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1500 questions
48
votes
2 answers
How do we get supersonic bullets?
I recently answered a question on the WorldBuilding forum about grenades and bullets. One of the things that came up was that I argued smokeless powder in a rifle round could detonate, but was challenged on that. Commenters said that smokeless…
Cort Ammon
- 48,357
48
votes
3 answers
Why is the information paradox restricted to black holes?
I am reading Hawking's "Brief answers". He complained that black holes destroy information (and was trying to find a way to avoid this). What I don't understand: Isn't deleting information quite a normal process? Doesn't burning a written letter or…
48
votes
6 answers
Why is electric field strong at sharp edges?
I learned about the coronal discharge, and the common explanation is because the electric field is strong where radius of curvature is small. But I haven't found anything yet that explains why electrons like to crowd at the peaks, and escape from…
Calmarius
- 8,100
48
votes
4 answers
In nuclear physics, what length year in seconds is used?
So I'm working on a nuclear physics problem and am looking at radioactive decay. The common unit used for very long decays is years within the literature. Is this the sidereal or tropical year? I want to use units of seconds but seeing as how these…
RocketTwitch
- 643
- 6
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48
votes
4 answers
Norton's dome and its equation
Norton's dome is the curve $$h(r) = \frac{2}{3g} r ^{3/2}.$$ Where $h$ is the height and $r$ is radial arc distance along the dome. The top of the dome is at $h = 0$.
Via Norton's web.
If we put a point mass on top of the dome and let it slide down…
countunique
- 1,663
48
votes
5 answers
Is the principle of least action a boundary value or initial condition problem?
Here is a question that's been bothering me since I was a sophomore in university, and should have probably asked before graduating:
In analytic (Lagrangian) mechanics, the derivation of the Euler-Lagrange equations from the principle of least…
Deep Blue
- 1,340
48
votes
1 answer
Why is the partition function called ''partition function''?
The partition function plays a central role in statistical mechanics.
But why is it called ''partition function''?
Arnold Neumaier
- 44,880
48
votes
4 answers
Do identical starting conditions always lead to identical outcomes?
My friend and I are discussing whether or not physical phenomena are deterministic. Let's say, for example, that we have a 3-dimensional box with balls inside of it upon which no gravitational forces are acting. The balls each have their own size,…
Sandi
- 583
48
votes
9 answers
Why is air not sucked off the Earth?
People said outside earth is a vacuum. But the air does not get sucked from the Earth's surface. Some said it is due to gravity and some said the speed of air molecules are not high enough to escape. We know vacuum will suck air like your vacuum…
Weidong Tong
- 609
48
votes
5 answers
Book covering differential geometry and topology for physics
I'm interested in learning how to use geometry and topology in physics. Could anyone recommend a book that covers these topics, preferably with some proofs, physical applications, and emphasis on geometrical intuition? I've taken an introductory…
user7757
48
votes
5 answers
Why are muons considered to be elementary particles in the Standard Model?
According to this article, a muon decays into one electron and two neutrinos.
According to this article, elementary particles or fundamental particles are particles "whose substructure is unknown, thus it is unknown whether it is composed of other…
48
votes
1 answer
Why is there no theta-angle (topological term) for the weak interactions?
Why is there no analog for $\Theta_\text{QCD}$ for the weak interaction? Is this topological term generated? If not, why not? Is this related to the fact that $SU(2)_L$ is broken?
Henry
48
votes
5 answers
Why is Helium so hard to liquify?
By the end of the 19th century all gasses had been liquefied apart from helium (He). What is it about helium that makes it so hard to liquefy compared to the other gases? And why does it need to be pre cooled in the Joule-Kelvin expansion?
48
votes
3 answers
Why don't galaxies orbit each other?
Planets orbit around stars, satellites orbit around planets, even stars orbit each other. So the question is: Why don't galaxies orbit each other in general, as it's rarely observed? Is it considered that 'dark energy' is responsible for this…
user98038
48
votes
5 answers
What causes this pattern of sunlight reflected off a table leg?
My friend noticed an interference-like pattern around the table leg. However, we do know that interference patterns of sunlight produces rainbow colours. What seems to be happening here?
Luke
- 481