# 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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### Can a body ever experience acceleration this strong?

Using the Schwarzschild radius formula, I approximated the Sun's Schwarzschild radius to be $3\text{ km}$. Now assuming I have a body (not a human body) which is at a distance of $10\text{ km}$ from ...
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### 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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### 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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### 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 $8.8 \ mm$ (Its Schwarzchild radius) to turn its ...
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### 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 ...
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### 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 ...
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### 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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### 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 ...
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### 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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### 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, ...
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### 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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### 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 ...
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### 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 ...
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### 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?
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### Could Galileo ever prove that $g$ is the same for a feather and a hammer?

Do you think he could have possibly found a way to make a feather and a hammer touch ground simultaneously, as they should, according to gravity? Can you make up a device that avoids air resistance? ...
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### 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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### Solar vs lunar gravity: inverse square law

Bearing in mind the moon's perfect fit when eclipsing the sun, were the two orbs to have the same density, would they not each exert the same gravitational pull on the Earth? (Much like a dim star in ...
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### Angular momentum

The satellites are revolving around a planet in circular orbits by themselves (no external source to maintain their speed). Are we able to make them revolve in elliptical orbits by themselves? Do ...
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### 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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### 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 ...
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 ... 1answer 82 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 ... 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 ... 0answers 63 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 ... 1answer 390 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 ... 1answer 170 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 ... 2answers 2k views ### Why does the acceleration g due to gravity not affect the period of a vertically mounted spring? For a vertically mounted spring, I was looking at the formula  T= 2\pi \sqrt{m/k} for a period. Why doesn't the gravitational acceleration g factor in? 1answer 159 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 ... 2answers 50 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 ... 1answer 97 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 ... 0answers 93 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 ... 2answers 760 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 ... 2answers 1k 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 ... 1answer 386 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 ... 2answers 198 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 ... 4answers 708 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, ... 3answers 659 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 ... 1answer 94 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 ... 3answers 86 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 ... 1answer 62 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 ... 1answer 194 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 ... 5answers 469 views ### The gravitational constant G theoretically? We know all that the gravitational constant isG=6.67428±0.00067\times 10^{-11}m^{3} kg^{-1} s^{-2}.$$But can we calculate it theoretically? 0answers 153 views ### How does matter interact with spacetime? [closed] It's easy to see how matter interacts with itself but how does it interact with spacetime which is "not" matter? Einstein showed us that mass and energy cause a curvature in spacetime, which intern ... 1answer 245 views ### Is General Relativity based on a Symmetry? In short: Is there any kind of symmetry one can start with to derive general relativity (GR)? Longer: Einstein had the opinion that GR was the generalisation of special relativity, because instead of ... 0answers 22 views ### What is the rate at which the field of an electron spreads? [duplicate] There are 2 steps in the creation of a force field: (1) first the object comes into existence (2) then the field appears. In the case when a photon splits into an electron and an antielectron, these 2 ... 1answer 54 views ### Is the moment of zero velocity identical for objects of differing mass? Assume we have two perfect spheres of identical size but different mass (e.g., one made of steel and one made of enamel) and an environment where the only forces in play are gravity and velocity ... 2answers 163 views ### Perceived acceleration in an artificially modulated gravitational field It's intuitive that while accelerating in a locally constant gravitational field, there is no perception of acceleration, since the body accelerates uniformly. What if a body were in a rapidly ... 1answer 539 views ### Can an object spin in a perfect horizontal circle During a lab I was doing involving uniform circular motion, i was required to spin a rubber stopper horizontally via a rope. At the bottom of the rope, there was some weights. After analyzing the ... 2answers 511 views ### What is conformal gauge? I often see in physics articles on gravity such notion as conformal gauge and Weyl transformation. They use Conformal gauge to change coordinates to transform metrics from arbitrary$$ds^2=g_{\mu ...
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 ...