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seen Oct 20 at 3:22

Just interested in physics.


Sep
24
awarded  Autobiographer
Apr
17
comment Complete list of nuclear fusion reactions
@Calmarius You can still retrieve the PDF using Internet Archive.
Jan
29
awarded  Yearling
Aug
20
awarded  Nice Answer
Jul
15
comment Stress in a thick-walled pressure vessel
@AlanSE At the risk of being a bit inaccurate by ignoring the compression, the constant stress assumption fails in the thick wall case "because" the inner part of the wall needs to deform more for a given radial deformation. If the inner radius of the vessel is 20 cm and the outer radius is 40 cm, both circumferences will grow ~6.3 cm if the radius of the vessel increases by 1 cm. But 6.3 cm is a much bigger relative deformation (strain) for the inner section of the vessel.
Jul
15
comment Stress in a thick-walled pressure vessel
@AlanSE The right solution will depend on the yield criteria you adopt. For metals, a good one is the von Mises yield criterion: $\sigma_y = \sqrt{\tfrac{1}{2}[(\sigma_{rr} - \sigma_{tt})^2 + (\sigma_{rr} - \sigma_{tt})^2 + (\sigma_{tt} - \sigma_{tt})^2]} = |\sigma_{rr} - \sigma_{tt}|$
Jul
13
comment Stress in a thick-walled pressure vessel
I have written an answer summarizing the solution process and trying to avoid "tricks". Please ask me if you are unsure about any of the steps.
Jul
13
answered Stress in a thick-walled pressure vessel
Jul
11
comment Stress in a thick-walled pressure vessel
Yes, the Poisson's ration appears in the radial stress ($\sigma_{RR}$) because it "connects" tangential and radial stresses. The tangential stress distribution doesn't depend on $E$ because the deformations are linear on $E^{-1}$ and the stress distribution only depends on relative deformations (remember that these are "infinitesimal" deformations).
Jul
10
comment Stress in a thick-walled pressure vessel
Check section 4.1.4 of this reference. (If elasticity cannot be assumed, the problem is much more complex.)
Jun
29
comment radiation thermodynamics paradox
@Trimok Yes. But, as the purple ray is being reflected by a plane of symmetry of both ellipsoids, it will behave as a ray coming from $B$ (the mirror image of $A$).
Jun
29
comment radiation thermodynamics paradox
@Maxim I think the problem you are having comes from assuming point-like bodies, as that gives you infinite radiance. See this article for more details.
Jun
29
comment radiation thermodynamics paradox
@Trimok The rays coming from $A$ and reflected by the vertical surface will seem to come from the other focus, $B$.
May
4
comment is the nature of particle beam weapons in science fiction true to the reality of particle physics?
Charged particle beams can propagate through non-trivial distances in the atmosphere by a combination of self-focusing and holeboring. I think that 20 meters were demonstrated and kilometers were hoped for (beam instabilities are the main problem). A very good technical summary of the "state of the art" in "directed energy weapons" during the 80s can be found in the Report to The American Physical Society of the study group on science and technology of directed energy weapons.
Jan
29
awarded  Yearling
Jan
20
comment What happens to 5 electrons on a sphere?
@Nathaniel Yes, it's a stable equilibrium because it has minimum energy. The N=5 case is one of the few instances of the Thomson Problem where an exact solution is known.
Jan
17
comment Could a planet ever end up with a doughnut hole in it?
@JohnRennie I think the projectile would have to be hyperdense to bore through the planet. Very roughly speaking, it's mass should be greater than the one of the column of rock that is being displaced. For small impactors with size $D$ boring through the Earth's diameter, their density needs to be about $5\mathrm{\frac{g}{cm^3}}\frac{12000 \mathrm{km}}{D}$.
Jan
12
comment What happens when you heat vodka in a microwave?
This answer and the linked one at Chemistry.SE are wrong. Vodka is about 40% ethanol by volume and will boil at about 84 °C. This can be seen clearly at this plot. We can check that by noticing that 40% V/V is a molar fraction of 0.17 and using Paul's more academic resource :)
Jan
8
comment What properties do you need for building a tower?
@AlanSE "If payload is usefully large, material cost is prohibitive." Not necessarily so: a taper ratio of 4 (the one mentioned in the paper) is quite reasonable and I don't see any basis for the "1 km" figure (the paper gets a ~150 tons elevator for a 1 ton lifter). The feasibility of the whole project seems to me linked with getting nanotube-based materials having strengths somewhat near their predicted values. As these materials would be quite useful by themselves, I'm somewhat optimistic about the whole idea.
Jan
8
comment What properties do you need for building a tower?
@AlanSE There is no need for a specific thickness, though even borderline realistic materials require tapering. This is a nice overview of the physics.