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Questions tagged [gravity]

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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Vertical Circular Motion and Tension

I stumbled across this problem on a physics test, and I'm convinced the correct answer (C) is wrong. The explanation simply states that all the signs are correct, and in circular motion, inward ...
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0answers
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There are a component proportional to speed in gravitional force?

Reading some introductory texts to general theory of relativity (GR), I understand that the gravitational force between two objects has, in addition to the component in the direction of the vector ...
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1answer
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$dS^d$ and $AdS^d$ are conformally equivalent

I have seen in the book by Zee (Einstein Gravity in a Nutshell) the following metric for $dS^4$: $ds^2 = \frac{1}{\cos^2 \tau} \big( - d\tau^2 + d\psi^2 + \sin^2 \psi \, d\Omega_2^2 \big) $, (Eq. IX....
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Black holes with different pulls?

Would a relatively small black hole attract objects in a different way than supermassive ones?
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1answer
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Does the Higgs VEV depend on the gravitational potential?

If I take the non-relativistic limit of the Klein-Gordon equation in a curved space-time I will get a contribution from the Newtonian gravitational potential. Shouldn't this gravitational contribution ...
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What would happen if the universe had no gravity? [closed]

My question is what would the universe be like without gravity.
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1answer
87 views

Zero-energy universe - What is nothing?

I am a layman, so excuse me in advance for the stupidity of my questions, and I hope you can answer them in a way that I can understand. I have read, here and there, that the Universe might have a ...
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Please explain gravity? [closed]

If spacetime is like an elastic plane holding planetary masses that sink into pockets creating "gravity" and planetary orbits around large masses, then how do we explain the "gravitational force" we ...
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1answer
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Gravitational redshift and Energy of a photon

Let us assume that we have a large gravitational field, then the gravitational redshift can be expressed as, $$\frac {v_{\infty}} {v_e} = (1-r_s/R_e)^{1/2}$$ In this equation $v_{\infty}$ represents ...
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Calculate velocity at a distance or time when accelaration is varying under gravity [closed]

How to calculate velocity at a distance or in time of an object under varying acceleration ? e.g. object falling to earth from a height of 10,000 km ; say it starts at initial velocity 0, what is the ...
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2answers
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The chicken-egg nature of bending a photons pathway

I'm a chemist not a physicist, so I have a loose relationship with its laws. I read that light (specifically a photon) doesn't have mass, gravity doesn't act on a photon to alter its trajectory. ...
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How to calculate mass from time dilation and radius?

I'm new to this site and I'm been searching the internet all over for this solution. I'm looking for an equation, that if given the time dilation at the surface of an object and its radius, it will ...
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Mass bending spacetime: what is the mechanism? [duplicate]

What is the mechanism that causes mass to bend spacetime? Merely inferring that it does is not an answer. How it does it is the answer.
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1answer
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Why is the heat death, rather than a Big Crunch, the most accepted theory of the ultimate fate of the Universe?

The Heat Death is accepted by most as the end of the Universe, but how can that be? Wouldn't the Big Crunch make a lot more sense? I mean, even if everything in the Universe is spread out uniformly ...
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Why clouds float on the sky and why blue or black [closed]

Why do clouds stay on the sky and not downwards, doesn't gravity pull everything to earth? What makes clouds to turn blue or black. How does the sky become blue and not any other colour on daylight?
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1answer
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What are some differences between electric force and gravitational force? [closed]

What are some differences between electric force and gravitational force? I mean, electric force is stronger, and electric force can be neutralized, but what else is there?
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Windowless space station: rotation vs. gravity ring vs. frame dragging?

