Gravity (from Latin gravitas 'weight'), or gravitation, is a natural phenomenon by which all things with mass or energy—including planets, stars, galaxies, and even light—are attracted to (or gravitate toward) one another. On Earth, gravity gives weight to physical objects, and the Moon's gravity causes the ocean tides. The gravitational attraction of the original gaseous matter present in the Universe caused it to begin coalescing and forming stars and caused the stars to group together into galaxies, so gravity is responsible for many of the large-scale structures in the Universe. Gravity has an infinite range, although its effects become weaker as objects get further away.
Gravity is most accurately described by the general theory of relativity (proposed by Albert Einstein in 1915), which describes gravity not as a force, but as a consequence of masses moving along geodesic lines in a curved spacetime caused by the uneven distribution of mass. The most extreme example of this curvature of spacetime is a black hole, from which nothing—not even light—can escape once past the black hole's event horizon. However, for most applications, gravity is well approximated by Newton's law of universal gravitation, which describes gravity as a force causing any two bodies to be attracted toward each other, with magnitude proportional to the product of their masses and inversely proportional to the square of the distance between them.
Gravity is the weakest of the four fundamental interactions of physics, approximately 1038 times weaker than the strong interaction, 1036 times weaker than the electromagnetic force and 1029 times weaker than the weak interaction. As a consequence, it has no significant influence at the level of subatomic particles. In contrast, it is the dominant interaction at the macroscopic scale, and is the cause of the formation, shape and trajectory (orbit) of astronomical bodies.
Current models of particle physics imply that the earliest instance of gravity in the Universe, possibly in the form of quantum gravity, supergravity or a gravitational singularity, along with ordinary space and time, developed during the Planck epoch (up to 10−43 seconds after the birth of the Universe), possibly from a primeval state, such as a false vacuum, quantum vacuum or virtual particle, in a currently unknown manner. Attempts to develop a theory of gravity consistent with quantum mechanics, a quantum gravity theory, which would allow gravity to be united in a common mathematical framework (a theory of everything) with the other three fundamental interactions of physics, are a current area of research.
I’m writing a 3DOF sim for a rocket. I’m having a hard time visualizing centrifugal effects from the Earth's rotation on the rocket while it is moving along the rail (acceleration > 0).
I know that once it has left the rail I no longer need to account for it since it’s in the ECI frame and...
In this 1965 paper by Weinberg, https://journals.aps.org/pr/abstract/10.1103/PhysRev.138.B988, he describes a quantum field theory of the graviton in a Coulomb-like fixed gauge, where the free graviton has only space-space components and is traceless. This of course makes the field dynamics...
~ Shower Thoughts ~
Twin A is in a spaceship, Twin B is in a spaceship. Both in 'deep space'.
B follows a highly elliptical geodesic which goes around a planet (or black hole) with strong gravity, very far away.
When they meet again, who is younger and why?
I genuinely don't know what this...
My understanding, possibly my ignorance, is that dark matter is calculated to exist from observations that there's not enough matter to fit observations if the current theory of gravity is right.
Is it possible to put into a nutshell why the case for dark matter together with current...
Hello!
The paper I study is related to string theory and modified gravity theories topics.
As they say in page 5 “The four-dimensional effective theory now follows by substituting Eq. (13) into the original action, Eq. (4)”
I wonder how did they drive a 4- dimensional effective metric...
Hi,
On this link: https://physicsteacher.in/2020/07/11/the-formula-for-acceleration-due-to-gravity-at-height-h-with-derivation/
They prove the formula for acceleration due to gravity at height h, which is: g1 = g (1 – 2h/R).
There are similar articles online.
When they go through the last...
Hi Pf
I am reading the origin of gravity written by Erik Verlinde
I have the same problem with a paper written by Jacobson on the same subject.
They want to deduce gravity from the holographic principle and thermodynamics.
I suppose that they cannot talk about geodesic at the beginning. the...
In a textbook, which is not in Englisch language unfortunately, I found a passage saying that intrinsic curvature of spacetime is just a specific definition. The alternative definition is that spacetime is flat, whereas clocks and rods have variable lengths - which is just Feynman’s bug...
I didn't pay them much attention until I saw the PBS Space Time's one,
and I went "What!? Really!? Am I missing something, here, like a brain?
Have they all gone crazy"? And then I remembered where I first saw it:
In Brian Greene/Horn/e's 2011 The Hidden Reality book.
On P14 there is a...
