Gravity from Latin gravitasmeaning 'weight' [1]or gravitationis a natural phenomenon by which all things with mass or energy —including planetsstarsgalaxiesand even light [2] —are brought toward or gravitate toward one another. On Earthgravity gives weight to physical objectsand the Moon 's gravity causes the ocean tides. The gravitational attraction of the original gaseous matter present in the Universe caused it to begin coalescingforming stars —and for 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 increasingly weaker as objects get further away. Gravity is most accurately described by the general theory of relativity proposed by Albert Einstein in which describes gravity not as a forcebut as a consequence of the curvature of spacetime caused by the uneven distribution of mass. The most extreme example of this curvature of spacetime is a black holefrom which nothing—not even light—can escape once past the black hole's event horizon.

Gravity is the weakest of the four fundamental interactions of physics, approximately 10 38 times weaker than the strong interaction10 36 times weaker than the electromagnetic force and 10 29 times weaker than the weak interaction. As a consequence, it has no significant influence at the level of subatomic particles.

The ancient Greek philosopher Archimedes discovered the center of gravity of a triangle. The Roman architect and engineer Vitruvius in De Architectura postulated that gravity of an object did not depend on weight but its "nature".

In ancient India, Aryabhata first identified the force to explain why objects are not thrown outward as the earth rotates. Brahmagupta described gravity as an attractive force and used the term "gurutvaakarshan" for gravity. Modern work on gravitational theory began with the work of Galileo Galilei in the late 16th and early 17th centuries.

In his famous though possibly apocryphal [11] experiment dropping balls from the Tower of Pisaand later with careful measurements of balls rolling down inclinesGalileo showed that gravitational acceleration is the same for all objects.

This was a major departure from Aristotle 's belief that heavier objects have a higher gravitational acceleration. Galileo's work set the stage for the formulation of Newton's theory of gravity. InEnglish mathematician Sir Isaac Newton published Principiawhich hypothesizes the inverse-square law of universal gravitation.

In his own words, "I deduced that the forces which keep the planets in their orbs must [be] reciprocally as the squares of their distances from the centers about which they revolve: and thereby compared the force requisite to keep the Moon in her Orb with the force of gravity at the surface of the Earth; and found them answer pretty nearly.

Where F is the force, m 1 and m 2 are the masses of the objects interacting, r is the distance between the centers of the masses and G is the gravitational constant.

Newton's theory enjoyed its greatest success when it was used to predict the existence of Neptune based on motions of Uranus that could not be accounted for by the actions of the other planets. A discrepancy in Mercury 's orbit pointed out flaws in Newton's theory.

By the end of the 19th century, it was known that its orbit showed slight perturbations that could not be accounted for entirely under Newton's theory, but all searches for another perturbing body such as a planet orbiting the Sun even closer than Mercury had been fruitless. The issue was resolved in by Albert Einstein 's new theory of general relativitywhich accounted for the small discrepancy in Mercury's orbit. This discrepancy was the advance in the perihelion of Mercury of Although Newton's theory has been superseded by Einstein 's general relativity, most modern non-relativistic gravitational calculations are still made using Newton's theory because it is simpler to work with and it gives sufficiently accurate results for most applications involving sufficiently small masses, speeds and energies.

The simplest way to test the weak equivalence principle is to drop two objects of different masses or compositions in a vacuum and see whether they hit the ground at the same time. Such experiments demonstrate that all objects fall at the same rate when other forces such as air resistance and electromagnetic effects are negligible.Gravityalso called gravitationin mechanicsthe universal force of attraction acting between all matter.

It is by far the weakest known force in nature and thus plays no role in determining the internal properties of everyday matter. On the other hand, through its long reach and universal action, it controls the trajectories of bodies in the solar system and elsewhere in the universe and the structures and evolution of stars, galaxies, and the whole cosmos.

Gravity is measured by the acceleration that it gives to freely falling objects. Thus, for every second an object is in free fall, its speed increases by about 9.

At the surface of the Moon the acceleration of a freely falling body is about 1. The works of Isaac Newton and Albert Einstein dominate the development of gravitational theory. The launch of space vehicles and developments of research from them have led to great improvements in measurements of gravity around Earth, other planets, and the Moon and in experiments on the nature of gravitation.

Newton argued that the movements of celestial bodies and the free fall of objects on Earth are determined by the same force. The classical Greek philosophers, on the other hand, did not consider the celestial bodies to be affected by gravity, because the bodies were observed to follow perpetually repeating nondescending trajectories in the sky. Those Aristotelian concepts prevailed for centuries along with two others: that a body moving at constant speed requires a continuous force acting on it and that force must be applied by contact rather than interaction at a distance.

