All Questions
16
questions
0
votes
1
answer
521
views
How do you integrate by parts when you have a triple integral?
I'm studying how particles of equal mass behave in a spherical cluster held intact by gravity. I will assume that the mass density $\rho(R)$ of the cluster is a function of the magnitude of the ...
- 1,222
0
votes
1
answer
66
views
If we increase and decrease gravity periodically, what is the name of the resulting wave?
The gravitational waves, as in the case of merging black holes, expand like water ripples in spacetime (this is how they are depicted in general), and they affect everything in the vicinity, including ...
- 720
2
votes
1
answer
97
views
Minimum mass of asteroid that could knock moon away [closed]
What is theoretically the minimum mass of asteroid, coming from deep space, that could knock the Moon away from Earth? The asteroid is not hitting the Moon, just interacting with it by gravity.
I was ...
1
vote
1
answer
29
views
Doubt regarding solving an integration for radial flow of matter around a star in Newtonian gravity
The spherically symmetric flow of matter around a star in Newtonian gravity is governed by the equation
$$v\frac{dv}{dr}+\frac{1}{P+\rho}\frac{dp}{dr}+\frac{1}{r^2}=0$$
The equation of state is chosen ...
- 2,015
1
vote
2
answers
794
views
How is the External Field Effect in MOND conceptually distinct from Newtonian gravity and GR?
I’m trying to understand the External Field Effect (EFE) in Modified Newtonian Dynamics (MOND) and how it is conceptually distinct from GR and Newtonian gravity. More specifically, the descriptions I ...
- 167
0
votes
2
answers
490
views
Pressure (gravitational) applied by a hollow, uniform, spherical shell on an object inside it [duplicate]
1) It's a well-known result that the net force inside a hollow, uniform spherical shell is zero at all points.
However, for a spherical shell with finite mass inside of it, we say that the mass ...
- 85
1
vote
1
answer
160
views
What imparts the linear motion or the tangential velocity to natural satellites like the moon to enable them to be in orbit around the earth?
From Newton's laws of motion & gravitation, it is clear that the force of gravitation provides the necessary centripetal force which acts along the line joining the centre of masses of the earth &...
- 21
1
vote
2
answers
388
views
Confusion regarding the concepts and derivation of Hydrostatic Equilibrium for a star
First and foremost I am sorry for this; in order to make the questions clear I must first upload the lecture notes from my institution $^\zeta$ for the derivation of Hydrostatic Equilibrium:
$^\...
- 2,470
5
votes
2
answers
289
views
Why do stellar boundaries exist?
What theorems or insights are available to decide wether a star with some given interdependence between its density, pressure and temperature distributions ought to have a boundary at a finite ...
- 643
0
votes
1
answer
253
views
Why aren't Roche limit and the difference in gravitational acceleration the same?
The definition of the Roche limit is: "The distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the ...
- 3,916
0
votes
3
answers
499
views
Negative density solutions of the Poisson's Equation for polytropic gas (aka Lane-Emden Equation)
Question: What does it mean by when we obtain negative density for specific region in space as a solution of Poisson's equations?
I will explain the situation as detailed as possible, so that anyone ...
- 193
48
votes
5
answers
23k
views
Why aren't there spherical galaxies?
According to the Wikipedia page on Galaxy Types, there are four main kinds of galaxies:
Spirals - as the name implies, these look like huge spinning spirals with curved "arms" branching out
...
- 615
10
votes
4
answers
1k
views
How is the most accurate value of $G$ measured?
How do modern scientists update the measurement of $G$, the gravitational constant? Is CODATA the authority on this measurement and the experiment?
- 171
4
votes
1
answer
235
views
Galaxy rotation curve and dark matter
I am reading "The Essential Cosmic Perspective" by Jeffrey O. Bennett, Megan O. Donahue, Nicholas Schneider, Mark Voit.
In Chapter 14, it is stated that an evidence of the presence of dark matter in ...
- 4,014
25
votes
2
answers
2k
views
How do spiral arms form?
Why aren't all spinning galaxies shaped as discs as my young mind would expect? I understand how the innermost parts of a galaxy spin faster than the outer parts, and that could explain why some ...
- 1,127