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Aug
31
comment What is special about quantum entanglement?
possible duplicate of Quantum entanglement vs classical analogy
Aug
31
comment Is Wikipedia's definition of angular velocity incorrect?
@kalkanistovinko Yes I suppose they would! That's a topic for another question. It's explained poorly in the wiki article en.wikipedia.org/wiki/… . But in practice I've never seen that definition of angular velocity used -- you usually specify a frame with Euler angles, construct a base change matrix $R(\theta,\varphi,\psi)$ in which all particles of the rigid body are fixed, and go from there.
Aug
31
answered Is Wikipedia's definition of angular velocity incorrect?
Aug
30
comment Solving for the firing rate of a model neuron
@dbliss oh. Well then maybe Oussama's use of erfi is correct.
Aug
30
comment Solving for the firing rate of a model neuron
Seeing as how the author refers to $\sigma_V$ as the "effective standard deviation in the voltage", and seeing as how yes, $\mathrm{erf}(x)=c \int e^{-u^2}\mathrm{d}u$. I'd guess that it should be $\int e^{-x^2}(1+\mathrm{erf}(x))\mathrm{d}x$. It could be a typesetting issue because that seems like too big of an error.
Aug
29
answered Tensors, indices and matrix notation - is there a common convention?
Aug
29
comment How to determine the angular velocity of a particle with mass $m$ and charge $q$ in a constant B field?
That's just the form of the equation. $\bf f$ could be the momentum or the velocity (doesn't matter), and $A$ should have units of inverse time.
Aug
29
comment How to determine the angular velocity of a particle with mass $m$ and charge $q$ in a constant B field?
I don't think you've shown sufficient research effort in this problem! Two tips: 1. the question likely intends for you to set $\vec{E}=0$. 2. restrict $\vec{v}=\vec{p}/m$ to a plane, and write out the resulting system of ordinary differential equations to the two by two matrix equation $\dot{\bf f}=A\cdot \bf f$ where $f$ is your two row column vector, and $A$ is a matrix.
Aug
27
awarded  Informed
Aug
25
comment modelling the sound wave of a guitar string with an equation
@ja72 fun simulation!
Aug
25
answered modelling the sound wave of a guitar string with an equation
Aug
23
comment Symmetrical twin paradox without changing direction
@yashar the "too long didn't read" version of this is that coordinate systems are fickle immaterial things. A sees B's clock moving slower, and B sees A's clock moving more slowly. It's not unphysical because at any "instant" in time, the ships are separated and it's physically impossible for them to communicate. The only way to get a physical prediction is to have them meet up at a later time and compare results!
Aug
20
comment How does light focused through a lens create bokeh (blurry background)?
@WayneWerner whatever you do, you can't think of photons as billiard balls! This is how you should think of diffuse reflection physicsclassroom.com/Class/refln/u13l1d6.gif . It's also the case that the amplitude decreases, and it's also true that the number of directions the rays are travelling is increasing, and it's also true that this is due to the fact that light is composed of waves, but it's a handy and immensely accurate approximation to treat the waves as rays.
Aug
20
comment How does light focused through a lens create bokeh (blurry background)?
@WayneWerner Well... the quantum way to look at it is that yes, photons are bosons and bosons are allowed to occupy the same state (2nd paragraph here: en.wikipedia.org/wiki/Boson ). The classical way to look at it is that light is just ripples on a pond and there is no such thing as a photon. It turns out that when you get $10^7$ ripples per meter moving at the speed of light, ripples on a pond can be approximated awfully well as rays.
Aug
20
revised How does light focused through a lens create bokeh (blurry background)?
embedded the wrong image
Aug
20
answered How does light focused through a lens create bokeh (blurry background)?
Aug
18
comment If there is no definite speed in the universe, only relative speeds, how does energy increase when velocity approaches the speed of light?
@NeilGraham I think you should be more careful and I do still think this answers your question. The train of thought, "if energy goes to infinity as you approach the speed of light, let's just measure our energy to find our absolute speed in the universe" is a fallacy, but it's just as tempting in classical mechanics due to the square in classical kinetic energy. The title (specifically the phrase "if there is no definite speed in the universe") is what made me address that misconception. There's nothing to say to the bulk of the OP but "yes, that's right, energy is relative".
Aug
18
answered If there is no definite speed in the universe, only relative speeds, how does energy increase when velocity approaches the speed of light?
Aug
17
answered Special Relativity - two beams of light in opposite direction
Aug
15
answered Electric potential of a sphere at a point on its surface