# All Questions

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### A coincident stack of D3 branes vs a shell of them

I would in general like to understand how to derive the low energy metrics to describe D-brane configurations. Any pedagogic reference which explains the method? In particular I have in mind these ...
442 views

### Why do surfaces act like barriers for electrons?

Say you have a conductor, filled with free electrons. The nuclei have a weak pull on the valence electrons so they are moving around in the conductor. But the electrons don't leave the solid. If you ...
3k views

### Which one conducts heat better: water, or air?

If we were to assume room temperature of 20C, which of the fluids conducts heat better, and why?
122 views

### How the tube of a tire get puncture?

How the tube of a tire(cycle) get puncture? I mean, the inner tube is protected by an outer tube, so how is it possible to make a hole in the inner tube? And in general the puncture hole is very small....
91 views

530 views

### light beams of the sun [closed]

We receive sunlight on earth surface. What type of light beams are these? Random/Parallel/Converging/Diverging I think it should be Diverging as Sun is radiating these beams away. But in one ...
125 views

### Can an EM wave be represented in terms of dipole? [closed]

The direction of propagation is represented by a line. A positive charge moves along this line at velocity c. A negative charge moves along a line that is perpendicular to the direction of propagation ...
161 views

### Earliest terrestrial planet?

If I've understood correctly, the heavier elements needed for terrestrial planets such as iron can only form in supernova. If that is indeed true, how long since the beginning of the universe would ...
437 views

### Why does $E=\nabla\phi$ follow from $\nabla\times E=0$?

I understand that using one of Maxwell's equations, $$\vec{\nabla} \times \vec{E}(\vec{x})=0,$$ it can be said that $$\vec{E}(\vec{x})=-\vec \nabla \phi(\vec{x}).$$ However, I can't find or ...
33 views

### How do you extract cosmological constants like $k$ or $H_0$ from the fluctuations in the CMBR (WMAP)?

I am currently reading a book about Astrophysics and also tried to find some information about it on the web, but was not able to figure it out. That is why I ask you guys now. Any help is greatly ...
64 views

### Why does a floating balloon in an accelerated train moves forward but the pendulum moves backward? [duplicate]

If we bring a floating balloon in a accelerated train, the balloon will move forward in the same direction as the acceleration as shown in the following figure. However if a pendulum is hung on ...
171 views

### Thermodynamics, temperature below 0 Kelvin [duplicate]

I read a news article about how they were able to create a negative temperature, below absolute zero, and my question is how does this work? I know that there are different definitions of temperature,...
131 views

### Linear Operators and their representations

I am currently learning Quantum mechanics on a slightly advanced level. I am curious in knowing if there are Linear Operators (Linear Maps) in the Hilbert Space (finite dimensional ones) that don't ...
305 views

### Given Velocity formula and told to find acceleration, I need help please. [closed]

I tried to do this problem by taking the derivative and it did not get me the right answer. I also tried the Vdv=Ads integration and could not get the right answer. Please Help
200 views

### What does the absence of hydrodynamic entrance effects do to the momentum conservation Equation?

I am trying to do a homework problem where we re-write the mass, momentum and energy conservation formulas for downward flow in a vertical pipe and it says "where all hydrodynamic entrance effects ...
733 views

### Curvature of Spacetime

I have been exploring for some time both the Special and General Relativity, hoping to glean at least a conceptual grasp of their basic tenets. In reading the book "Gravitation" by Misner, Thorne and ...
416 views

### Rigid body problem in 2d

I have some questions about this exercise: In an horizontal plane, a $OA$ bar with mass $m$ and length $a$ moves, with another bar $AB$ (same mass, double length) attached in the point A. In the ...
287 views

### Rigid body problem

I have some doubts about the next excercise: A bar of length $2a$ and mass $m$ moves freely with both of its extremes on a ring of radius $\sqrt2a$. The ring can rotate freely in a certain ...
I have a very specific doubt about the next exercise: I have the crane of the picture: With a force $F_a=-K\varphi$ Applied on the point B, perpendicular witch AB, and another force $F_b=-K\psi$ ...