Imagine a windowless hollow cylinder, with an observer sitting inside on the curved wall. Something is pulling the observer against that wall. The observer can also walk easily along the inner ...
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0answers
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Deriving kinematic equations for the slingshot maneuver using Lagrangian mechanics

I have an assessment/investigation coming up in my math class for which I plan to derive the equations of motion for the slingshot maneuver/gravity assist using Lagrangian mechanics. So far, I am ...
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2answers
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How Does Gravitational force work on Light? [duplicate]

m'=$m_0/\sqrt{1 -v^2/c^2}$ How Will the gravitational force work on Light ?
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0answers
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What causes Gravity/Gravitation? [duplicate]

What causes Gravity/Gravitation? We know that mass causes gravitational force which is a non-contact force but is there anything physical that leads gravitational pull. Here, by physical I mean ...
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1answer
26 views

Calculate stroke force and angle for a ball rolling on a surface to reach a destination position (optimal golf putt)

I'm trying to find an algorithm which calculates the optimal golf putt (angle, force) given a 3D surface (obtained through Poisson reconstruction), start and destination position (hole). I guess that ...
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1answer
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How does this magnetic toy work?

A nephew of mine has this toy called "Gravitrax". It is basically a track building kit - you can build various tracks for iron balls which go through the track thanks to gravity. However, there is ...
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1answer
58 views

Modification of Gravitational force's law and related problem

I'm facing a problem which states the following: Suppose that it is discovered that Newton's law of gravitation is incorrect and that the force $F$ on test particle of mass $m$ due to a body of ...
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Where is the energy transfer from a metal ball falling from a magnet?

A ball falls from a magnet but the magnet still exerts an upwards force against gravity yet the ball falls anyway. However, the ball slows down and thus the sound when it hits the floor is less ...
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1answer
64 views

Why would it take so great distance for an object to have the acceleration due to gravity?

Let's suppose that in a certain reference frame, a hyperbolic coordinate is used, i.e. $(x,t)=(r\cosh \omega, r \sinh \omega)$. On each hyperbola $r=R$, where $R>0$ is a constant, the proper ...
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1answer
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Direction of gravity

General Relativity explains the path a falling body makes (ex. An apple falling toward the center of the Earth) as a geodesic in curved spacetime. What explains the direction the apple falls? In other ...
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1answer
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Does a field gain energy or does the mass/charge?

My textbook reads that when a mass/ charge is moved into the influence of a gravitational/ electric field the field gains/ loses energy rather than the mass/ charge. How can this be explained?
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How to explain gravitational lensing to a layman?

This question is about how to explain gravitational lensing to a layman, not about exact theoretical correctness. I am accustomed to explaining optical refraction in terms of wavefronts and the fact ...
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0answers
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Large objects in the universe sized by gravitational and electrical constants

In a famous talk (from 14:30 to 15:00) Nima Arkani-Hamed tells us that what "fixes the size of the earth" is the equilibrium between the gravitational pressure against the atomic pressure of the atoms ...
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2answers
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Shouldn't the infinite range of gravity be seen as troubling? [closed]

I've heard a lot of talk that infinities emerging in physics equations usually indicate that a mistake has been made somewhere and given that gravity seems to be a perennial mystery shouldn't there be ...
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0answers
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Gravity as a fundamental force [duplicate]

In Dr.Einstein's theory of General Relativity-- he goes to great lengths to explain how the force of gravity is merely an illusion and in the case of the falling apple-- it is the earth accelerating ...
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3answers
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Does light get acceleration due to gravity? [duplicate]

When an object, say a ball, is attracted by the black hole it gets acceleration due to gravity. Suppose light is moving towards the black hole vertical to it... then does it gain acceleration due to ...
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4answers
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Question on equivalence of acceleration and mass with respect to gravity

Layman’s question here. Let’s say I’m standing on the inside rim of a rotating space station spun at right rate to produce earth-like gravity. Does the spinning warp space time? If so, how can a small ...
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3answers
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Surviving a fall into a balloon-filled pit [closed]

My brothers and i have been debating this question for years. If an average person fell at terminal velocity into the center of a pit that is 1 mile deep x 1 mile in diameter, and the pit is filled ...
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1answer
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Could the gravitational field be a cloud of particles not curved spacetime? [closed]

The more I've been looking into quantum gravity the more I've been coming to the conclusion that the graviton must be some sort of particle within a cloud of identical particles surrounding massive ...
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0answers
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Why $\kappa = 8 \pi G$ in $D$ dimensional spacetimes?

