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"head" in the name refers to the Mathematica concept. That was the first account I created. I kept the name.


2d
comment What's the meaning of linear medium in electromagnetism?
@JohnM You got it backwards here and in a comment to your answer. $D$ gives you $\rho_f$ and $E$ of course gives you the total $\rho$. Since $\rho_f$ is zero, $\nabla \cdot D$ must be zero, but since we are linear, $\nabla \cdot P$ must be zero as well.
2d
comment What's the meaning of linear medium in electromagnetism?
It is not clear to me that the absence of source charges implies that $\vec{M} = \vec{P} = \vec{0}$. In fact, I would have thought that the electromagnetic wave would disturb the medium and cause a polarization to form. Similarly, a magnetic field would induce a magnetization. However, with no free sources I would expect $\vec{\nabla} \cdot \vec{P} = \vec{0}$.
Aug
18
revised No need Power to fly ? Paradox
added 117 characters in body
Aug
18
comment No need Power to fly ? Paradox
@Thomas No, a blimp really can float with doing anything. Here is a picture of a blimp in case there was any confusion about what a blimp was. Also the blimp was just supposed to be one example of how something can float without expending energy. It's not the only way. I have edited my answer so the second paragraph gives an example of levitating something denser than air without using energy.
Aug
18
answered No need Power to fly ? Paradox
Aug
14
revised Multiplying propagators
added 6 characters in body
Aug
14
answered Multiplying propagators
Aug
12
comment Conservation of Energy vs Conservation of Momentum in Rotational Dynamics
Would you be able to see why energy isn't conserved if you put a initially stationary block on a large moving surface and they both move together at a new velocity?
Aug
10
comment Reciprocal Lattice of a non-bravais lattice
@kalkanistovinko you should add a detailed explanation of your previous comment as an edit to your question.
Aug
1
comment Does “converge” mean intersecting and producing image when we are taking about convex lenses?
Yes, the lens will bend the light rays so they all converge to a single point. To add a wrinkle, I will say that the location of this point will depend on what angle the incoming rays from infinity make with the lens.
Jul
31
answered Does “converge” mean intersecting and producing image when we are taking about convex lenses?
Jul
31
revised What do “ℜe” and “A*” mean?
added 110 characters in body
Jul
29
comment Moment of Inertia and Rotational Dynamics?
@1110101001 angular momentum is different from $mv$, which is usually called momentum, but can be called linear momentum to distinguish it from angular momentum. Describes motion going around a point, and the further away from the point you are, the more angular momentum you have. This is why there is an $R$ in the formula.
Jul
15
comment Using Dynamics to find the Net Force of a object upon two other masses acting upon it
From this I gathered that the main source of confusion was how to add forces acting in different directions to get the net force. Looking at the picture of his work supports this conclusion. Therefore I would say his question is really just about that concept. Now you could say that we should only allow questions when the asker himself is able to identify the specific concept he is having trouble with, but I think that will leave out a lot of potential questions. I think as long as the asker makes a good effort to narrow the scope of the question as much as they can, then its specific enough.
Jul
15
comment Using Dynamics to find the Net Force of a object upon two other masses acting upon it
For what its worth, I don't think this question should be closed, at least not for the reason stated. It's clear that LostAtPhysics showed effort to solve the problem, so it shouldn't be closed for that reason. However, you could object that there is no concept singled out as the topic of the question, but I would say there is: The question says (v2) "Overall I can do questions like these in linear, yet when it comes to this question is stumped me, would be have like a vector (direction) in the final answer?" ...
Jul
15
revised Using Dynamics to find the Net Force of a object upon two other masses acting upon it
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Jul
15
answered Using Dynamics to find the Net Force of a object upon two other masses acting upon it
Jul
11
awarded  Revival
Jul
6
reviewed Leave Open Why can arbitrary two qubit density matrices be expressed in this form?
Jul
1
answered Can the entropy of a part of a closed system become less than the initial entropy of the entire system?