I am an Engineer, so I have some knowledge about physics and calculus. I've seen relativity in university years ago but only briefly. The majority of my knowledge in SR and GR, which isn't much, are from reading, studying and searching online myself.
I have seen some definitions that I am not...
It's a puzzle. I have decomposed vector v by using formulas known from physics: m*g*sin(theta) and m*g*cos(theta).
I got: ##\vec v = (5, 5*\sqrt{3})##
But it has been marked as wrong. Consequently, the rest of my calculations is not correct. Could you tell me, why?
Hi, I need to prove that the tension for artificial satellite consists of two points of mass m/2 connected by a light rigid rod of length , the tension in the rod is -
$$ T=\frac{3}{4}\frac{Gmm'l}{a^3}-\frac{1}{4}\frac{Gm^2}{l^2} $$
the satellite is placed in a circular orbit of radius a>>l...
As per the summary I don't understand why physicists talk as if gravitons are inevitable, when gravity is just curved spacetime? Why would curved spacetime have a particle?
The equivalence principle states that a person stood on Earth would experience “gravity” the same as if he was in an elevator in space traveling at 1g. I get this. but when Einstein was first exploring this, I read he came to the realisation that a person free falling on Earth (if in a vacuum)...
I was reading Einstein's 1911 paper named "On the Influence of Gravitation on the Propagation of Light" when stated the formula for frequencies measured by observers at different fixed positions (heights) on Earth surface. One observer is at the origin of some coordinate system and measures a...
I imagined all possible quantum gravity theories in one set, of which only one is correct, and further divide these theories into testable and non-testable. Question: how do we know if the correct theory is currently testable? If it is testable, then we need to come up with new theories until...
OK. Gravity is not a force it is a contraction or curvature of space.
I was free-falling and now I hit the ground. Why don't I float through the universe, or go upward instead of still trying to go downward.
Because I hit the ground, and now there is no force(like gravity) and my free-falling...
Let's say an object far far away from the Earth free falls in gravitational field. At Earth's surface free falling object gains kinetic energy E_1.
Let's say an electron far away from the proton free falls in electromagnetic field. At Bohr's radius free falling electron gains kinetic energy E_2...
Hello,
I am very new to the concept of center of gravity and I have a question. I wanted to know if the center of gravity of an object is always in the same location in 3-D space. For example, if I was able to find the center of gravity for cylinder/rectangle when its lying flat on a horizontal...
Let's say a mass is gently laid on top of a massless spring. The spring compresses.
There is a change in the height of the mass. Therefore, there is a change in the gravitational potential energy: a decrease.
The compressed spring now has potential energy (it has gained energy).
The change...
If I hold a ball above the ground, it has potential energy. Once gravity pulls on it, it becomes kinetic. What is gravity and how does it convert one kind of energy to another?
I had a bit of a brainwave regarding artificial gravity (outside of rotation or linear acceleration). As we know "gravity plating" is just a plot device to allow sci-fi films and shows to escape the pain of having to accurately recreate a zero-g environment. But then I thought: gravity is a...
It's one of those staples of sci-fi. Unless you're going for a hard sci-fi that uses rotation or linear acceleration to achieve simulated gravity, your starship will likely use "gravity plating". As we all know, gravity plating is just a plot device, usually for shows with more limited budgets...
Could I please ask for help with the following:
Given: The centre of gravity of a uniform solid right circular cone of vertical height h and base radius a is at a distance 3h/4 from the vertex of the cone.
Such a cone is joined to a uniform solid right circular cylinder of the same material...
With a pure die, all odds are equal. With a pure die, the center of gravity is exactly in the middle of the die. But what if the center of gravity is not in the center? How are the odds then. For example, how do the odds become if the center of gravity is exactly on the line that runs through...
Hi all,
I've recently become transfixed with the idea of magnetism and gravity.
I have two main questions I'd like to see discussed more.
1. Would it be possible to harness magnetic fields for energy on earth/for acceleration between planets?
1a. Has this been studied at all?
2. Why do we...
So far, I have found g of the foreign planet, Vf of the ball, and realize that GPEiA = KEfA (Am I right here?). Thus, since GPEiA = GPEiE, GPEiE also equals KEfE. I also understand that, since the same impulse is applied to catch the objects, both objects have the same momentum at the moment...
My name is Dilip (James) I am fascinated by physics and have written three books on the subject, which indicates my level of interest.. My latest book “The Electromagnetic Universe: A New Physics” is available on Amazon and describes in detail: a new theory on the propagation of light, a new...
