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| visits | member for | 2 years, 3 months |
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| stats | profile views | 176 |
Just interested in physics.
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May 4 |
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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. |
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Jan 29 |
awarded | Yearling |
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Jan 20 |
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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. |
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Jan 17 |
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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 $5\mathrm{\frac{g}{cm^3}}\frac{12000 \mathrm{km}}{D}$. |
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Jan 12 |
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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 :) |
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Jan 8 |
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What are the Constraints on Building a Tower to Space? @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. |
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Jan 8 |
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What are the Constraints on Building a Tower to Space? @AlanSE There is no need for a specific thickness, though even borderline realistic materials require tapering. This is a nice overview of the physics. |
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Dec 29 |
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Writing $\dot{q}$ in terms of $p$ in the Hamiltonian formulation @Kitchi I think Lubos is using the standard definition. |
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Dec 12 |
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Usage of helium in MRIs @user14445 Extracting helium from Jupiter would be far easier (though that doesn't mean a lot). |
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Dec 10 |
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What matter in the original atom bomb is converted to energy? @TobiasKienzler "a neutron can annihiliate anti-hydrogen, but it decays into hydrogen first and then its proton and electron annihilate the anti-proton and positron" I think you can have direct proton-antineutrom "annihilation" via strong interactions. |
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Dec 5 |
awarded | Talkative |
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Dec 4 |
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Nuclear fusion: what causes this “resonance” peak? Nice answer, but the symbol of boron is $\mathrm{B}$, $\mathrm{Be}$ is beryllium. |
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Dec 3 |
awarded | Constituent |
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Nov 26 |
awarded | Caucus |
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Nov 19 |
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What is the cause of the normal force? @ChrisGerig I remember your elaboration in this thread. You were just asking us to take your word for it, as you didn't give any explanation (remember that I gave a lot of sources, FWIW). Please write a more detailed answer so we can address this issue properly (i.e. not by saying "I'm right!"). |
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Nov 13 |
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What velocity must an aircraft achieve for its shock wave to transform to plasma? @AlanSE I think that $v_1$ and $T_1$ are in the right place. Using $T_1 = 300K$, $T_2 = 10^5K$ and $v_2 = 0$ I get $v_1^2/2 + (1.4/0.4) \cdot 300 \cdot 300 = (1.4/0.4) 300 \cdot 10^5$. Leaving $v_1$ alone in the LHS: $v_1 = \sqrt{2(1.4/0.4)300(10^5-300)} \approx \sqrt{2.1 \cdot 10^8} \approx 1.4 \cdot 10^4 \frac{m}{s}$, matching your result reasonably well. |
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Nov 12 |
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What velocity must an aircraft achieve for its shock wave to transform to plasma? @AlanSE You are using the wrong Bernoulli Equation :) The compressible version must be used for transonic and supersonic flows but it's easier to get the temperature directly by using conservation of enthalpy: $c_p\,T+\frac{v^2}{2} = c_p\,T_s$. |
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Nov 12 |
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What velocity must an aircraft achieve for its shock wave to transform to plasma? @AlanSE I'm also doing a calculation based on adiabatic compression. Can you post your detailed calculation? Your approach in the other post looks right, but you must be making some trivial arithmetical mistake here, because at 0.05 c even electrons have energies of almost 1 keV. |
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Nov 12 |
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What velocity must an aircraft achieve for its shock wave to transform to plasma? I don't understand your first calculation. Ignoring real gas effects, the stagnation temperature is 10 eV at a Mach number of $\sqrt{(400-1)\frac{2}{1.4-1}}\approx 44$, a speed of about 13 km/s. Though, for communication purposes, ionization starts being significant at Mach 10. |
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Oct 25 |
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Could someone jump from the international space station and live? It could be something like MOOSE. |
