The Greek philosopher Aristotle (384–322 BC) hypothesized, following previous traditions, that the material world was made up of four elements: earth, water, air, and fire. For example, a rock was made mostly of Earth with some water, air and fire, a cloud was made mostly of air and water with some Earth and fire. Each element had a natural or proper place in the universe to which it spontaneously inclined; Earth belonged right in the center, water in a layer covering the Earth, air above water, and fire above air. Each element had a natural tendency to return to its place, so that, for example, rocks fell towards the center and fire rose into the air. This was one of the first explanations of gravity: that it was the natural tendency of the heaviest elements, Earth and water, to return to their correct positions near the center of the universe. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essay For centuries, Aristotle's theory was thought to imply that objects with different weights should fall at different speeds; that is, a heavier object should fall faster because it contains more central tendency elements, Earth, and water. However, this is incorrect. Objects of different weights fall at the same speed. (This statement, however, is only an approximation, since it assumes that the Earth is perfectly stationary, which it is not. When an object is dropped, the Earth accelerates "upward" under the influence of their mutual gravitation , just as the object "falls", "and meet somewhere in the middle. For a heavier object, this meeting occurs slightly earlier than for a light object, and therefore, heavier objects actually fall slightly faster of light ones. In practice, however, the Earth's motion is not measurable for "fallen" objects smaller than planetary size, and therefore it is correct to say that all small objects fall at the same rate, regardless of their mass.) Aristotle's model of the universe also included the moon, the sun, the Earth, visible planets, and fixed stars. Aristotle assumed that these were external to the layer of fire and were made up of a fifth element, ether or quintessence. Latin expression quinta essentia, or fifth essence, used by the medieval translators of Aristotle). Celestial bodies surrounded the Earth attached to nested ethereal spheres centered on the Earth. To maintain these movements no forces were needed, since everything was considered perfect and immutable, having been set in motion by a Prime Mover: God. Aristotle's ideas were accepted in Europe and the Near East for centuries, until the astronomer Polish Nicolaus Copernicus (1473 –1543) developed a heliocentric (sun-centered) model to replace the geocentric (Earth-centered) one that had been the dominant cosmological concept since the time of Aristotle. (Non-European astronomers unfamiliar with Aristotle, such as the Chinese and Aztecs, had developed their own geocentric models; no heliocentric model existed before Copernicus.) Copernicus' model placed the sun at the center of the universe, with all planets orbit the sun in perfect circles. This development represented such a radical change from the previous model that it is now called the Copernican Revolution. It was an ingenious intellectual construct, but it still didn't explain why the planets revolved around the sun, in the sense of what made them do so. While many scientists sought to explain these celestial motions, others sought to understand terrestrial mechanics. It seemed like a matter ofcommon sense that heavier objects fell faster than light ones: drop a feather and a rock and see which hits the ground first. The flaw in this experiment is that air resistance affects how quickly objects fall. How about another experiment, in which air resistance plays a smaller role: observing the difference between dropping a large rock and a small rock? This is an easy experiment to perform and the results have profound implications. As early as the 6th century AD Johannes Philiponos (c. 490–566) stated that the difference in landing times was small for objects of different weight but similar shape. Galileo's friend, the Italian physicist Giambattista Benedetti (1530–1590), in 1553, and the Dutch physicist Simon Stevin (1548–1620), in 1586, also considered the problem of falling rocks and concluded that the rate of fall was independent of weight. However, the individual most closely associated with the problem of falling bodies is the Italian physicist Galileo Galilei (1564–1642), who systematically observed the movements of falling bodies. (It is unlikely that he actually dropped weights from the Leaning Tower of Pisa, but he wrote that such an experiment could be performed.) Because objects accelerate (accelerate) rapidly as they fall, and Galileo was limited to naked-eye observation from part of Using the technology of his time, he studied the slower movements of pendulums and bodies rolling and sliding along an inclined plane. From the results obtained, Galileo formulated the law on falling bodies. This states that, regardless of air resistance, freely falling bodies accelerate with a constant acceleration (rate of change of velocity) independent of their weight or composition. The acceleration due to gravity near the Earth's surface is indicated with the symbol g and has a value of approximately 32 feet per second per second (9.8 m/s2). This means that 1 second after release a falling object is moving at approximately 10 m/s; after 2 seconds, 20 m/s; after 10 seconds, 100 m/s. That is, after falling for 10 seconds, it falls fast enough to cross the length of a football field in less than a second. Writing v for the speed of the falling body and for the time elapsed since the beginning of the free fall, we have v = gt. Galileo also determined a formula to describe the distance d at which a body falls in a given time: d = ½gt2That is, if you drop an object, after 1 second it will have fallen about 5 m; after 2 seconds, 20 m; and after 10 seconds, 500 meters. Galileo did a great job describing the effect of gravity on objects on Earth, but it wasn't until the English physicist Isaac Newton (1642-1727) studied the problem that we understood how universal gravity works. AND. An old story says that Newton suddenly understood gravity when an apple fell from a tree and hit him on the head; This story may not be exactly true, but Newton said that a falling apple helped him develop his theory of gravity. Newtonian Gravity Newton's universal law of gravitation states that all objects in the universe attract all other objects. So the sun attracts the Earth, the Earth attracts the sun, the Earth attracts a book, a book attracts the Earth, the book attracts the desk and so on. The gravitational attraction between small objects, such as molecules and books, is generally negligible; the gravitational pull exerted by larger objects, such as stars and planets, organizes the universe. It is gravity that keeps us on Earth, the Moon in orbit around the Earth, and the Earth in orbit around the sun. Newton's law of gravitation also states that the strength of the attractive force depends on the masses of the two objects. The mass of an object is a measure of how muchmaterial it has, but it is not the same as its weight, which is a