Tagged Questions

Gravity is an attractive force that affects and is effected by all mass and - in general relativity - energy, pressure and stress. Prefer newtonian-gravity or general-relativity if sensible.

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23 views

Archimedes' Principle questioned?

Let's say we have a body A of mass 'm', and a trough of water some 'h' metres deep. Let's say the body sinks in water. Let the weight of the body in air be W Newtons. I "place" it in water. It ...
0
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0answers
22 views

Relation between $f(R)$ gravity and Tensor–vector–scalar (TeVeS) gravity

We know that there is a relation between f(R) gravity and scalar-tensor gravity. By applying the Legendre-Weyl transform, we can receive brans-dicke gravity from $f(R)$ gravity. If we start with the ...
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0answers
33 views

Is there an equivalent force to magnetism for gravity? [duplicate]

In electrodynamics, one way to explain the origin of the magnetic field is to analyze how a moving particle perceives a static electrical field under the effects of special relativity. Due to the ...
4
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1answer
30 views

What is the effect of water 'climbing' over a cup via a wet string? [duplicate]

I have noticed (with tea bags, etc) that if you get the whole string wet, and you have one end inside the cup and the other end below the level of the liquid (outside), the water drains down the ...
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1answer
42 views

Is the event horizon also the boundary region of the mass contained by a black hole?

I was recently playing around with classic Newtonian mechanics and calculated that the Earth would have to be compressed to a spherical region of 8.8 millimeters (Its Schwarzchild radius) to turn its ...
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1answer
61 views

Einsteins gravity Space time

I'm having a difficulty in understanding Einstein's version of gravity as "space time curvature" (I could be a total physics idiot, please point out !). How does an object on the surface of earth ...
0
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2answers
45 views

Black Holes and Light [duplicate]

It is said that photons have zero rest mass so how can gravitational force of a black hole affect light? And if photons do have some effective mass while traveling at speed of light then only can a ...
1
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1answer
54 views

What is the identity of Dark Matter?

What is the identity of dark matter? Is it a particle? Is it the lightest superpartner (LSP)? Do the phenomena attributed to dark matter point not to some form of matter but actually to an ...
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0answers
42 views

Light Speed in a vacuum [on hold]

I saw recently that the speed of light is 186,000 miles/second, then later read about the speed of light 'in a vacuum', obviously meaning space. Does this mean that gravity or air effects the speed ...
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0answers
12 views

Muscular energy transformations with regards to gravity

Maybe this has more to do with biology than I know, but I have been wondering just what kind of energy muscles use and convert when lifting a weight off the ground or lowering a weight back to the ...
5
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1answer
92 views

Explanation for “if all accelerated systems are equivalent, then Euclidean geometry cannot hold in all of them”

I'm doing an EPQ (mini college research paper) on gravity, and I found a site that explained things in simple terms. I am having trouble understanding how Einstein came to his revelation space-time ...
21
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8answers
6k views

Is it possible to create a parachute large enough to stop all velocity?

This idea came to me while playing Kerbal Space Program. I noticed that the larger my parachute was, the slower my rocket would fall back down to Kerbin. I would like to know if it is possible to ...
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0answers
20 views

Generalization of Feynman's derivation of the formula for gravitational potential energy in FLP volume I

In the Feynman Lectures on Physics, Feynman derives the formula for gravitational potential energy by applying a line of reasoning he borrowed from Carnot on reversible weight lifting machines (FLP I, ...
5
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1answer
214 views

Is it possible for photon to run in circle by its own gravity?

I have heard that gravity came from energy and momentum so photon has gravity too. Then there are theory state that photon has energy tied to frequency. So if a photon has very very high frequency ...
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2answers
69 views

How is strong time dilation consistent with weak tidal forces?

Nolan's latest film, Interstellar, takes pains to explain to lay audience members that the passage of time slows in the presence of strong gravitational fields (as per Einstein's theory of General ...
1
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1answer
52 views

Potential Energy of Springs and Gravity, and Work of a force

I am currently really stuck on this problem and I am not sure how to actually solve it, I think I can reason it out logically but I do not know how to show it. Here it is and under I will show what I ...
0
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1answer
43 views

How can black holes be point-like but have a size?