These ideas were generally held until the 16th and early 17th centuries, thereby impeding an understanding of the true principles of motion and precluding the development of ideas about universal gravitation. The 17th-century German astronomer Johannes Kepler accepted the argument of Nicolaus Copernicus which goes back to Aristarchus of Samos that the planets orbit the Sunnot Earth.

Using the improved measurements of planetary movements made by the Danish astronomer Tycho Brahe during the 16th century, Kepler described the planetary orbits with simple geometric and arithmetic relations.

He realized that bodies that are uninfluenced by forces continue indefinitely to move and that force is necessary to change motion, not to maintain constant motion. In studying how objects fall toward Earth, Galileo discovered that the motion is one of constant acceleration. He demonstrated that the distance a falling body travels from rest in this way varies as the square of the time.

As noted above, the acceleration due to gravity at the surface of Earth is about 9.

Galileo was also the first to show by experiment that bodies fall with the same acceleration whatever their composition the weak principle of equivalence. Article Media. Info Print Print. Table Of Contents. Submit Feedback. Thank you for your feedback. Gravity physics. Written By: Kenneth L. Nordtvedt Alan H.

Cook James E. See Article History.

## What Is Gravity?

Get exclusive access to content from our First Edition with your subscription. Subscribe today. The planets describe elliptic orbits, of which the Sun occupies one focus a focus is one of two points inside an ellipse ; any ray coming from one of them bounces off a side of the ellipse and goes through the other focus.

The line joining a planet to the Sun sweeps out equal areas in equal times. The square of the period of revolution of a planet is proportional to the cube of its average distance from the Sun. Load Next Page. More About. LiveScience - What is Gravity? Articles from Britannica Encyclopedias for elementary and high school students.Gravityalso called gravitationin mechanicsthe universal force of attraction acting between all matter.

It is by far the weakest known force in nature and thus plays no role in determining the internal properties of everyday matter. On the other hand, through its long reach and universal action, it controls the trajectories of bodies in the solar system and elsewhere in the universe and the structures and evolution of stars, galaxies, and the whole cosmos. Gravity is measured by the acceleration that it gives to freely falling objects.

Thus, for every second an object is in free fall, its speed increases by about 9. At the surface of the Moon the acceleration of a freely falling body is about 1. The works of Isaac Newton and Albert Einstein dominate the development of gravitational theory. The launch of space vehicles and developments of research from them have led to great improvements in measurements of gravity around Earth, other planets, and the Moon and in experiments on the nature of gravitation.

Newton argued that the movements of celestial bodies and the free fall of objects on Earth are determined by the same force.

The classical Greek philosophers, on the other hand, did not consider the celestial bodies to be affected by gravity, because the bodies were observed to follow perpetually repeating nondescending trajectories in the sky.

Those Aristotelian concepts prevailed for centuries along with two others: that a body moving at constant speed requires a continuous force acting on it and that force must be applied by contact rather than interaction at a distance. These ideas were generally held until the 16th and early 17th centuries, thereby impeding an understanding of the true principles of motion and precluding the development of ideas about universal gravitation.

The 17th-century German astronomer Johannes Kepler accepted the argument of Nicolaus Copernicus which goes back to Aristarchus of Samos that the planets orbit the Sunnot Earth. Using the improved measurements of planetary movements made by the Danish astronomer Tycho Brahe during the 16th century, Kepler described the planetary orbits with simple geometric and arithmetic relations. He realized that bodies that are uninfluenced by forces continue indefinitely to move and that force is necessary to change motion, not to maintain constant motion.

In studying how objects fall toward Earth, Galileo discovered that the motion is one of constant acceleration. He demonstrated that the distance a falling body travels from rest in this way varies as the square of the time. As noted above, the acceleration due to gravity at the surface of Earth is about 9. Galileo was also the first to show by experiment that bodies fall with the same acceleration whatever their composition the weak principle of equivalence.

Article Media. Info Print Print. Table Of Contents. Submit Feedback. Thank you for your feedback. Gravity physics. Written By: James E. Faller Kenneth L. Nordtvedt Alan H. See Article History. Get exclusive access to content from our First Edition with your subscription. Subscribe today. The planets describe elliptic orbits, of which the Sun occupies one focus a focus is one of two points inside an ellipse ; any ray coming from one of them bounces off a side of the ellipse and goes through the other focus.

## Newton's Law of Gravity

The line joining a planet to the Sun sweeps out equal areas in equal times. The square of the period of revolution of a planet is proportional to the cube of its average distance from the Sun. Load Next Page. More About. LiveScience - What is Gravity?

Articles from Britannica Encyclopedias for elementary and high school students.Gravity is the force that attracts two bodies toward each other, the force that causes apples to fall toward the ground and the planets to orbit the sun.

