mmc
Reputation
1,503
Top tag
Next privilege 2,000 Rep.
 Mar 17 awarded Enlightened Mar 17 awarded Nice Answer Jan 29 awarded Yearling Apr 30 comment Can two spaceships go fast enough to pass straight through each other? The claim in the video is wrong, as clarified later: COMMENT FROM TONY: "My comments at 1.30 were a bit hasty. Cosmic rays do not pass through the earth. Most collide in the upper atmosphere. I was actually thinking of the LHC safety debate. If they exist, mini blackholes produced by cosmic ray collisions on earth would slip straight through because the earth isn't dense enough. But not the protons themselves." Jan 29 awarded Yearling Dec 18 awarded Nice Answer 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.