David H

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	558 reputation	bio	website		visits	member for	10 months
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May 17	comment	How can I determine whether the mass of an object is evenly distributed? Perhaps using a hydrostatic balance and Archimedes' Principle?
May 14	comment	Chain of balls on an inclined plane +1 for Stevinus alone. I first saw that argument in Feynman's books, and Never has so simple and short a bit of text and a scribbled picture brought me so much joy.
May 14	comment	Absorption of Water Well, I know the answer is "yadda-yadda the surface tension of a spherically symmetric water droplet in space will be uniform, but on Earth the droplet's deformed shape results in non-uniform surface tension, blah-blah water's a polar molecule, handwave and a miracle and it's that water is gonna act weird in space." (OK, that might be admittedly light on the physical details...)
May 13	comment	Is there any correlation between mass-energy equivalence and Maxwell's 4th equation? Einstein's mass-energy relation does indeed pop out of Maxwell's equations and classical electrodynamics... almost: E=4/3mc^2. Conflicting interpretations of the formula also abounded, from the mass of the EM-field fluid continuum to the hypothesis of an electromagnetic mass mechanism as the origin of mass as the inertia of a charge's electrostatic field (Higgs' great-great-great-aunt).
May 13	answered	MRI's and Electromagnetic Radiation
May 13	answered	If there were two equal masses with one mass in the middle which way would gravity pull it?
May 12	answered	Crackling of Speakers-Audio
May 9	awarded	Citizen Patrol
May 8	comment	Is the universe fundamentally deterministic? A subtle point about the TDSE: it is deterministic in the sense of differential equations, and the only thing it determines is the wave-function. If the wave-function itself is tantamount to reality, then quantum mechanics (and any quantum mechanical universe) can be said to be deterministic. If, on the other hand, the wave-function is merely a probability amplitude for classical state variables, then reality is stochastic. Deterministic randomness is not deterministic.
May 7	comment	Gauss' law and an external charge Glad to hear it, and thank you for the compliment.
May 7	answered	Gauss' law and an external charge
May 7	comment	Finding Surface Tension of water at certain Temperature and Pressure @Link On an unrelated note, whoever wrote this question is either evil or an idiot just for using pounds-per-square-inch pressure units but dynes and centimeters for force and distance units.
May 7	comment	Finding Surface Tension of water at certain Temperature and Pressure Is it safe to assume we're dealing with a water droplet? I'd like to model the problem as a spherical bead of water in air whose radius is a function of temperature (and pressure). Also, I'm fairly certain whether or not the Young-Laplace equation is applicable hinges on this point.
May 1	comment	QM formalism is one big confusion - lack of geometrical explaination with images David Griffiths' Introduction to Quantum Mechanics is one of the best (arguably the best) intros to QM that money can buy. I'd also encourage reading his preface to the book (it can be found online at amazon or google books) for some insight into the pedagogical problems of QM. But consider yourself forewarned: there is no road to quantum mechanics.
May 1	comment	What does it mean for electric current to be a scalar? @Nathaniel But the charge in such a circuit IS moving. And the amount of charge to pass a given point of the circuit over a time dt is dq = Idt. The time derivative in Ben's definition is valid, but it is implied that you have a surface in mind on which the derivative is being taken.
May 1	comment	What does it mean for electric current to be a scalar? I might go so far as to say when direction in important, current density is ALWAYS technically what's being considered, but this is only obvious after being introduced to the Dirac delta function.
May 1	comment	What does it mean for electric current to be a scalar? @MichaelBrown Not necessarily. If electric current is defined as the dot-product of a current density vector and vectorial area, the result is a scalar quantity.
May 1	comment	How would you determine whether an object is at equilibrium? By comparing the object against the definition of equilibrium of course. Could you be more specific about what you're having trouble understanding?
Apr 27	comment	Similarity between the Coulomb force and Newton's gravitational force It's always possible in situations like this that the reason for same equations is same underlying nature, and physicists are always on the lookout for scenarios like that. But it's very specious reasoning to jump to that conclusion automatically, as is the case here. The two equations are only approximately valid, and it's entirely coincidental that the first order approximation of one equals that of the other. The same happens with ideal springs & pendulums, which obey the same eq.'s IF you use the small-angle approximation for the pendulum.
Apr 27	comment	How universal gravitation falls short For astronomers, one important reason GR has to be used for strong gravitational fields is to explain gravitational lensing of starlight. Newtonian gravity has no effect on light (since light is massless), which would make the observed bending of light around quasars anomalous. An explanation requires taking into account the curvature of space.