The more massive an object is, the stronger its gravitational pull. Gravity is one of the four fundamental forces, along with the electromagnetic, strong and weak forces. It is what causes objects to have weight. When you weigh yourself, the scale tells you how much gravity is acting on your body. The formula for determining weight is: weight equals mass times gravity. On Earth, gravity is a constant 9. Historically, philosophers such as Aristotle thought that heavier objects accelerate toward the ground faster.

But later experiments showed that wasn't the case. The reason that a feather will fall more slowly than a bowling ball is because of the drag from air resistance, which acts in the opposite direction as the acceleration due to gravity. He found that gravity acts on all matter and is a function of both mass and distance. Every object attracts every other object with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them.

The equation is often expressed as:. Newton's equations work extremely well to predict how objects such as planets in the solar system behave.

Newton published his work on gravitation inwhich reigned as the best explanation until Einstein came up with his theory of general relativity in In Einstein's theory, gravity isn't a force, but rather, the consequence of the fact that matter warps space-time. One prediction of general relativity is that light will bend around massive objects.

Live Science. Please deactivate your ad blocker in order to see our subscription offer.Newton's law of gravity defines the attractive force between all objects that possess mass. Understanding the law of gravity, one of the fundamental forces of physicsoffers profound insights into the way our universe functions. The famous story that Isaac Newton came up with the idea for the law of gravity by having an apple fall on his head is not true, although he did begin thinking about the issue on his mother's farm when he saw an apple fall from a tree.

He wondered if the same force at work on the apple was also at work on the moon. If so, why did the apple fall to the Earth and not the moon? Along with his Three Laws of MotionNewton also outlined his law of gravity in the book Philosophiae naturalis principia mathematica Mathematical Principles of Natural Philosophywhich is generally referred to as the Principia. Johannes Kepler German physicist, had developed three laws governing the motion of the five then-known planets.

He did not have a theoretical model for the principles governing this movement, but rather achieved them through trial and error over the course of his studies. Newton's work, nearly a century later, was to take the laws of motion he had developed and applied them to planetary motion to develop a rigorous mathematical framework for this planetary motion. Newton eventually came to the conclusion that, in fact, the apple and the moon were influenced by the same force.

He named that force gravitation or gravity after the Latin word gravitas which literally translates into "heaviness" or "weight. In the PrincipiaNewton defined the force of gravity in the following way translated from the Latin :. This equation gives us the magnitude of the force, which is an attractive force and therefore always directed toward the other particle.

As per Newton's Third Law of Motion, this force is always equal and opposite. Newton's Three Laws of Motion give us the tools to interpret the motion caused by the force and we see that the particle with less mass which may or may not be the smaller particle, depending upon their densities will accelerate more than the other particle.

### Gravity is More Than a Name

This is why light objects fall to the Earth considerably faster than the Earth falls toward them. Still, the force acting on the light object and the Earth is of identical magnitude, even though it doesn't look that way. It is also significant to note that the force is inversely proportional to the square of the distance between the objects.

As objects get further apart, the force of gravity drops very quickly. At most distances, only objects with very high masses such as planets, stars, galaxies, and black holes have any significant gravity effects. In an object composed of many particlesevery particle interacts with every particle of the other object.

Since we know that forces including gravity are vector quantitieswe can view these forces as having components in the parallel and perpendicular directions of the two objects.

In some objects, such as spheres of uniform density, the perpendicular components of force will cancel each other out, so we can treat the objects as if they were point particles, concerning ourselves with only the net force between them. The center of gravity of an object which is generally identical to its center of mass is useful in these situations. We view gravity and perform calculations as if the entire mass of the object were focused at the center of gravity.

In simple shapes — spheres, circular disks, rectangular plates, cubes, etc. This idealized model of gravitational interaction can be applied in most practical applications, although in some more esoteric situations such as a non-uniform gravitational field, further care may be necessary for the sake of precision.

Sir Isaac Newton's law of universal gravitation i. Instead of calculating the forces between two objects every time, we instead say that an object with mass creates a gravitational field around it.

The gravitational field is defined as the force of gravity at a given point divided by the mass of an object at that point.If you're seeing this message, it means we're having trouble loading external resources on our website. To log in and use all the features of Khan Academy, please enable JavaScript in your browser. Donate Login Sign up Search for courses, skills, and videos. Science Physics Centripetal force and gravitation Newton's law of gravitation. Introduction to gravity.

