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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Calculate the time taken for a body to hit earth's surface [on hold]

Find the time taken for an iron cube of dimensions 0.1m × 0.1m × 0.1m, whose density is 7.85 g/cm³ to reach the earth's surface released from a height of 10m from earth's surface, provided gravity = 9....
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Gravity as a repulsive force

As we know that universe is expanding, why can't we think the gravitational force as repulsive in nature in the realms of dark matter and dark energy?
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Schwarzschild Radius of a Galaxy

If an ultra compact/dense Galaxy has a Schwarzschild radius same as it is own radius, how can it be observed from the outside of the Galaxy?
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How can light reach its speed? [duplicate]

The mass of a photon is said to be 0, But light (photons) get attracted due to gravity of a black hole, that means photons have a mass which is very very very small. And we know that any object with ...
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If gravity is a wave, what medium does it pass through? Is there a medium that it can not pass through?

I listened to a podcast discussing gravity and had this question. My apologies if it has been aptly answered somewhere else.
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Is there a Shapiro time delay to a signal sent to the outer solar sytem and back?

The Shapiro time delay to a light signal has two equal halves, according to Clifford Will, the space part and the time part. The time part is just the slowing effect of gravitational time dilation. ...
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Why can't gravity force an object past the speed of light? [duplicate]

I hope this question is not a duplicate (it doesn't seem to be) and that it is appropriate for this site. If we had a universe with only two bodies. One is ultra massive, and the other is very small. ...
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How does an object gain energy when it enters a potential field?

Conservation of energy says total change in energy is $0$. Let's assume a small object is in outer space (maybe like a football). It moves towards a planet (assume planet has no atmosphere for ...
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Gravitational field of an infinite cylinder [closed]

What is the expression, in terms of distance r from the axis, for the gravitational field around an infinite length cylinder of density D and radius R? The field around an infinitesimally thin wire ...
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Is there a cosmological constant $\Lambda$ in 2D?

It is well known that the Einstein tensor is trivially 0 in any two dimensions spacetime: \begin{equation}\tag{1} G_{\mu \nu}^{(2)} \equiv 0. \end{equation} Thus, it is not possible to formulate the ...
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Can I use the expansion of the universe to create or destroy energy?

Imagine I have a setup like this: (The objects are not attached to each other. The red objects are much heaver than the black one, and the setup is balanced.) This setup has a non-zero ...
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Does general relativity predict that the gravitational field of a black hole would be uniform about its center of mass?

Black holes are typically represented by extreme distortions of space, where space is so bent, that it reaches a single point (i.e., the singularity). Further, they are almost always depicted as ...
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How does a sudden acceleration change split gravity waves into a pair of counter-propagating components? (“instantaneous time mirror”)

The Nature article Time reversal and holography with spacetime transformations is cited by IFL Fluid Dynamics' Waves contain lots of information which includes the GIFs below. While the paper is ...
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Is there an experiment showing that the warping of space affects the shape of an electrostatic field? [duplicate]

General relativity assumes that space itself is warped by gravity, which is what gives rise to the shapes of the paths followed by objects subject to a gravitational field. If space itself truly ...
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Newton's Universal Law of Gravitation doubt

The Universal Law of Gravitation states that the module of the force, $F$ is $$F = \frac{GmM}{r^2},$$ where $m$ and $M$ are the mass of the two objects and $r$ is the distance between the two objects....
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Gravitational field of sun [closed]

Let there be a solar system without Earth. Now what happen if we place the Earth suddenly on its actual position does the gravitational force of sun acts on it immediately because of its pre ...
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Mounting spots for a rectangular shaped object on a wall for optimal weight distribution [closed]

I want to mount a rectangular metal plate (45 cm height, 32 cm width, 0.5 cm thickness) to my wall. For that I plan to glue magnet plates to my wall. A proper weight distribution on the mounting ...
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Spacetime - reasons for “unification” of space and time - why we imply some unity in the term spacetime?

Why do we say spacetime, as though stressing that this is some kind of unity: space together with time? I understand a little that mathematically we have 4 variables in the same (Einstein) equation(s)...
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Force acting on an object traveling upwards (ignoring air resistance) [closed]

Could someone please help explain to me which formula should be used for a question like this... A ball with the mass of 166.1 g is thrown vertically upwards with an initial velocity of 18.5 m/s. It ...
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Can two black holes on a collision course, escape the merger?

Let's consider in this hypothesis that the two black holes have similar masses and approximation speed 0,99 c. At the maximum approximation point, only their event horizons are invaded slightly. The ...
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Is there a need to consider the relativistic mass when calculating gravitational potential energy?

The equation for gravitational potential energy is: $$V(r) ={GMm\over r}$$ In the reference frame where $M$ is stationary. I am wondering for mass $m$ here do we need to consider its relativistic ...
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What is the skin depth of space?

It is known that space behaves like a 3D rubber sheet, when mass is present, and bends. Gravity can be explained by the curvature of this bending. Water forms a highly flat surface that also behave ...
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Riemann Dual Tensor and Scalar Field Theory

I'm trying to find the component equation of motion for the action in a paper. The action for the system is, $$S=\frac{m_P^2}{8\pi}\int d^4x\sqrt{-g}\bigg(\frac{R}{2}-\frac{1}{2}\partial_\mu\phi\...
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How do we prove gravity exist? [closed]

In order to prove gravity in it's basic form, we use a fact like an object released above ground will travel downwards. This I suppose is using the rule of inference where in theory it says There is ...
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Is it possible for a particle to decay via gravity?

Is it possible for a particle to decay via gravity? I know gravity is immensely weaker than the other forces, but all the other forces interact with particles. Do we need an understanding of quantum ...
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Does the static gravitational field move with the source instantly?

