All Questions
153 questions
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How much does the radioactive core of a nuclear bomb shrink due to compression by conventional explosives before it goes critical?
I saw a claim online that the conventional explosions used to detonate a nuclear bomb "significantly" compress the radioactive core, but I can't find any numbers indicating by how much the ...
- 183
3
votes
1
answer
2k
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Average Density of the Universe over Time
For reference:
Referring to a question I posted in Feb. 2015:
Could Hyper-Massive Black Holes be due to Dark Matter in the Early Universe?
User10851 answered, and I replied. He then replied to my ...
- 189
3
votes
3
answers
2k
views
Lift and drag coefficients on other planets
The question I'm trying to answer seemed simple: how hard would it be to fly on a planet with lower gravity but also thinner atmosphere compared to Earth. If the answer could hint me at how much ...
- 33
3
votes
1
answer
728
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Exoplanet Mass-Radius Diagram
I'm currently studying the following diagram:
But I'm not entirely sure I understand what's going on. Is it just, that most exoplanets discovered, is pretty much made up of Hydrogen and Helium ? And ...
- 2,577
3
votes
2
answers
1k
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Do supermassive black holes contain a singularity?
According to wikipedia "... the average density of a supermassive black hole
(defined as the mass of the black hole divided by the volume within its Schwarzschild radius)
can be less than the ...
user98038
2
votes
1
answer
376
views
Is there some special cutoff density after which spacetime "collapses" and forms a black hole?
With crude calculations following densities can be approximated:
Given that radius of proton is $1.75×10^{−15} m$ and it's mass is $1.67 × 10^{-27}kg$, this gives density of proton to be $\dfrac {1....
- 1,447
2
votes
1
answer
4k
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Calculating the eccentricity of an exoplanet
I'm wondering how to calculate the eccentricity of an exoplanet by its radial velocity vs. phase graph. To clarify my question I will take an exoplanet called WASP-14b 2 as an example (http://...
- 27
2
votes
0
answers
975
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Standing Waves in a String of two Linear Densities
Given a string with sections of two linear densities like this:
Does the point where the two linear densities meet have to be a node if a standing wave is produced? Are there alternatives? Could I ...
2
votes
2
answers
2k
views
Is there an upper theoretical density limit?
Is there an upper theoretical density limit - density being mass in a finite volume, and the part consisting mass has also some finite volume, just with large space in between (particles, atoms, ...).
...
- 181
2
votes
4
answers
11k
views
Why is density an intensive property?
I am still trying to understand what are intensive and extensive
properties. Possibly someone can give a pointer to a decent text (preferably on the web), as I
am not too happy (to say the least) with ...
- 3,808
2
votes
1
answer
325
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Catenary curved string with differing linear densities - Linear density distribution
Do you guys know if there's some kind of weight distribution that fits a problem like this one?
Let there be a string with two different sections with different linear densities, like the following ...
2
votes
1
answer
280
views
How are changes in density of matter as universe expands measured?
In this answer John Rennie says about deciding whether something observed by astronomers is matter:
... when we say matter we means anything that scales like matter as the universe expands.
This ...
- 29.6k
2
votes
3
answers
690
views
Ocean density vs atmosphere density
I understand that the density of the oceans on Earth in on average constant regardless of the depth. It is 1020 kg/m^3 at the surface and 1050 kg/m^3 at deep waters.
I understand too that this is not ...
- 29.3k
2
votes
1
answer
2k
views
Does the Relative density of water change based on the state it is in
Will Relative density of water change based on the state it is in? Ie solid, liquid, gas.
What causes this change(if any) in Rd?
- 1,251
2
votes
3
answers
19k
views
Dirac delta function and volume charge density
I just got introduced to the Dirac delta function and one of the questions was to express volume charge density $\rho({\bf r})$ of a point charge $q$ at origin. I saw that the answer is related to ...
- 2,565
2
votes
2
answers
1k
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Whats at the center of a neutron star?
What is nuclear pasta? Somebody told me that the inside of a neutron star was made of nuclear pasta. Also is the inside of a neutron star some sort of liquid?
When you have a massive glacier the ice ...
- 835
2
votes
2
answers
6k
views
Finding the expression for probability density (the Klein Gordon equation)
Source: Quantum Field Theory for the Gifted Amateur by Tom Lancaster,
Stephen J. Blundell.
I am struggling to understand the logical step from the outline of the 'proof' in the footnote, to the fact ...
