views

What is difference among the consepts: bulk free energy, cohesive energy, Gibbs free energy, binding energy?

I am working on size dependent surface energy of metals. I need to know the different energies between the atoms. Different papers use different energies but their meanings seem alike, therefore they ...
0
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1answer
41 views

A conceptual question about work

Consider an object of mass $5kg$ on a plane. If a horizontal force of $10N$ acts on the object for a time interval of a second. Calculate the work done on the object by the force given that the force ...
0
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1answer
37 views

Rheological Definition of Friction

I was listening to a record at our university about friction and its rheological definition. For the first moment I thought that its the normal definition Friction is the force resisting the ...
0
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0answers
27 views

What does 'fully excited' actually mean?

In statistical mechanics you often hear the phrases such as 'when the degrees of freedom are fully excited then....'. An example would be the validity of the equipartition theorem. But what is the ...
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2answers
81 views

Definition of translational , rotational , rolling motion

What are the exact definitions of pure translational , pure rotational and rolling motion ? I am a class 11th student ... I find it difficult to exactly make a difference between translational , ...
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4answers
36 views

Is impedance the ratio of voltage to current at any instant?

My book defines impedance as the ratio between the amplitude of voltage to that of the current (Vmax to Imax), I just wanted to make sure whether its also true for the instantaneous values of voltage ...
0
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1answer
43 views

Work done by a force

What is the difference between work done by a force and work done against that force? Is it true that work by a force = -(work against that force)?
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1answer
85 views

What is difference between 9.8 N (Kgwt) and 9.8 m/s^2 (g)?

What is difference between 9.8 N (Kgwt) and 9.8 m/s^2 (g)? What is difference between kg weight and gravity? I am not so good in physics so please explain in a lot of detail (but sorry for this silly ...
2
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1answer
53 views

Why does this formula for the partition function not include the multiplicity?

I am having problems understanding the formulas used for describing the partition functions and the probability distributions for canonical ensembles. In the first case I have two formulas for the ...
3
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3answers
208 views

Why is the definition of inertial mass circular?

On Wikipedia, the definition of inertial mass is: Inertial mass is the mass of an object measured by its resistance to acceleration. And, can be evaluated using $F = ma$, Newton's second law....
3
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1answer
54 views

Reversible process, equivalence of two definitions?

There are two common definitions of a reversible process: A reversible processes is quasistatic with no dissipation. And A process where an infinitesimal change in conditions would reverse ...
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1answer
81 views

What is the difference between the Lorentz force and the ponderomotive force? [closed]

I understand that Lorentz force is due to motion of moving charged particle in a magnetic field, and I imagine that ponderomotive force is mechanical version analogy to a person surfing on a wave ...
0
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1answer
23 views

Clarification regarding Homogenous systems in Thermodynamics

So i have just started with Classical Thermodynamics..and i am reading Thermodynamics by Bruno LinderSo I could not understand what he means in this portion- If intensive properties are uniform ...
4
votes
2answers
43 views

Does a central force have to be independent of angle?

When defining a central force, some sources, like Wikipedia, say that the magnitude of the force only depends on the distance $r$: In classical mechanics, a central force on an object is a force ...
0
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2answers
142 views

How are unitary matrices and unitary random matrices associated with physics or quantum mechanics? [closed]

Please forgive my ignorance, my back ground is not physics. I am looking for distance measure between two unitary matrix (for my work). So my starting point is where else unitary matrix is applicable?...
4
votes
2answers
90 views

Difference between two types of pressure

When I first studied thermodynamics the concept of pressure was defined by means of the fundamental relation $S = S(U,V,N)$ for simple systems. The pressure was thus defined by $$P = -\left(\dfrac{\...
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2answers
241 views

Differences between eigenstates, bound states and stationary states [closed]

I am not very clear about the differences between eigenstates, bound states and stationary states.
2
votes
1answer
108 views

What is the difference between thermodynamic and empirical temperature?

When I've studied Thermodynamics I did so in Callen's book and there the author talks about temperature as a single thing, which mathematically is simply defined as: $$T = \dfrac{\partial U}{\partial ...
1
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1answer
49 views

Scientific definitions of “moment (of)” and “instant”? [closed]

What are the scientific definitions of "moment (of time)" and "instant"? Are they different with their definitions in everyday language? I also don't know the definitions in everyday language, of ...