Presumably all singularities are the same size: a point. What makes one black hole bigger than another? Also, a singularity will only have the same mass (or less) than the collapsed star, so what ...
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1answer
29 views

Reduced mass and energy representation?

If we have a gravitational system where $m<<M$ then we can write down the energy of $m$ as follows: $$E=\frac{1}{2}m(\frac{dr}{dt})^2+\frac{L^2}{2mr^2}-\frac{GMm}{r}$$ but if $m$ is not small ...
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1answer
65 views

Does the gravity affect voltage in a circuit?

The electric current is a flow of electrons, which have mass (small, but it is still a mass). So, considering a planar circuit, do the properties of the electric current (voltage, intensity) change ...
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0answers
38 views

Modelling gravitational potential of a galaxy

I am interested in modelling the gravitational potential of a disc-shaped galaxy with radius $R$, i.e. solving the 2D Poisson equation numerically by Gauss-Seidel relaxation: $$\nabla^2 \phi = 4\pi G ...
0
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1answer
55 views

Is there anything interesting about an n-body system whose potential energy is constant?

I am curious about a system of gravitating bodies with particular characteristics. Before describing the characteristics of the system, let me say what I want to know about it: Can this be a ...
0
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0answers
22 views

electrostatic force vs gravitational force [duplicate]

I have question regarding the forces caused by electrostatic charges and gravity. From the explanation here in the link just by existing an object with mass distorts space-time. Will there be a ...
0
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0answers
16 views

What are laws regarding falling meteorites and the angles of descent?

I would like to know if there is any math / laws of physics governing the angle that a meteor falls on our planet. I doubt it falls towards the center of the planet, from what footage I've seen, it ...
1
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1answer
37 views

Different density objects falling on Earth's atmosphere?

So I finally understood why 2 objects with different masses, free falling from the same distance reach Earth's surface at the same time (due to the objects Inertia). Now when the objects fall in the ...
7
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1answer
139 views

Can light gravitationally affect itself?

Consider a electromagnetic wave in a vacuum. From my understanding of general relativity, The wave has momentum, and thus generates a gravitational field in all directions. The gravitational field ...
0
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1answer
54 views

Gravitational force and Electromagnetic force?

I found this interesting note in one of my textbooks, The enormous strength of the electromagnetic force compared to gravity is evident in our daily life. When we hold a book in our hand, we are ...
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2answers
37 views

Human between two bodies of large mass

What if a person is put between two identical dense spherical bodies (well, not as dense as black holes, but definitely 100 times more than earth) with mass $M_1=M_2=10^{30}kg$, exactly in the middle ...
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1answer
52 views

Gravitational work

As far as I know gravitational work is independent from the path of the object, and I have an object that goes up on a inclined plane to a certain height, and than, after the object reaches the edge ...
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0answers
36 views

Conditions for a diagonal induced metric?

Let $M$ be a manifold of dimension $n$ with a (say Lorentzian) metric $g$, that is diagonal in some choice of local coordinates. Let $S$ be manifold of dimension $k<n$ , embedded in $M$ by some ...
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2answers
87 views

length contraction in a gravitational field [duplicate]

As space-time is distorted in a gravitational field, relativistic effects such as time dilation and length contraction take effect. Time dilation is explained simply enough: closer to the source of ...
1
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2answers
492 views

Gravitational time dilation caused by a galaxy, and by

In a word, if you are sitting on the Earth, if I'm not mistaken you are experiencing Time Dilation compared to being in deep solar system space. Due to the mass of the Earth. However. We're all ...
3
votes
1answer
104 views

How to explain centripetal force in terms or relativity

At the end of a video of dropping a ball and feathers in a vacuum, Brian Cox explains that the Ball and Feathers, as understood in terms of General Relativity, aren't falling. (apologies I can only ...
0
votes
1answer
60 views

Why can't my body draw a tiny piece of paper despite gravitation pull it exerts on the piece of paper?