- 1,081
2
votes
3
answers
558
views
Confusion about solution of the Callan-Symanzik Equation
In the QFT textbook by Peksin and Schroeder, they apply a hydrodynamic-bacteriological analogy to derive the solution of the Callan-Simanzik equation. Yet I'm confused with the integration boundary on ...
- 57
2
votes
2
answers
2k
views
Calculating air density lapse with altitude (specifically, pressures)
This might be a bit more of an engineering question, but I'm calculating air density drop-off with altitude, and I'm having some problems calculating the pressure (I'll run through my method). This ...
- 133
2
votes
0
answers
102
views
What orbit does a planet have to have to be a planet? [closed]
Other questions have dealt with where you could find a planet-like object, and what a planet has to consist of to count as a planet. But this question is directed as what orbit or path an object would ...
- 121
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2
answers
2k
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Why won't a block less dense than water fully submerge?
Suppose we have an object of volume $1\, \mathrm{m^3}$. Mass of that object is $500\, \mathrm{kg}$, which means that the density of the object is $500\, \mathrm{kg/m^3}$.
If the object is in water it ...
- 71
1
vote
2
answers
2k
views
Point masses and infinite densities
Point masses are masses who don't have volume. It is said that they are infinitly dense, but I though division by zero is undefined hence you can't define the density for a point mass because $\rho=m/...
1
vote
1
answer
171
views
How to understand critical density?
In Cosmology, critical density is given by setting $\Lambda = 0$ and $k = 0$, in other words, a universe without dark energy and zero curvature. According to my understanding and Wikipedia, this ...
- 453
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vote
1
answer
336
views
How to produce a 3D density map of a time-depenent system of particles?
I have a time-dependent system of varying number of particles (~100k particles). In fact, each particle represents an interaction in a 3D space with a particular strength. Thus, each particle has $(X,...
- 173
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vote
3
answers
767
views
Is the nuclear mass density constant for different nuclei and shape of the nucleus
I was reading the Wikipedia page on nuclear density when I came upon a problem. (I haven't yet been introduced to nuclear physics.)
What form does the nucleus have? Is it spherical? How is it possible ...
user279106
1
vote
1
answer
275
views
How to get the density parameter of $\Lambda$CDM as a function of time?
For flat universe, the Friedmann equation is given by:
$$
H^2 = \frac{8\pi G}{3} ~~ \left( \rho_m + \rho_r + \rho_\Lambda \right).
$$
From this thread:
What is the equation for the scale factor of ...
- 405
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vote
0
answers
123
views
Milky Way Density Profile
Does anyone know where I can find either a printed version or, better, a downloadable dump of data giving me the density profile of the Milky Way? I'm looking to put together a quick-and-dirty ...
user32023
1
vote
1
answer
2k
views
Confusion about the probability cloud
What is the meaning of the electron probability cloud?
I understood it to mean that the electron has a probability to be found in a certain postion before measurement, but now after reading ...
- 253
1
vote
1
answer
792
views
Bernoulli Principle and pressure function
I have read in several places now that for compressible flow the Bernoulli equation
$$ \frac {v^2}{2}+ P\ + \Psi = \text{constant}$$
requires:
$$P=\int {dp\over \rho(p)} $$
But I don't get what ...
- 920
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vote
1
answer
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views
How does the gravitational field behave inside a star?
The interior gravitational field of a star with constant density is given by
$ds^{2}=-\left(\frac{P_{c}+\rho_{0}}{P(r)+\rho_{0}}\right)^{2}dt^{2}+\frac{dr^{2}}{1-\frac{8\pi\rho_{0}}{3}r^{2}}+r^{2}d\...
- 789
1
vote
2
answers
352
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On Planets orbiting binary stars
Several years ago a discovery was made of planet orbiting a star of a binary system (two stars orbiting each other). Since binary star systems are plentiful in our galaxy, I presume we will be ...
- 1,577
1
vote
1
answer
395
views
If mass density curves space-time, then why isn't density (at each $x$, $y$, $z$) considered a dimension in space-time? [closed]
From http://science.howstuffworks.com
"Theodor Kaluza theorized that a fourth spatial dimension might link general relativity and electromagnetic theory. But where would it go? Theoretical ...
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vote
3
answers
236
views
Why is an event horizon dependent on density?
It’s frequently stated in Astronomy documentaries that you could replace the sun with a black hole of equivalent mass and the orbital mechanics will continue as normal. This works fine in dispelling ...