I have read this question: How fast does gravity propagate? where hawkeye says: So what does that mean? It means that the "speed of gravity" is the speed of light … technically. Changes in ...
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What will happen if a body with equal mass of water is submerged in water, will it drown or float?

Suppose I were to take some object that will have a mass that is equal to that of water and I put it in water, will it submerge or float assuming that the object is solid?
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Can gravitational waves be visualized in the elastic rubber sheet model of gravity?

To visualize the space curvature related to gravity, there is a model or analogy using an elastic sheet and a weight. A rubber sheet is spanned like a very soft trampoline, and a heavy ball on it ...
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Are there good reasons why special relativity should motivate geometrised gravity in a way that Newtonian mechanics does not?

I have studied a bit of Newton Cartan theory recently, the geometrised version of Newtonian gravity in which gravity is due to the curvature of spacetime, but is Newtonian (simultaneity is absolute). ...
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How do we know that Dark Energy has homogeneous energy density? [duplicate]

I’ve read places that the density of Dark Energy is the same throughout the universe. If the expansion density of dark energy were the same throughout the universe, why don’t all galaxies have the ...
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Does static gravity follow spacelike geodesics?

Thank @KyleKanos for a suggestion, I am rephrasing this question to hopefully make it more clear. According to this publication in Physics Letters, 2000: Aberration and the Speed of Gravity: It is ...
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What will happen if a vacuum chamber gets moved with a person inside?

Imagine that a person would be put inside a vacuum sphere, just like the spheres that NASA uses for simulating space's zero gravity. That person would be hovering right in the center of the sphere. ...
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How is the electromagnetic/gravitational force transmitted? [duplicate]

So I was thinking about how a positive and a negative charge (or positive/positive, negative/negative) interact. I have read previously about how photons carry the electromagnetic force. However, how ...
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How does gravitational potential energy work in Relativity?

According to relativity, the universe is expanding, at an accelerating rate. Far away planets and stars will soon leave our observable universe. But as planets and stars leave our observable universe, ...
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Clock ticks faster with altitude

Suppose there are two identical drones in a weak and uniform gravitational field that are initially at the same place and with synchronized clocks. Assume drone A first rises vertically for 10 ...
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To a distant observer, is the trajectory of an object falling toward a black hole always reversible?

I think (pure intuition) that every object that falls towards a massive center, in the condition of free fall, should always be able to return to the starting point. Even if this massive center is a ...
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Feynman diagrams for gravity

Feynman rules is the basic tool to compute amplitudes in perturbation theory for a QFT. Here, I am trying to understand perturbation theory in GR around the flat space metric, in terms of Feynman ...
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Ricci scalar in terms of vierbein and spin connection

I have been trying to derive the following form for the Ricci scalar in terms of vierbein and spin connection $$R=(e^{\mu a}e^{\nu b}-e^{\mu b}e^{\nu a})(\partial_\mu \omega_{\nu ab}+\omega_{\mu a}^{\...
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The sign of $d\mathbf{r}$ when integrating the universal gravitational force in order to define gravitational potential energy [duplicate]

I am trying to find gravitational potential energy by integrating the gravitational force: $$\mathbf{F}(\mathbf{r}) = - \ G \frac{Mm}{|\mathbf{r}|^2} \ \hat{\mathbf{r}}$$ where $\mathbf{r}$ is the ...
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Scalar coupled to Gauss-Bonnet invariant vs Horndeski theory

So here it is a somewhat tormenting question. The first statement will be a little specific but then I will make clear what the jargon indicates. How can we show that a Lagrangian made of a scalar ...
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Definition of “specific gravity”

I've learnt that a specific quantity is an extensive quantity divided by the mass. How does the definition of specific gravity fit into this scheme?
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Gravitino Equation of motion in second-order formalism

In Freedman and Proeyen's text on supergravity they derive the equation of motion for the gravitino using the second order formalism. However, I'm not exactly clear as to how they use partial ...
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How is spacetime warped by a massive object?

I was going through this question (Why don't planets have Circular orbits?) related to planetary orbits. In the accepted answer it is stated that orbits are actually conic sections. Given this ...
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What is the difference between Einstein aether theory and general relativity?

I'm currently doing some research on the various aether theories and I stumbled upon Einstein aether theory. It appears to be much different from Luminiferous aether and be more closely related to ...
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Can we feel gravity while free falling?

I watched some video saying that there is still gravity in space station and almost same strong as it on Earth, the reason they feel no gravity is because the centrifugal force of the orbit is same as ...
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If time slows in the presents of gravity, how does the conclusion that the universe’s expansion is accelerating account for this?

I am sure there is a simple explanation for this and I’m not a physicist so hoping someone here can help answer this for me. Time is retarded by gravitational field. Expansion in all directions ...
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Is it possible that the acceleration of the growth of our universe is driven by the overall gravitational pull of neighboring universes [closed]

I am painfully unequipped when I think about my my knowledge of physics. Still, I am enthralled, as many of us are, by the mysteries and complexity of the universe, both known and unknown. I was ...
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How would classical physics explain the sorting of objects according to density in a gravitational field?

I'm wondering how classical physics (i.e., using Newtonian Gravity) would explain why it is that objects are sorted by density in the presence of a gravitational field?
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Finding suitable element to perform integration upon [closed]

Is there any precise (proper) method or technique to specify the element on which integration will be performed. Is it the same for all properties like moment of inertia, gravitational potential, ...
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Negative masses [duplicate]

If there existed negative masses how would one use Newton's law of gravitation (since it doesn't spit out the direction of the gravitational force but only the magnitude like Coulomb's law)? Would ...