What is Gravity: Definition and 993 Discussions

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.

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  1. F

    I Mond - gravity as two separate forces?

    Is it possible that MOND might be standard gravity plus an additional force that is inversely proportional to distance. One that is also a property of matter? I ask, because viewing this way begs the questions: If two, then why not more, and if more then perhaps a series with a hope of...
  2. P

    Misunderstanding Work-energy theorem and center of mass properties

    To do this apparently, you need to use the work-energy theorem. You can calculate work done by gravity easily. However it was said that work done by the reaction forces from the hinge is zero, I don't get why. Reaction Force from the hinge is an external force on the rod, and all external...
  3. H

    Gravitational acceleration magnitude - confused

    The given answer is g/4. But when I substituted R/4 into the radius, I get 16GM. Am I just using the wrong equation altogether? He also said that you also got g/4 if the distance was 2R.
  4. K

    I How does gravitational self-interaction affect the Milky Way Galaxy?

    It is believed that gravity interacts with itself. I assume that gravity between stars increases. Does gravitational self-interaction change the galaxy's shape or increase the rotation curves of stars?
  5. G

    The period of a simple pendulum

    While not exactly correct, we will continue to use Newtonian gravitational force and tension force in the lab frame. We will not concern ourselves with GR, besides the approximation is reasonable for low velocity and small mass. In the lab frame, the forces acting on the pendulum is weight and...
  6. domv95

    B Pumping water upwards into a large water tank

    Hi everyone! Not sure if this is the right place, but here we go: I am layman when it comes to physics, so I might be overthinking this completely, but I need to make sure that the physics stack up before I do the actual pipes etc. I have a 1000-litre water tank and below it I have a smaller...
  7. L

    I Question about gravity being a distortion of spacetime

    people say that gravity is not a force, that it's rather a distortion of space-time so objets that go to a gravitational center are actually just moving through space in linear direction but there is a problem in this theory shouldn't objects go to the center in constant speed? why are they...
  8. U

    I Why normal force is not equal to gravity?

    We learned that normal force equal in magnitude as mg and opposite direction, it is reaction force to gravity. 1.If normal force is not equal gravity, isnt this violate newton 3. law action=reaction? 2. If gravity is higher than normal force , this system has net Force non zero,it means...
  9. C

    I Effects of gravity on planetary core structure

    Hello everyone! I joined this forum to reach out to people much smarter than me. After searching the matter, i realize this subject has been beaten to death for a VERY long time. But, as of yet, I have not stumbled upon a definitive answer to my question. Flipping through my news feed...
  10. jeff einstein

    B Am I attracted to a hydrogen atom 1 billion light years away?

    Just a quick question, are we attracted by a hydrogen atom billions of light years away by gravity or is my gravity completely masked by earth's gravity and so I am not attracted by the hydrogen atom? I am talking on a very very very small and incomprehensible scale.
  11. S

    I Dark energy contributing to, or modifying, mass estimates?

    I have found some papers (like this one: https://www.aanda.org/articles/aa/full_html/2009/45/aa12762-09/aa12762-09.html) which say that dark energy increases the potential energy in a system of a quasi-stationary gravitationally bound many-body system. It also says that because of this, the...
  12. hyksos

    I Observation of the effect of gravity on the motion of antimatter

    The Alpha Experiment at CERN has finally produced a paper on whether antimatter falls towards the earth under gravity. The research confirms that antimatter acts identically to regular matter in regards to gravity. Observation of the effect of gravity on the motion of antimatter. Anderson...
  13. mfb

    I Antimatter falls down - with (0.75 ± 0.13 ± 0.16)g

    Took longer than expected, but now we have a result. Observation of the effect of gravity on the motion of antimatter Antimatter falls down with (0.75 ± 0.13 (statistical + systematic) ± 0.16 (simulation)) g, compatible with the expected 1 g and ruling out -1 g.
  14. Y

    Questions About the Atmospheres of Various Sizes of Artificial Planets

    I have some questions about the atmospheres of various sizes of artificial planets. A lot of this could apply to natural planets from the size of of Mercury to a radius about 1.5x Earth. My questions are about a larger range of sizes, from the smallest in my question in the following paragraph...
  15. J

    Where to discuss new theories.

    Hi, Fairly new to the forum, but trying to post some ideas on new ways to look at gravity, but not sure where the correct place is. Where is the best place to discuss new ideas please? John.
  16. haushofer

    I Remark on centrifugal force in Heino Falcke's black hole book

    Dear all, currently I'm reading the Dutch translation of Heino Falcke's "Light in the Darkness: black holes, the universe and us" as a preparation on a course on black hole I'm giving later this year. In part 1 it contains a remark about space telescopes, and the author imagines us to travel...
  17. jeffinbath

    I Surely the 2nd Law of Thermodynamics is defied by gravity?