Mass and weight clarification. Gravity for astronauts in orbit. Would a brick or feather fall faster? Acceleration due to gravity at the space station. Space station speed in orbit. Introduction to Newton's law of gravitation. Current timeTotal duration Google Classroom Facebook Twitter. Video transcript We're now going to learn a little bit about gravity. And just so you know, gravity is something that, especially in introductory physics or even reasonably advanced physics, we can learn how to calculate it, we can learn how to realize what are the important variables in it, but it's something that's really not well understood.

### Newton's Law of Universal Gravitation

Even once you learn general relativity, if you do get there, I have to say, you can kind of say, oh, well, it's the warping of space time and all of this, but it's hard to get an intuition of why two objects, just because they have this thing called mass, they are attracted to each other. It's really, at least to me, a little bit mystical. But with that said, let's learn to deal with gravity. And we'll do that learning Newton's Law of Gravity, and this works for most purposes. So Newton's Law of Gravity says that the force between two masses, and that's the gravitational force, is equal to the gravitational constant G times the mass of the first object times the mass of the second object divided by the distance between the two objects squared.

So that's simple enough. So let's play around with this, and see if we can get some results that look reasonably familiar to us. So let's use this formula to figure out what the acceleration, the gravitational acceleration, is at the surface of the Earth. So let's draw the Earth, just so we know what we're talking about. So that's my Earth.

And let's say we want to figure out the gravitational acceleration on Sal. That's me. And so how do we apply this equation to figure out how much I'm accelerating down towards the center of Earth or the Earth's center of mass?

The force is equal to-- so what's this big G thing? The G is the universal gravitational constant. Although, as far as I know, and I'm not an expert on this, I actually think its measurement can change. It's not truly, truly a constant, or I guess when on different scales, it can be a little bit different. But for our purposes, it is a constant, and the constant in most physics classes, is this: 6.

**Minute Physics: What is Gravity?**

I know these units are crazy, but all you have to realize is these are just the units needed, that when you multiply it times a mass and a mass divided by a distance squared, you get Newtons, or kilogram meters per second squared.

So we won't worry so much about the units right now. Just realize that you're going to have to work with meters in kilograms seconds.Nearly every child knows of the word gravity. Gravity is the name associated with the mishaps of the milk spilled from the breakfast table to the kitchen floor and the youngster who topples to the pavement as the grand finale of the first bicycle ride.

Gravity is the name associated with the reason for "what goes up, must come down," whether it be the baseball hit in the neighborhood sandlot game or the child happily jumping on the backyard mini-trampoline. We all know of the word gravity - it is the thing that causes objects to fall to Earth. Yet the role of physics is to do more than to associate words with phenomenon. The role of physics is to explain phenomenon in terms of underlying principles.

The goal is to explain phenomenon in terms of principles that are so universal that they are capable of explaining more than a single phenomenon but a wealth of phenomenon in a consistent manner. Thus, a student's conception of gravity must grow in sophistication to the point that it becomes more than a mere name associated with falling phenomenon. Gravity must be understood in terms of its cause, its source, and its far-reaching implications on the structure and the motion of the objects in the universe.

Certainly gravity is a force that exists between the Earth and the objects that are near it. As you stand upon the Earth, you experience this force. We have become accustomed to calling it the force of gravity and have even represented it by the symbol F grav. Most students of physics progress at least to this level of sophistication concerning the notion of gravity. This same force of gravity acts upon our bodies as we jump upwards from the Earth.

As we rise upwards after our jump, the force of gravity slows us down. And as we fall back to Earth after reaching the peak of our motion, the force of gravity speeds us up. In this sense, the force gravity causes an acceleration of our bodies during this brief trip away from the earth's surface and back.

In fact, many students of physics have become accustomed to referring to the actual acceleration of such an object as the acceleration of gravity. Not to be confused with the force of gravity F gravthe acceleration of gravity g is the acceleration experienced by an object when the only force acting upon it is the force of gravity.

On and near Earth's surface, the value for the acceleration of gravity is approximately 9. It is the same acceleration value for all objects, regardless of their mass and assuming that the only significant force is gravity. Many students of physics progress this far in their understanding of the notion of gravity. In Lesson 3, we will build on this understanding of gravitation, making an attempt to understand the nature of this force.

Many questions will be asked: How and by whom was gravity discovered? What is the cause of this force that we refer to with the name of gravity? What variables affect the actual value of the force of gravity? Why does the force of gravity acting upon an object depend upon the location of the object relative to the Earth?

How does gravity affect objects that are far beyond the surface of the Earth? How far-reaching is gravity's influence? And is the force of gravity that attracts my body to the Earth related to the force of gravity between the planets and the Sun? These are the questions that will be pursued. And if you can successfully answer them, then the sophistication of your understanding has extended beyond the point of merely associating the name "gravity" with falling phenomenon.

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