5 votes

In stress-strain diagrams, why is the dependent axis ($y$) used to represent stress and the independent axis ($x$) to represent strain?

Because the equipment used to measure stress-strain curves works by applying a strain that increases linearly with time and measuring the stress. The strain is the independent variable. The problem ...
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4 votes

Why does the Strange Quark have Strangeness -1?

From the Wikipedia article Strangeness: The terms strange and strangeness predate the discovery of the quark, and were adopted after its discovery in order to preserve the continuity of the phrase; ...
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3 votes

Fourier Transform of a Wave Packet

Mathematicians usually put $1/\sqrt{2\pi}$ before each integral. Physicists usually leave it out of the integral like your first equation, but then divide the inverse integral by $2\pi$
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2 votes
Accepted

Lower vs Upper indices in stress energy tensor

The functional derivative flips the up/downdedness of the index, which has to be the case, because if you have an expression like [for any two arbitrary tensors $A$ and $B$]: $$\frac{\delta}{\delta A_{...
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2 votes

In stress-strain diagrams, why is the dependent axis ($y$) used to represent stress and the independent axis ($x$) to represent strain?

Besides the very good answer by John Rennie, a couple of other notes: You can choose whatever axis orientation you deem appropriate, it is based on conventions, and people often do what other people ...
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2 votes

The definition of Time and second

The goal of SI is to define base units in terms of physical quantities that are very repeatably the same, and easy to measure with extreme precision. One of the most precise types of measurements in ...
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  • 2,359
2 votes

What is parity useful for in physics?

The parities of two atomic states (or nuclear states) factor into the selection rules for transitions between them mediated by electromagnetism, which in turn influences decay rates. Consider a ...
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2 votes
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Why is this a negative frequency?

The naming of positive and negative frequency is a convention related to your Fourier transform conventions. In quantum mechanics, a more physical distinction is between positive and negative energy ...
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  • 33.3k
1 vote
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Arbitrarity of $i$ in the propagator

how arbitrary can the factor in front of the propagator be? You are free to choose whatever definition you like, whether it is: $$KG=\delta$$ or $$KG=-\delta$$ or $$KG=i\delta$$ or $$KG=-i\delta\;,.$$...
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  • 7,006
1 vote

What is a rigorous and general definition of the parity operator?

Parity is a discrete spacetime symmetry under which the spatial coordinates $x^i$ transform as $x^i \rightarrow -x^i$ and the time coordinate transforms as $t \rightarrow t$. Various fields also ...
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1 vote

Why does the Strange Quark have Strangeness -1?

The sign of additive quantum numbers is set up to a convention. Take for example the case of the electric current: we use to say in electronics that current is a positive flow of charge, that is a ...
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1 vote

Covariant vs. contravariant definition of the Energy-Momentum tensor

The metric $g_{\mu\nu}$ can be used to raise and lower indices. In this case you are lowering the indices on the SEM tensor, that is converting it from contravariant to covariant. So the answer is ...
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