- 113
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1
answer
555
views
If you were to have an empty balloon, would it float?
Okay, I really have three questions, but I can only post once every 40 minutes and they’re all related.
Say I have a balloon with the inside supported by a collapsible frame and made from a very ...
- 183
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vote
1
answer
385
views
Density of states function for fermion gas (or gas in a box model): relation with degeneracy and modes
Consider a free fermion gas in a (cubic) volume $V$. The "density of states" function is
$$g(p)=\frac{V}{2 \pi^2 \hbar ^3} p^2$$
This is an example of a more general type of "gas in a box" model .
...
- 2,637
1
vote
0
answers
115
views
Do all black holes have the same mass density? [duplicate]
I have been reading about black holes in Stephan Hawking's 'A Brief History of Time'. Then I was reading more about it on Wikipedia; I am making a school project on it. I do not really remember what ...
- 11
0
votes
2
answers
403
views
Derivation of pressure gradient stellar equation [duplicate]
I am trying to understand how to derive the following formula:
$\frac{dP(r)}{dr}=-\frac{GM(r_<)\rho(r)}{r^2}$
The notes are as follow:
Consider a star with COM and a shell:
$P_1 - P_2 = -{\...
- 988
0
votes
3
answers
625
views
Center Of Mass Troubles
I understand the concept of Center Of Mass(com), but I am having a difficult time interpreting the equation of the simplified case of one-dimension.
The book I am reading defines the position of the ...
- 453
0
votes
1
answer
248
views
Measure liquids density: hydrometer
It is possible to measure liquids density with an hydrometer:
I want to figure out how this work. Hence I wrote:
$$\sum F = 0\\[1em] F_b = F_w \\[1em] \large \delta^{l}\,g\,V_h^l = m_o\,g \hspace{...
user153036
0
votes
3
answers
897
views
Materials known to have higher density than Osmium at at high pressure and/or low temperature
This question is basically a modification of this.
It's well known that Osmium is the densest material you can find at room temperature and pressure. I am curious at higher pressures (and lower ...
- 3,019
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1
answer
294
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Speed of Sound greater in denser Materials linked to restoring force
I recently read a book that explained why the speed of sound is faster through a medium with a higher density.
It said -
In a denser medium, the particles are closer together. This means that ...
- 233
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2
answers
244
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In a mixture of two immiscible liquids, why does the denser liquid always settle down?
The same is not true for solids, e.g. an iron block kept over a wooden block.
- 105
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1
answer
437
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Pressure,Pascals Law [duplicate]
Lets say that i have 2 different types of containers, one cuboidal and another one conical.
and lets say both these are massless and fully contain water.
**1.**In the cuboidal one the pressure ...
- 71
0
votes
1
answer
3k
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Deriving buoyant force from first principles [duplicate]
How can buoyant force be derived from most basic laws of fluids ?
I can think of easy one. Consider such scheme of body floating deep in water :
Where $dA$ is elementary downward directed surface ...
0
votes
1
answer
1k
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is gravitational force dependent on objects radius?
I was told that objects with the same mass but different sizes (or densities) will have a different gravitational pull. Is this true? If so, why?
- 11
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3
answers
3k
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Why is charge $= nALe$?
Sorry if this question is a bit broad but I can't find any info on this by just searching.
The equation $q = neAL$ where $L$ is the length of a conductor, $A$ is the area of cross section, $n$ is the ...
- 87
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1
answer
800
views
Is density of a material related to penetration?
Theoretical question. If you have a standard projectile with mass 1 and velocity 1 and fired it at different materials would the penetration be related to the density of the material? I'm not trying ...
0
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2
answers
2k
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How do Astronomers Measure the Mass of a Planet using Doppler Spectroscopy?
So, I am making an essay explaining planet-hunting techniques. And while I do get how astronomers measure the semi-major axis of a planetary orbit (using Kepler Laws), I don't get how they measure the ...
- 835
0
votes
0
answers
171
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How can a singularity have infinite density? [duplicate]
I've heard many times that black holes have infinite density at their center, but I've also heard astronomers quote mass estimates for black holes. How could the singularity have infinite density if ...
- 131
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5
answers
1k
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Why is Earth's gravitational pull only at its center?
Why is Earth's gravitational pull only at its center?
Does acceleration due to gravity remain constant wherever I go in and on the Earth?
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