I want to derive an acceleration in the case for a stationary mass in the gravity field.
I found the total energy in the GR is provided by a simple equation:
https://en.wikipedia.org/wiki/Schwarzschild_geodesics
## E = mc^2\sqrt{1 - rs/r} * \gamma ##
So, this is easy to provide acceleration...
Could I please ask for help with the following question. Part 2 is my problem. I have no idea how to begin, any hints would be much appreciated:
1) Prove that the center of gravity of a uniform triangular lamina is the same as that of three equal particles placed at the vertices of the lamina...
At 0:51 in this simulation video there seems to be a brief moment upon merging that these singularities produce what appears to be negative gravitational "Spike?" that seems to bulge spacetime in an opposite metric compared to normal spacetime flatness. Is this a quirk of the simulation or is...
A recurring topic in particle physics, is the possibility of a "desert" above the electroweak scale: no new physics (new particles, new forces) until the grand unification scale or the Planck scale. It's important to remember that the Higgs boson mass was correctly predicted three years in...
I'm re-watching Star Trek TNG and I just started the episode where they encounter Scotty aboard a ship that's crashed into a Dyson sphere.
That got me thinking. What would the mass and external surface gravity of a Dyson Sphere be? I've done the math myself, but I'd appreciate someone double...
Could I please ask for help with the following:
A lamina ABCD is in the form of a trapezium in which DC is parallel to AB, AB = 2a, CD = a and AD = h and the angle BAD is 90 degrees. Find the position of the centre of gravity of the lamina from the edges AD and AB.
The lamina is placed...
[Mentor Note: thread split off from a different thread]
https://www.physicsforums.com/threads/heavier-objects-fall-faster.1002022/
Since seeing this thread yesterday, I have been trying to derive the time equation for the collision of two masses due to Newtonian gravity. Unfortunately, this...
Newton thought of gravity as action at a distance. Einstein showed that gravity is the curvature of space-time. Einstein's General Relativity is the best answer, but neither discusses gravity at the quantum level. I tend to think of gravitational force to be the result of matter interacting with...
I remember being told that heavier things fall at the same rate as lighter things when everything else is equal. But this guy does a demonstration of a Xenon balloon falling faster than a Neon balloon. Why? Also an overloaded plane may not be able to fly, but it has the same aero dynamics, it...
Could I please ask for help as to why I disagree with a book answer on the following question:
Answer given is book is $$\frac{1}{2}(a+b)$$
Here's my proposed method:
Prior to this question there is an example of a similar question:
And here is the answer:
So, to solve my question I...
Does intervening mass between two stars decrease the gravitational attraction between these two stars?
Is gravity a local phenomenon in that local mass interacts with the surrounding gravitational field caused by the local star and distant stars?
Conceptually I have never really understood Ohm's law, other than using it in calculations. My brain just cannot understand why this equation works.
I was wondering if there is some sort of gravity equivalent to perhaps conceptualise it a bit better.
Voltage: would be the acceleration due to...
I would like to understand if matter accumulates into the gravity well of the Earth?
After so many years of circling the sun I have this imagination of tritium, nitrogen, and ozone accumulating underneath the Earth into it's traditional orbit, and accumulating over time.
Is this possible and...
I'm reading through Stephen Hawking's 'A Brief History of Time' and came across this sentence in the second chapter:
" If the law were that the gravitational attraction of a star went down faster or increased more rapidly with distance, the orbits of the planets would not be elliptical, they...
Hi everybody
I saw quite a nice Youtube vid about general relativity and how gravity bends spacetime and therefor redirects angular momentum into the center of gravity. I thought the first time I begun to understand the concept but immediatly the questions poped up.
The video basically says...
Assuming that the dark matter problem is finally solved by some kind of MOND, how that would change the scene for quantum gravity?, how that would change the proposed/expected properties of the Graviton (massless spin 2 particle)?
This video explains gravity in a way I haven't encountered before (regardless of how irritating the presenter may be). Nevertheless, I find it hard to believe that a squirrel falls from a tree to the ground due to gravitational time dilation between its head and its feet. The amount is so...
Hi, I’ve got a question.
it seems that engineers, structural mainly, don’t seem to understand how gravity works regarding an endless stationary (steel) ring/bridge around the equator.
Many if not all have said that such a ring would be modeled as two half circles pushing into each other. So the...