measure of how much force a given mass experiences in a given gravitational field; a given rock, for example, will have the same mass everywhere in the universe but will weigh more on Earth than on the Moon. We don't feel the gravitational forces of objects other than Earth because they are weak. For example, the gravitational force of attraction between two friends who weigh 45.5 kg (100 lb) and are 1 m (3 ft) apart is only about 3 × 10−8 N = 0.00000003 lb, which corresponds approximately the weight of a bacterium. (Note: The pound is a measure of weight – the gravitational force experienced by an object – while the kilogram is a measure of mass. Strictly speaking, then, pounds and kilograms cannot be substituted for each other as in the previous sentence. However, near the Earth's surface weight and mass can be approximately equated because the Earth's gravitational field is approximately constant; treating pounds and kilograms as proportional units is therefore standard practice in this condition.) The gravitational force between two objects becomes weaker if the two objects are moved apart and stronger if they are brought closer; that is, the force depends on the distance between objects. If we take two objects and double the distance between them, the force of attraction decreases to a quarter of its previous value. If we triple the distance, the force decreases to one ninth of its previous value. The force depends on the inverse square of the distance. All these statements derive from a simple equation: for two objects having masses m1 and m1 respectively, the intensity of the gravitational force acting on each object is given by: F = Gm1m2/ r2, where r is the distance between the centers of the objects and G is the gravitational constant (6.673 × 10−11N m2/kg2.) Note that the gravitational constant is an extremely small number; this explains why we only feel gravity when we are close to a large mass (for example, the Earth). Newton also explained how bodies respond to forces (including gravitational forces) acting on them. His second law of motion states that a resultant force (i.e. a force not canceled out by an opposing force) causes a body to accelerate. The magnitude of this acceleration is inversely proportional to the mass of the object. This means that under the influence of a certain force, more massive objects accelerate more slowly than less massive objects. Alternatively, to experience the same acceleration, more massive objects require more force. Consider the gravitational force exerted by the Earth on two rocks, the first with a mass of 2 pounds (1 kg) and the second with a mass of 22 pounds (10 kg). Since the mass of the second is 10 times that of the first, the gravitational force on the second will be 10 times that on the first. But a 10 kg mass requires 10 times more force to accelerate it, so both masses accelerate towards Earth at the same speed. Ignoring the acceleration of the Earth towards the rocks (which is extremely small), it follows that equal speeds of fall for small objects are a natural consequence of Newton's law of gravity and the second law of motion. What happens if you throw a ball horizontally? If you throw it slowly, it will hit the ground a short distance. If you throw it faster, it will land further. Since the Earth is round, it will curve slightly away from the ball before it lands; the farther the throw, the greater the amount of curve. If you could throw or throw the ball at 18,000 miles/h (28,800 km/h), the Earth would move away from the ball by the same amount that the ball falls. The ball would never come close to the ground and would be in orbit around the Earth. Gravity still accelerates the ball at 9.8 m/s2towards the center of the Earth, but the ball never comes close to the ground. (This is exactly what the Moon does.) Furthermore, the orbits of the Earth and other planets around the Sun and all movements of stars and galaxies follow Newton's laws. This is why Newton's law of gravitation is called "universal"; describes the effect of gravity on all objects in the universe. Newton published his laws of motion and gravity in 1687, in his seminal Philosophiae Naturalis Principia Mathematica (Latin for Mathematical Principles of Natural Philosophy, or Principia for short). When we need to solve problems related to gravity, Newton's laws are usually sufficient. There are, however, some phenomena that they cannot describe. For example, the motions of the planet Mercury are not exactly described by Newton's laws. Newton's theory of gravity, therefore, needed modifications that would require another genius, Albert Einstein, and his theory of general relativity.General RelativityGerman physicist Albert Einstein (1879–1955) realized that Newton's theory of gravity he had problems. He knew, for example, that Mercury's orbit showed inexplicable deviations from what Newton's laws predicted. However, he was worried about a much more serious problem. Since the force between two objects depends on the distance between them, if one object gets closer, the other object will feel a change in gravitational force. According to Newton, this change would be immediate, or instantaneous, even if the objects were millions of kilometers away. Einstein saw this as a major flaw in Newtonian gravity. Einstein believed that nothing could travel instantaneously, not even a change in force. Specifically, nothing can travel faster than light in a vacuum, which has a speed of around 300,000 km/s. To solve this problem, Einstein had to not only revise Newtonian gravity, but change the way we think about space, time, and the structure of the universe. He stated this new way of thinking mathematically in his general theory of relativity. Einstein said that a mass bends space, like a heavy ball denting a rubber sheet. Furthermore, Einstein argued that space and time are intimately linked to each other, and that we do not live in three spatial and temporal dimensions (all four quite independent of each other), but rather in a space continuum -four-dimensional temporal, a perfect fusion of the four. It is therefore not "space", naively conceived, but space-time that deforms in reaction to a mass. This, in turn, explains why objects attract each other. Consider the sun sitting in space-time, imagined as a ball resting on a sheet of rubber. It curves space-time around itself in the shape of a bowl. Planets orbit the sun because they roll through this distorted space-time, which curves their motions like that of a ball rolling inside a shallow bowl. Gravity, from this point of view, is the way in which objects influence the movements of other objects by influencing the shape of space-time. Einstein's general relativity makes predictions that Newton's theory of gravitation does not. Since particles of light (photons) have no mass, Newtonian theory predicts that they will not be affected by gravity. However, if gravity were due to the curvature of space-time, light should also be affected in the same way as matter. This proposition has been verified as follows: during the day, the sun is too bright to see the stars. However, during a total solar eclipse, the solar disk is blocked by the Moon and you can see stars appearing in the sky near the sun. During..