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Feb
9
comment How Much Weight Would This Put On The Legs Of A Desk?
It's symmetric, so 17.5 pounds will be on the left two legs of the top desk, 17.5 pounds on the right two legs of the top desk, 17.5 pounds on the left two legs of the bottom desk, 17.5 pounds on the right two legs of the bottom desk. ie, if you put a scale under any one of the (total eight) legs it will read 17.5lbs.
Feb
8
comment Why the 3D heat equation is $\frac{\partial u}{\partial t}=\alpha\nabla^2u$
What is insufficient about the Wikipedia article on the heat equation? en.wikipedia.org/wiki/Heat_equation
Jan
28
comment Symmetry and Group theory book
I think you need to specify what field you're interested in to get good answers! If you're interested in symmetry in crystallography, you'll need a different answer than for symmetry in particle physics. Or, you could be talking about how conservation of energy/momentum/angular momentum follow from symmetry laws in classical and quantum mechanics, which is a fact that requires almost no group theory!
Jan
20
comment Why is there “ringing” at the violet end of a rainbow but not the red end?
Oh towards the center of it! I was looking at either end.
Jan
20
comment Why is there “ringing” at the violet end of a rainbow but not the red end?
I see: Before violet it looks to me like the rainbow colors blend into the background colors. After violet the rainbow stops. Could you elaborate on what seems weird?
Jan
10
comment How far away from the Sun does space start to be cold?
I like the answer but a perfect spacesuit would still radiate like a blackbody!
Dec
31
comment How do I describe the order of events in spacetime?
@StanShunpike I mean you can transform the vector $(0,1)$ into the vector $(\gamma \beta,\gamma)$ under a Lorentz transformation. Since $\gamma>0$ and $\beta$ can be positive or negative, you can let the timelike coordinate be positive of negative. The time coordinate of $(0,1)$ can change sign depending on your reference frame. The time coordinate of $(1,0)$ cannot change sign depending on your reference frame. You should prove this for general timelike/spacelike/lightlike vectors.
Dec
29
comment How do I describe the order of events in spacetime?
@StanShunpike Yep, I should have said "timelike or lightlike". I think you should figure out the answer on yourself though. Hit ${\bf x-y}$ with a Lorentz transformation and figure out when the time coordinate can change sign.
Dec
29
comment Does Quantum Mechanics say that anything is possible?
@Shing You meant to comment on aquidturtle's post.
Dec
19
comment Initial wave function for a particle in an infinite square well
The solution you worked out does not have the property that $|\psi|^2=(\mathrm{constant})$ everywhere, so it doesn't meet the supposition of the problem.
Dec
15
comment Can you break causality if you are able to instantaneously perceive a distant system?
Assuming you meant: "He assumes that it had been sent time $D/c$ before", that sounds like exactly the OPPOSITE situation of what the original poster cares about.
Dec
6
comment Why Electrostatic field has a continuous curve?
There is nothing very meaningful here. A field line is a curve $\vec{x}(t)$ for which $\vec{x}'(t)=\vec{E}(\vec{x}(t))$ for some vector field $\vec{E}$. In order to satisfy this equation, $\vec{x}'(t)$ must exist, so it can't have any breaks.
Dec
6
comment Elastic Collision of Infinitely Large Body and an Infinitely Small Body
You get indeterminates of the form $(\infty-\infty)/0$!!! Just solve the problem and take the limit $M\to\infty$, $m\to 0$ AFTER you find the solution.
Dec
6
comment Using the fourier series to analyze the motion of a finite string
The trick is to multiply by $\sin(k \pi x/L)$ and integrate from $x=0$ to $L$.
Nov
29
comment Geodesic equation (free particle)
Can you add more details? You can't always choose a coordinate system where the geodesic equation is trivial everywhere.
Nov
29
comment Does voltage always cause current?
@user4205580 and that's because we don't care about the situation where a vanishingly small voltage gives you an infinite current, or where you can have a nonzero current with no EMF. If you're trying to consider such a case you're going to run into difficulties, because that's just not what Kirchoff's laws were meant for... but I am considering the case with a vanishingly small but nonzero resistance in the wire. In that case, my statement (both voltage drops are zero at the same moment in time implies current is zero) is true.
Nov
29
comment Does voltage always cause current?
@user4205580 and if the voltage drop across the inductor and the voltage drop across the source are both zero, in a real circuit you build, then the current is zero.
Nov
29
comment Does voltage always cause current?
@user4205580 There are lots of little nuances and this is the reason people usually talk about LR or LC or RC circuits. If you just have an "L" circuit, the current will be as high as possible until the "R" of the wire itself, the battery, or the inductor comes into play, and you have to treat things as a three component circuit. If you really did have zero voltage across $L$ and zero voltage across the battery (and you measure these things in a lab, so think of a real circuit here), there would be no current!
Nov
28
comment How much work can the human body do before dying of exhaustion
Hi, this isn't the usual meaning of the word "potential energy" (because you're talking about useful work). I think it would help if you changed your title to "How much work can the human body do before dying of exhaustion", since that's the real question.
Nov
27
comment How can light follow curved spacetime?
The lesson of special relativity is that there IS no frame of reference where a photon is stationary. Not that such a frame "is a point".