    As our sun and the other stars evolved from gravitationally led aggregations of hydrogen gas which permeated our early universe then that is an example of a high entropy system becoming a low entropy system and the so-called "arrow of time etc." was reversed?
  18. J

    I Gyroscopic precession of a spin-stabilized bullet

    Hi! I am trying to understand the physics behind the gyroscopic phenomenon called spin drift. Spin drift occurs to bullets that are spin-stabilized over the course of their flight. Spin drift starts with an induced rotation in a spin stabilized bullet. As a bullet flies through the air...
  19. feynmansorange

    Verifying the acceleration of gravity in our lab (help with error please)

    Data and graphs here (the time is measured every 1/30 of a second, but for some reason, Google Sheets thought 1/30 wasn't a number so its in decimals!!! very confusing!!! so sorry about that!). why is the slope only 4.68, should it not be 9.81m/s^2? is the slope of m/s^2 not supposed to be...
  20. L

    B Busting the myth about achieving artificial gravity by rotating a body

    Movement isn't required to achieve gravity, even the artificial kind. Standing on a rotating body requires you to be actually physically standing on it. How is this achieved? Well, by gravity of course. Unless you are tied/stuck to the rotating body, but in that case there is no gravity pushing...
  21. Kinker

    I Does gravity cause quantum decoherence?

    Does gravity cause quantum decoherence? In the microscopic world, gravity seems to act weakly, but in the macroscopic world, it seems to act strongly. Is this the boundary between the microscopic world and the macroscopic world? So a phenomenon like quantum tunneling can occur in the microscopic...
  22. ejacques

    B Why do we feel gravitational acceleration from the Earth and not from the Sun?

    The acceleration near the earth, due to the force of gravity is g. Now every particle when moving in a curve trajectory had a centripetal acceleration towards the center (say the sun) a=(v^2)/R. If this is true why we measure weight only with the account of g? I guess when R is big it might be...
  23. L

    Converting gravity into pressure inside the very center of a planet

    Gravity in the very center of the planet must be zero because all other atoms are pulling evenly around the center. We have a deap gold mine in South Africa and uranium waste storage Norway. Has the force of gravity shown change at these depths. Even at the bottom of the Pacific ocean gravity...
  24. S

    B Explaining Variations of Free-Fall Gravity on Earth?

    I stumbled upon gravitational variations that do not coincide with altitude, latitude or lunar interference. The only logical explanation I have for this would be the geology or makeup of the earth below those locations. But what is the variation of the Earths makeup below the crust? From my...
  25. P

    When is the acceleration due to gravity negative and when is it positive?

    I know to break it down into its x and y components and then use Pythagorean: Acceleration in the x direction is Fx/m ---> (7.50 x 10^6*cos55) / (4.50 x 10^5 kg) = 9.56 m/s^2 Acceleration in the y direction is: (Fy - mg)/m ---> ((7.50 x 10^6*sin55) - (4.5 x 10^5* 9.8 m/s^2)) / (4.5 x 10^5 kg)...
  26. Ken G

    B Beware of "truthiness" in astronomy in seemingly reliable websites

    It is widely known that the more massive a main sequence star, the higher its luminosity, which is called the mass-luminosity relation. This relation is so important, many authors and educators cannot resist the temptation to include an explanation for it. While I applaud that sentiment...
  27. Demystifier

    A Thermodynamics/gravity/CFT correspondence

    Some evidence (Bekenstein, Hawking, Jacobson, Verlinde, ...) points to the idea that gravity is really a thermodynamic theory in disguise. Other evidence (Susskind, Maldacena, Witten, ...) points to the idea that gravity is dual to a lower dimensional conformal field theory (CFT). Are these two...
  28. jaketodd

    B What does 3-dimensional space deform into, in the presence of gravity?

    I have no expertise in this area, other than rudimentary concepts. The following might apply if the visualization of space, as depicted below, represents actual reality, but I don't know for sure. Please help me understand better, you guys! 2-dimensional space The curvatures deform into the...
  29. S

    B How does gravity's warping of spacetime appear first among the forces?

    The Wikipedia section below says that gravity is the earliest to appear out of the fundamental interactions, but wasn't high energy in every area already curving its local spacetime surroundings? https://en.m.wikipedia.org/wiki/Chronology_of_the_universe#The_very_early_universe
  30. dwinkley

    Rock falling down a tunnel

    all i could accomplish was calculating the distance between P and the ceiling in a horizontal line(6)
  31. S

    B Does the inverse square law equalize gravity at different spots?

    Inverse square law would reduce the gravity from the parts of Earth that are farthest from our feet. It'll also reduce the gravity from Earth's center by a lesser amount, but would that be lesser enough so the gravity 20 kilometers under our feet is stronger than the core's gravity or even the...
  32. shunyadragonvv

    I Quantum decoherence and the emergence of space/time and gravity

    Quantum decoherence. and the emergence of continuous space/time and gravity In another forum I have experienced a lot of combative dialogue asserting that continuous time/space is a property of the smallest Quantum scale. My present knowledge indicates this not true, and that the goal of the...
  33. Bertin

    Studying Comparing Candidate Theories of Quantum Gravity

    Hi, you all, I have been for a couple of semesters interested in quantum gravity as a problem, but truth is I never have been properly introduced to any of the candidate theories. Actually, there are multiple candidates and I would like to compare them. The question then is the following: do...
  34. Mohmmad Maaitah

