# 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.

4,479 questions
Filter by
Sorted by
Tagged with
25 views

### How gravity would affect light "perceived" speed?

Here is the experience: A photon P travels from X to Y through tube drilled inside the earth from the North Pole to the South Pole. A photon Q travels from A to B, which is the exact same segment ...
47 views

### The behaviour of acceleration of a ball in motion

I have a few doubts on how the acceleration of a ball works when we throw it up in the air, In my mind, I could imagine it in some different "real world" cases, So I am listing them all down,...
99 views

### How can we be sure that gravity is not the reason for the perceived expansion of the universe? [closed]

The question has been partially discussed here but I believe the following formulation is a bit different. What experience could disprove the notion that the universe is not expanding despite the pull ...
97 views

### Black holes are not infinitely dense [closed]

If a black hole is infinitely dense, then therefore its terminal velocity in a determined field of gravity would be greater than the speed of light which is impossible, therefore a blackhole is not ...
25 views

### Do materials sort in order of density in space the way they do near Earth?

Obviously, if you drop materials into water near the surface of the Earth, they sort according to densities, with the least dense materials near the surface, and the most dense materials near the ...
56 views

### Proving relation between curvature of spin-connection (in 3 dimensions)

The curvature of the spin connection $\omega^{ij}:=\omega^{ij}_{\mu}\mathrm{d}x^{\mu}$ is defined by $$F^{ij}:=\mathrm{d}\omega^{ij}+{\omega^{i}}_{k}\wedge {\omega^{kj}}$$ As usual, in $3$-dimensions, ...
82 views

### Where does Okun's formula come from?

The formula is $(16)$ in Lev B. Okun's paper "The Concept of Mass". It gives the force acting on a light body by a heavy spherically symmetric mass according to general relativity. ...
22 views

### How we can visualise that gravitational potential, instead gravity is responsible for time dilation?

How we can visualise that gravitational potential, instead gravity is responsible for time dilation here on Earth? How possibly 'point A' can know that it should add some time dilation because at '...
436 views

### Speed due to gravity [duplicate]

Assuming a miraculous arrangement of a string of black holes for slingshot purpose, and a solid ball slinging by these black holes one after the other, gaining speed with each slingshot, is it ...
17 views

### Gravitational Potential Energy and mass [duplicate]

I understand when we lift an object (a mass), its potential energy increases. Also we know $E=mc^2$, does it mean when we lift an object in the gravitational field (like on earth), its mass increases?...
30 views

### If the Earth is constantly accelerating at 9.8m/s^2, according to General Relativity, won't Earth's speed be constantly increasing towards infinity? [duplicate]

As per Einstein's Theory of General Relativity, I understand that 'the Earth's ground is accelerating towards us at 9.8m/s^2. This is why we're falling towards the center of the Earth, as GR doesn't ...
65 views

### Does Newton's Theory with Retarded potentials give rise to the motion of perihelion of Planets

If we take into account the retarded potentials and the motion of the Sun(due to the planet), does Newton's Gravitational theory give rise to the motion of Perihelion of planets (qualitatively, not ...
75 views

### Is there a max terminal velocity much less than speed of light?

If you had an object falling into, say, a neutron star which had a very great gravity, is there some reason other than an atmosphere that would prevent the object from reaching any arbitrary speed up ...
51 views

### How does space change as the universe expands?

Consider a metre ruler. Despite the universe – and space itself – constantly expanding, the ruler maintains its size. If this ruler was alone in empty-ish space, other distant objects would appear to ...
145 views

### Are gravitational waves produced only when a mass accelerates? [duplicate]

Why? They are not a form of EMR, are they. Or are the rapidly changing ones the only ones we can detect?
175 views

### Why do gravitational waves move at the speed of light? [duplicate]

They are not a form of electromagnetic radiation, are they? So, why do gravitational waves move at the speed of light?
51 views

