# Questions tagged [conventions]

A convention is a set of agreed, stipulated, or generally accepted norms. It typically helps common efficiency or understanding but is not required, as opposed to a strict standard or protocol.

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### Is gravitational potential energy of body by $mgh$ negative? [closed]

Consider a 15kg object at 1m from earth ground level, is P.E = 15kg * 9.8m/s^2 * 1m = 147J or P.E = -Gm1m2/r^2 * h = -9.8 * 15kg * 1m= -147J after browsing for a while on debate of potential energy's ...
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### How Can there be a Gravitational Potential when there is NO Gravitational Field? [closed]

How does it make any logic that there exist a potential when there is no net field for example when we have a Hollow Sphere with mass we can find out the the gravitational *potential inside the sphere ...
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### What's the difference? $\nabla_\mu e_\nu=\Gamma_{\mu \nu}^\rho e_\rho~\text{ and }~\partial_\mu e_\nu=\Gamma_{\mu \nu}^\rho e_\rho~?$

What's the difference? $$\nabla_\mu e_\nu=\Gamma_{\mu \nu}^\rho e_\rho~\text{ and }~\partial_\mu e_\nu=\Gamma_{\mu \nu}^\rho e_\rho~?$$ In John Dirk Walecka's book 'Introduction to General Relativity',...
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### Why is work done by force $+mgh$ in the situation of throwing something up?

If there is a particle at point A(at rest) and a force moves it to point B(Above point A vertically)(final velocity = 0 at this point), the work done by gravity is $-mgh$. This I understand as the ...
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### Significant figures when product is more by a factor of 10

A rectangular plate has a length of 21.3 cm and a width of 9.8 cm. Calculate the area of the plate. A straightforward question, which you multiply and arrive at 208.74 cm2. Taking significant figures ...
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### $CP$-transformation for fermionic bilinears

I am trying to derive the transformation of the fermionic bilinear $\bar{\psi}\psi$ under $CP$ transformation. I know that $P$ acts as: $$\psi(t, \vec{x}) \xrightarrow{P} \gamma^0 \psi(t, -\vec{x})$$ ...
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### Equation for real/complex $\phi^4$ theory

On wikipedia (see this link), the Lagrangians of the $\phi^4$ equation for real AND complex scalar fields are given. One may derive the Klein-Gordon equation by inserting into the Euler-Lagrange-...
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### Why did we choose Coulomb's constant $k$ as $9\cdot10^9 \rm Nm^2/C^2$ while define unit of charge instead of any other number?

I understand that while defining charge, Coulomb had to choose any arbitrary value of $k$ to describe unit of charge. But, why did we chose $9\cdot10^9 \rm Nm^2/C^2$ as the value of $k$, but not any ...
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### Are there any non-SI base units? [closed]

Are there any base units that are not in the SI system? If so why aren't they? Edit: By this I don’t mean other systems that you can convert to (ie. Feet, Ounces, Hours, etc) I mean new base units ...
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### Hamiltonian flows and Poisson Brackets confusion

I have been using results from this paper in calculations. In sections 2.4 and 3.4 they perform a canonical transformation into new coordinates consisting of constants of motion. My question is ...
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### If helicity of photons is +1 then the light is right- or left-circularly polarized?

In particle physics, we say: a particle has +1 helicity (right-handed) if its momentum and spin are parallel, or it has -1 helicity (left-handed) if its momentum and spin are antiparellel. Now, if we ...
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### Why is the term 'Accuracy of 1 part in $x$' used?

My question stems from how we measure the accuracy of Caesium clocks. Most Caesium clocks are said to have an accuracy of '$1$ part in $10^{14}$.' I understand that the terminology means to convey ...
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### Why does Newtonian gravity necessarily imply that gravitational potential energy is negative? [duplicate]

Why can't the zero value of gravitational potential energy be set at a different point instead of infinite distance within Newtonian gravity? I am not asking why is gravitational potential energy ...
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### The sign of Work in Turbomachines point of view

As I know from Thermodynamics work done by the system is negative and equals to $U=Q-W$. In this picture it clearly describes the process in which when we take some pebbles from the piston the system ...
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### Why something connected to the ground has 0 potential?

I'm studying the concept of grounding of neutral wire in AC. I have done an extensive research on the internet to answer the question "Why the neutral wire has 0 potential ?". The answer is ...
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A common convention for the definition of the covariant derivative in the SM is $$D_\mu = \partial_\mu - i g_s \frac{\lambda^a}{2}G^a_\mu - \cdots$$ where $\lambda^a$ are the Gell-Mann matrices. In ...