    Work done by gravity and the minus sign

    Shouldn't work be minus when the man climbing up and force on him is down? shouldn't the power be also in minus? Can someone explain to me why is it positive please!
  35. Leonard Begy

    B Gravity waves and Planck's constant

    Can the energy of a gravitational wave be related to Plancks constant?
  36. Thytanium

    I Inertial Gravity in Relativistic Geodesics - Forum Discussion

    Hello friends of the Forum. I want to ask you why the inertial acceleration in free fall in the relativistic geodesic equations is assumed equal to zero in free fall and equal to 9.8 m/s at rest on the earth's surface. On the other hand, assuming that zero acceleration in free fall, what would...
  37. acertainayush

    Experimentally calculating the acceleration due to gravity

    [Mentors' note: No template because the thread was moved from the technical forums] TL;DR Summary: I need help with determining the value of 'g' using the data I have collected in the lab, using an apparatus consisting of light gates fixated on a stand, the positions of which can be varied...
  38. P

    B Space Pulled into Gravity Well: What Happens?

    Many explanations out there for gravity. One that I saw a few months ago explained that our gravity hete on earth is space moving toward the center of the earth, and we should be falling to the center of earth at a rate of 9.8 m/ s/s. But the surface of the earth is in the way... What happens to...
  39. jed1408

    I Can angular motion exist outside of a gravity field?

    I can't find the answer anywhere here's my question. can a centrifuge exist outside a field of gravity. More specifically, in a theoretical void of nothing without stars in view or any point of reference for comparison how could motion like spinning or acceleration exist?
  40. P

    Finding the apparent acceleration due to gravity versus latitude

    I began by drawing a diagram and resolving the forces. Since the question asked for 'apparent gravity' I tried to find the normal force. I started with the equations: $$\\(\frac{GM}{R^2}-N)sin\lambda-Fsin\lambda=m\omega^2Rcos\lambda$$ $$\\(\frac{GM}{R^2}-N)sin\lambda-Fcos\lambda=0$$ Solving...
  41. doudou

    I Gravity does negative or positive work with cosmos redshift?

    Based on the fact of observed cosmological redshift, scientists have proposed different ideas to explain. One interesting question is whether gravity does negative or positive work now: According to universe expanding in Big Bang theory (Lemaitre, 1927), obviously gravity does overall negative...
  42. J

    MTW Exercise 12.6: Formulating Newtonian Gravity Using Curvature

    Hello, I am attempting to work through problem 12.6 in MTW which involves formulating Newtonian Gravity using Curvature as opposed to the standard formulation. This is a precursor before standard GR. In it he states that the curvature tensor in this formulation is as follows...
  43. E

    How to calculate Earth speed of the Moon induced orbit?

    It didn't work and I don't know how to do it.
  44. H

    I Dark Energy Strength in Great Voids of Galaxies

    Assuming dark energy is fairly, uniformly distributed through out the cosmos, how strong is it, or how much energy is associate with it, out in the deepest, emptiest voids in space? I'm specificlaly refering to the great voids in between the great walls of galaxy clusters. I'm making the...
  45. T

    Model for Gravity -- What mechanism distorts space in the real case?

    Homework Statement:: Model for Gravity Relevant Equations:: Rg -Rg = G/(8pi*c^4)T The rubber table model for gravity can't quite translate to reality. One sees that a ball placed on the table distorts it, but this is due to it being in a uniform gravity field. Just what mechanism distorts...
  46. L

    B If gravity was a force wouldn't going back in time cause us to float?

    This might sound as a dumb and silly question but if you think about it, it makes sense. If we wrongly assume that gravity is a force just like any other, and given the fact that time is closely related to gravity and that gravitational time dilation is a thing, wouldn't reverse time travel...
  47. L

    B Where does the energy of gravity come from?

    Since gravity accelerate things (even with some people saying that it's not a force) there must be a consense that it adds energy to things. There are even hydroeletric power plants generating energy everywhere. So where does the energy of gravity come from? Is it spontaneous generation of energy?
  48. DANIELWR1998

    How do I find way of comparing the density of the Earth and the Moon?

    Surface acceleration is proportional to density and radius of planet (as 2 powers of R cancel with the volume) g(moon)/g(earth) = density(moon)*radius(moon)/density (earth)*radius(earth) = (1/4)*density(moon)/density(earth)
  49. C

    I How to Land a Tree Flat in 3/4 Rotation: Solving the Equations

    Can anyone tell me how to solve this problem? I have the stem of a tree that is X feet tall. It's just a cylinder as the top and all branches have been cut off. I want to cut off the top portion such that when it falls, it will do precisely a 3/4 rotation and land perfectly flat. What fraction...
  50. Malapine

    Interplanetary Portals vs Gravity

    For story purposes, I want a portal from the Earth to a sealed cave on the Moon. But how would gravity behave near such a portal? A test mass on the moon end feels 1.6m/s^2 acceleration towards the center of the Moon, a test mass on the Earth end feels 9.8m/s^2 acceleration towards the center...