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 Oct 2 comment Why is boron so good at neutron absorption? @user43087: The point John is making is that the neutron should not be considered an infinitesimal point; it is effectively delocalised (spread out) over a larger radius. The interaction is governed by the set of available energy states for the combined system, and here the nuclear forces can come into play. The 'cross section' is not a measure of how far away the neutron can be, but of the probability of the interaction taking place. As John points out, the size of different nuclei is largely unimportant. Sep 15 comment Is This Educational Model of Electron Any Good? The particle vs wave behaviour is not the electron changing from one to another. They are just analogies we can grasp which describe aspects of the electron's behaviour. When a large system interacts with an electron (e.g. you measure its position) no energy is necessarily given to or taken from the electron, but the interaction does affect the state of the electron. In deterministic interactions the electron has to yield a valid deterministic answer so the wavefunction is said to collapse - really this is not well understood but it is more like a selection. It is not a particle or a wave. Jul 1 comment How to emulate 40ft (12 m) of water? +1 for pressurised air above the water. Jun 29 comment Why can't the Navier Stokes equations be derived from first principle physics? Disagree on the first paragraph - we use useful equations covering bulk behaviour which could be derived from simpler principles because it is easier to use (where the derived relations are sufficient). $PV = nRT$ is not fundamental, and it breaks down for anything too far from an ideal gas. But no-one would throw it away when considering gas behaviour for a combustion engine, for example, because starting from first principles would get you to the same, or worse result given an amount of resource effort. Feb 3 comment Why do most formulas in physics have integer and rational exponents? Actually, as orion points out, there are important mathematical reasons for linear equations, so the value of the exponent is not arbitrary Dec 8 comment Electromagnetic waves should stop while encountering a conducting shell? I've added a little information on surface plasmon resonance, but if you have more questions, please do post a new question to attract good answers! Nov 28 comment A question about motion and time dilation @user43783: Congratulations, this is what General Relativity is all about. Begin by remembering that there is no absolute position, and no absolute time. Nov 25 comment Why does iron sink in molten iron instead of floating? I think your densities are wrong. At present you have a higher density of ice at 0 C, which would make it sink. Sep 30 comment Correct approach for calculating excited states of circular quantum dot under effective mass approximation I've added the equation, explanation, paper details and fixed the title so that people can see what it's about. Sep 15 comment How does an electron move around in an orbital? Is it “wave-like” or random? FYI, calculating the surface within which there is 90% probability is actually quite challenging unless the orbital is really simple and analytic. So usually what is plotted is just an isosurface on which the probability density is at a given figure, because that's much easier. Jul 23 comment Is speed of light and sound rational or irrational in nature? Regarding rationality of numbers in measurement, David Z's answer is spot on. If you are trying to grasp whether the universe prefers integers, then yes it does. Things like the resonance frequencies of strings are in strict integral relationships (f, 2f, 3f, 4f). Quantum physics is also based on integers; the idea of quanta itself is that nature is lumpy rather than continuous. Jul 10 comment Classic home experiments for an 8-year-old child Exactly. That's one of the things that I feel makes for a good learning experiment; there is space for multiple hypotheses, and people usually get it wrong the first time. I remember being sure it would miss, partly because it seemed impossible to hit it every time. Ping. Clatter. Gasp! Jul 9 comment Classic home experiments for an 8-year-old child Yes. The narrative is intended to point out that the gun is pointed directly at Kiki. The usual intuition mistake is to assume that bullets do not fall like any other projectile, or to ignore it. The set-up with the monkey letting go as the gun fires is supplying a separation of the motion vector. Clearly for a real gun to function correctly, its targets must compensate for the motion of the bullet. Jul 9 comment Would a craft travelling increasingly close to the speed of light appear to be decelerating? True, and this is the basis of many 'paradox' explanation articles. The question, however, was how it would appear to an observer, not to the traveller. To the observer watching someone accelerate near c, it is still an acceleration. Just because you move at some speed relative to me, it does not change my experience of time. Jul 4 comment Does serving food on a hot plate really keep it warm longer? Extract from the lab diary: "Trial 1, Apparatus: Roast chicken dinner (plate at 21.3deg), dining table, knife and fork..." Jul 1 comment Effective mass of a particle It can also be anisotropic (for example in Silicon) Apr 9 comment Force between two charged particles To clarify, do you have 2 charges (q0 and q1) both on the y axis, separated by distance d1? Apr 8 comment multibody problem and determinism I'm not confusing them, they are the same thing. We see from the double slit experiment that making the result deterministic changes the very probabilities of the evolution of the system. I'm probably not explaining it very well. Many worlds just says that the mechanism of multiple states evolving simultaneously is deterministic; i.e. the universe bifurcates at every instant. But it does not give an answer on how or why recombination happens. Yet some prefer it because it takes one of the question marks away (at the cost of adding an infinitely bifurcating universe). Apr 8 comment multibody problem and determinism I would say it is generally accepted. As we can only falsify non-determinism (i.e. we cannot prove that something has no cause, only that none has been found), there will always be some who dislike the idea of something unseen deciding whether the cat lives or not. However, unless you are in the field of many-worlds theories and so on, it is immaterial what you believe; the equations give you the non-determinism you will have to deal with. The most broadly accepted interpretation (Copenhagen) is that we do not know what happens in the midst of an interaction, only that it follows our equations Apr 8 comment Does interference take place only in waves parallel to each other? @rahulgarg12342: Hmm. The first is meant to show how the path difference varies over the plane, and the second is supposed to show how that results in reinforcement or dampening of the resulting waves, including where the dead zones are. If you understand that, how would you describe it? If not, how far do you follow it? Cheers!