Probably another question without an answer! ;-) In most books/papers I saw on General Relativity, everybody writes $\kappa = 8 \pi G_D$ in the right part of Einstein's equation, even for spacetimes ...
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2answers
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What is meant by “instantaneously aligned” with regards to rotating reference frames/coordinate axes?

I'm trying to understand a set of lecture notes I found online from MIT which describe the derivation of the equations of motion for an object in a rotating reference frame, this is for an essay on ...
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Warped space and gravity

I'm trying to combine in my head a warped space (e.g. an Alcubierre "warp bubble") with the curved space of gravity. How do the two interact? More specifically, if a warp bubble moves near a source of ...
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1answer
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Is ER= EPR a Hidden Variable Theory & If NOT Why?

John Bell showed that entanglement cannot be a local hidden variable theory in 1964. Leonard Susskind and Juan Malcadena have proposed that non-traversable wormholes, Einstein-Rosen bridges (ER) are ...
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1answer
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Gravity / Rate Of Fall Paradox on Moon Earth

It is well known that the rate of fall in the vacuum on the Earth is equal for all bodies, e.g. a feather and a gold bar, so on the moon as well. However, the moon has according to less mass also less ...
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4answers
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Gravity, falling bodies, and the equivalence principle

Why is that bodies in a box accelerating uniformally in space that is sufficiently removed from gravitational fields fall identically to bodies in a box located in a homogeneous gravitational field (e....
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1answer
46 views

Super Heavy Brown Dwarf

As I understand black holes, they're considered "black" because the escape velocity is, or exceeds, the speed of light. Alternatively, the escaping light is redshifted infinitely. The Schwarzschild ...
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2answers
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Time dilation, and curvature of space caused by two black holes of unequal masses

If you were to try to find the time dilation in a region of space near a black hole you would use the equation $$t_r=\sqrt{1-\frac{2GM}{rc^2}}$$ Would the time dilation from two black holes be this? ...
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1answer
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Velocity of object falling into black hole measured in Schwarzschild co-ordinates, depending on co-ordinate time

This is a followup question to the answer given to a similar question where the velocity is given as $$v(r) = \left(1 - \frac{r_s}{r}\right)\sqrt{\frac{r_s}{r}}c \tag{1}$$ where $r$ is the radius ...
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2answers
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Why can't we insert gravity in the special relativistic lagrangian?

I am a math student and I have taken four-five lessons about special relativity in a course about Lagrangian and Hamiltonian mechanics, so be patient with me if my question is stupid. My teacher says ...
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7answers
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If gravity is a pseudoforce in general relativity, then why is a graviton necessary?

As far as I’m aware, gravity in general relativity arises from the curvature of spacetime and is equivalent to an accelerated reference frame. Objects accelerating in a gravitational field are in fact ...
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0answers
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Does the Dirac sea have any mass or gravitational effect?

Dirac sea is a model for vacuum which considers the empty space as a sea full of negative-energy particles. Anti-particles are holes in this sea. Dirac sea is used to model Quantum field theory in the ...
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1answer
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Doesn't expansion of space not allow a perputual motion?

Well consider the following thought experiment: We have a tool to extract/transform "nearly all" potential gravitational energy into other forms of energy. We can theoretically make such an object (...
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1answer
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Principle of etremal aging

Say I'm in a region of space time where there is nothing and all of a sudden a star and a planet appeared out of nowhere ( for reference sake) is it due to the principle of extremal aging that the two ...
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1answer
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Does space expansion increase potential energy, and hence increase mass?

As space expands the gravitational potential Energy increases. So does the potential energy between Atoms and quarks and so on. I have read several times that these effects are so minuscule that they ...