### Can the surface of a neutron star move faster than light? How length contraction manifests there?

It is known that any Kerr black hole is surrounded by ergosphere, inside which everything rotates around the black hole faster than light. Since ergosphere is greater than event horizon, one can ...
283 views

### Mach's principle and a reference frame for acceleration

Although there are different statements of Mach's principle, one statement could be that acceleration is meaningless unless it can be defined relative to something. The distance stars provide a fixed ...
49 views

### Gravitational force acting on a small element on a sphere

I was given a formula that force acting on a small element on a sphere (made by connecting two hemispheres) to the center of the sphere equals: $$dF = \frac12 \cdot G\cdot m\cdot dm/r^2$$ I cannot ...
96 views

### Theory like general relativity for charges

According to GR, the distribution of mass curves spacetime and the curvature in spacetime then moves all masses in the spacetime in a specified fashion. Is there an analogous theory that says the ...
84 views

### What is the role of the dilaton in Jackiw-Teitelboim 2D gravity?

I read that the Einstein Hilbert action is topological in 2 dimensions. (What does that mean?). To write down a non-trivial action one introduces the dilaton field in JT gravity. Does this field have ...
67 views

### Will a massive particle be attracted to an infinitely long beam of light?

I was attended to Bonnor beams. A Bonnor beam is the general relativistic exact solution for the spacetime surrounding an infinitely long straight beam of light (it includes also the spacetime ...
176 views

### What would you see dropping a sufficiently strong chain with substantial length into a black hole?

Here's a visual representation of the scenario Here you can see we have a black hole on the left. The event horizon is the edge of the black hole. You are far away from the event horizon, and a chain ...
80 views

### If gravitational potential energy is negative, why does not it produce any repelling force? [closed]

The gravitational potential energy is usually considered negative. There is even zero-energy universe theory. If so, why it does not produce any repelling force? Or does it in form of cosmological ...
101 views

### What is the physical nature of a "force"?

This has application to E-M as well as gravity, but let me use the gravitational example as I think it's a bit more conceptually easy to grasp. Say we have two 1kg weights placed on the ground a metre ...
47 views

### The gravitational potential at the center of a solid ball (confusion)

I came across this question during my physics class. Suppose we have a solid, spherical planet with mass $M$ radius $R$ s.t. the density of this planet is uniform everywhere, then what is the ...
154 views

### What is the gravity at the event horizon of a black hole?

I've been reading and couldn't find any answer to this question. What is the minimum required gravity to prevent light from escaping?
17 views

### Conditions for when gravitational force acts on the centre of mass [duplicate]

In my physics material it says the gravitational force can be considered to be applied to the centre of mass of a body only in the following cases: 1.The effective gravitational field is homogeneous 2....
30 views

### Internal force and centre of mass

I have still not yet learn about the centre of mass of a system but I really do need to solve this question that keeps bugging me. I know that internal forces cannot accelerate the centre of mass of a ...
17 views

### Can matter be coaxed into becoming an anisotropic photon emitter?

Please allow me explain my line of reasoning: Consider an isotropic photon emitter at the origin of 3D space $(0, 0, 0)$. Also, consider a sphere-shaped photon absorber at position $(0, 100, 0)$, with ...
31 views

11 views

### If a Spacecraft Was To Launch From An Asteroid Would it Be Easier To Minimize The Required Escape Velocity By Launching At A 45 degree angle? [duplicate]

If a Spacecraft Was To Launch From An Asteroid Would it Be Easier To Minimize The Required Escape Velocity By Launching At A 45 degree angle? Or Would It Be Better To Head Straight Up?
36 views

### Does anyone know where I can find values of gravity?

Does anyone know where I can find the values of gravity at different places across the globe and at different heights? I am doing an assessment on gravity and can't find any data for the values of ...
147 views

### Curvature vs Gravity [closed]

I understand how and why spacetime curvature is responsible for the formation of geodesics and how objects follow those curved spacetime paths. "Matter tells spacetime how to bend and spacetime ...