We observe in static electricity experiments that a comb rubbed on dry hair lifts tiny pieces of paper.This is due to the attraction force exerted by the charge collecting on the surface of the comb ...
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4answers
512 views

Why is the relative weakness of gravity a problem?

In my physics classes, I remember it being repeated a few times that gravity is a much weaker force than the other three fundamental forces, and being told this is an open problem in physics. However, ...
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3answers
105 views

Gravity and Magnetism… One and the same? [duplicate]

I asked this in another question as if it was an answer so as to use a bit of that questions popularity and possibly get some clarifications on this but I also prefer to make a question of my own, so ...
2
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1answer
55 views

Help understanding centre of mass

So basically the question I posed myself a few days ago was for a torus shaped planet, what would the gravitational field be like inside the torus? After viewing some answers on the internet, (Gravity ...
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3answers
53 views

Should chemists at different altitudes factor in the specific gravity value when measuring mass via spring scale?

Forgive my ignorance but I've never found an answer to this question....In the spirit of independent confirmation/research replication, should chemists operating a different altitudes above sea level ...
2
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4answers
101 views

Conservation of potential energy for a wormhole

So you managed to build a stable traversable wormhole. Somehow you managed to acquire the exotic negative-tension materials with sufficient densities to make it all work. Now you place opening A of ...
1
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1answer
34 views

How would behave theoretical matter with negative mass? [duplicate]

I wonder if there is any possibility to evaluate theoretical characteristics of matter with negative mass? This is not thing of anti-mass but classical matter just with negated weight. I thing that if ...
3
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1answer
47 views

Gibbons-Hawking-York (GHY) boundary term for Schwarzschild metric

What is the simplest way to calculate Gibbons-Hawking-York boundary term for Schwarzschild metric? \begin{align} \int ...
5
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4answers
220 views

The gravitational constant $G$ theoretically?

We know all that the gravitational constant is $$G=6.67428±0.00067\times 10^{-11}m^{3} kg^{-1} s^{-1}.$$ But can we calculate it theoretically?
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0answers
69 views

How does matter interact with spacetime? [closed]

It's easy to see how matter interacts with itself but how does it interact with spacetime which is "not" matter? Einstein showed us that mass and energy cause a curvature in spacetime, which intern ...
3
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1answer
115 views

Is General Relativity based on a Symmetry?

In short: Is there any kind of symmetry one can start with to derive general relativity (GR)? Longer: Einstein had the opinion that GR was the generalisation of special relativity, because instead of ...
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0answers
22 views

What is the rate at which the field of an electron spreads? [duplicate]

There are 2 steps in the creation of a force field: (1) first the object comes into existence (2) then the field appears. In the case when a photon splits into an electron and an antielectron, these 2 ...
0
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1answer
39 views

Is the moment of zero velocity identical for objects of differing mass?

Assume we have two perfect spheres of identical size but different mass (e.g., one made of steel and one made of enamel) and an environment where the only forces in play are gravity and velocity ...
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2answers
61 views

Perceived acceleration in an artificially modulated gravitational field

It's intuitive that while accelerating in a locally constant gravitational field, there is no perception of acceleration, since the body accelerates uniformly. What if a body were in a rapidly ...
1
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1answer
94 views

Can an object spin in a perfect horizontal circle

During a lab I was doing involving uniform circular motion, i was required to spin a rubber stopper horizontally via a rope. At the bottom of the rope, there was some weights. After analyzing the ...
3
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1answer
102 views

What is conformal gauge?

I often see in physics articles on gravity such notion as conformal gauge and Weyl transformation. They use Conformal gauge to change coordinates to transform metrics from arbitrary $$ds^2=g_{\mu ...
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0answers
55 views

why ostriches cannot fly? physics explanation

I'm looking for a physics explanation concerning to the fact "The ostriches cannot fly". My statments: During the time interval $\Delta$t of the upward wingbeat, the ostrich drops a distance h ...
0
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0answers
21 views

What does one get when admitting negative mass? [duplicate]

So essentially, both gravitational and electric fields behave in exactly the same way. The only difference is that, while mass can only be positive, charge can be positive or negative. As I understand ...