Gravity

Aristotle

Aristotle was an ancient Greek philosopher. His contributions to gravity included the Aristotelian theory of gravity, made around 350 B.C., which was a flawed theory that stated all bodies move towards their “natural place.” According to Aristotle, all objects have a natural tendency to fall towards the centre of the Universe. Since the centre of the Earth coincides with the centre of the Universe, all objects also tend to fall towards the Earth's surface. For some objects, Aristotle claimed the natural place to be the centre of the earth, wherefore they fall towards it.

So, why do the planets not fall towards the Earth? Well, according to Aristotle, the planets are embedded in crystal spheres which rotate with them whilst holding them in place in the firmament. The natural place is the heavenly spheres, wherefore gases, steam for example, move away from the centre of the earth and towards heaven and to the moon. The speed of this motion was thought to be proportional to the weight of the object. He also thought there were two types of perfect motion, straight lines and circles; and that the only objects that couldn't move in straight lines were made of aether - the stars and planets. They, therefore, had to move in perfect circles.

Aristotle didn't believe in experiment. His logic told him that he was right. We still call the phenomenon he contemplated, gravity, which comes from the Latin word gravitas, meaning heavy.















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Galileo Galilei

Galileo was an Italian scientist of the late sixteenth and early seventeenth centuries. Galileo’s contribution, before Newton, to the knowledge of gravity was that objects with different masses – an apple and a melon, for instance – fall at the same velocity, will hit the ground at the same time.

In 1585 he conducted this experiment, in which he dropped the two objects of different masses from the Leaning Tower of Pisa, in Italy, to demonstrate that their time of descent was independent of their mass (excluding the limited effect of air resistance). He thus proved that objects fall at the same rate and accelerate as they fall. This was contrary to what Aristotle had taught: that heavy objects fall faster than lighter the body will rise in direct proportion to weight.

Galileo also performed experiments involving rolling balls down inclined planes, which proved the same thing: falling or rolling objects (rolling is a slower version of falling, as long as the distribution of mass in the objects is the same) are accelerated independently of their mass.

Galileo realized that bodies in air also experience buoyant forces (upthrust) that oppose the pull of gravity. If the buoyant forces overcome gravity, then the body will rise. If gravity is stronger, then the body will fall.
Galileo realized that gravity not only causes a body to fall, but also determines the motion of rising bodies and, furthermore, that gravity extends to the centre of the Earth. Galileo then showed that the motion of a projectile is made up of two components: one component consists of uniform motion in a horizontal direction, and the other component is vertical motion under acceleration or deceleration due to gravity. Galileo explained that the motion of a bird, like a projectile, has a horizontal component that is provided by the motion of the Earth and that this horizontal component of motion always exists to keep such objects in position even though they are not attached to the ground.
Galileo’s Equation
He had the brilliant idea of slowing vertical motion by measuring the movement of balls rolling down inclined planes, realizing that the vertical component of this motion is a uniform acceleration due to gravity.
He determined the correct mathematical law for acceleration: the total distance covered, starting from rest, is proportional to the square of the time. He also concluded that objects retain their velocity unless a force — often friction — acts upon them, refuting the generally accepted Aristotelian hypothesis that objects "naturally" slow down and stop unless a force acts upon them. Galileo's Principle of Inertia stated: "A body moving on a level surface will continue in the same direction at constant speed unless disturbed." This principle was incorporated into Newton's laws of motion (first law).
























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Isaac Newton


Sir Isaac Newton was an English physicist of the late 17th and early 18th centuries. He made substantial contributions to the laws of gravity. Newton formulated laws of universal gravitation and motion—laws that explain how objects move on Earth as well as through the heavens. Newton began with the laws of motion and gravitation he observed in nature, then used these laws to convert physics from a mere science of explanation into a general mathematical system with rules and laws.

Around 1665, he recorded his first thoughts on gravitation, inspired (according to legend) by observing the fall of an apple in an orchard. According to a report of a conversation with Newton in his old age, he said he was trying to determine what type of force could hold the Moon in its path around Earth. The fall of an apple led him to think that the attractive gravitational force acting on the apple might be the same force acting on the Moon. Newton believed that this force, although weakened by distance, held the Moon in its orbit.

Newton devised a numerical equation to verify his ideas about gravity. The equation is called the inverse square law of attraction, and it states that the force of gravity is related to the inverse square of the distance between the two objects (that is, the number 1 divided by the distance between the two objects times itself). Newton believed this law should apply to the Sun and the planets as well. He did not pursue the problem of the falling apple at the time, because calculating the combined attraction of the whole Earth on a small body near its surface seemed too difficult. He reintroduced these early thoughts years later in his more thorough work, the Principia, in 1687. In this work Newton stated the three universal laws of motion that were not to be improved upon for more than two hundred years. He defined the law of universal gravitation.

Isaac Newton's law of universal gravitation states the following: Every point mass attracts every other point mass by a force directed along the line connecting the two. This force is proportional to the product of the masses and inversely proportional to the square of the distance between them:


Newton’s Law of Gravitation












































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Albert Einstein

Albert Einstein was a German physicist of the 20th century. His contributions to gravity included, in November 1915, presenting a series of lectures before the Prussian Academy of Sciences in which he described his theory of gravity, known as general relativity. Einstein's equations joined space and time together. This theory of relativity reconciled Maxwell's equations of light with Newton's laws of motion. The final lecture ended with his introduction of an equation that replaced Newton's law of gravity. This theory considered all observers to be equivalent, not only those moving at a uniform speed. In general relativity, gravity is no longer a force (as it is in Newton's law of gravity) but is a consequence of the curvature of space-time.


Einstein’s Equation

This description of gravity cleared up a small mystery about free-fall. Massive objects have a lot of momentum, and are difficult to stop or turn around. That is described by inertial mass. Massive objects also create a strong gravitational force. That is described by gravitational mass.

Gravitational Mass measures how much gravity an object creates. Inertial Mass measures how an object reacts to a force. Newton had explained free-fall by equating these two types of mass, but it is not entirely clear why this should be the case. Einstein said that gravitational mass is the same as inertial mass, because gravity is the same as inertia.

One part of this problem was solved in 1908 when a painter fell off a roof. Einstein heard of the accident, and asked the painter what it felt like to fall. He found out that while falling freely, the painter felt weightless. Einstein realised that a body that is freely falling along a path under the influence of gravity is behaving similarly to an object moving in a straight line under inertia.

Most astronomers did not like Einstein's geometrization of gravity and believed that his light bending and gravitational red shift predictions would not be correct.









































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Summary

As early as around 350 B.C.E. the philosopher Aristotle was thinking about gravity. He decided that all objects were attracted to their "natural place." For some objects, that natural place was the heavenly spheres, explaining why steam rises for example. For some objects, the natural place was the center of the universe (at that time, the center of the Earth), thus explaining why heavy things fall. Aristotle thought that heavier objects were more strongly attracted to their natural places than light objects.

Aristotle's theory was superseded by the work of Galileo Galilei.

In 1585, Galileo tried dropping objects with differing weights to see what happened, actually conducting experiments which Aristotle did not. He supposedly dropped some of his weights from the leaning tower of Pisa. Galileo found that if he dropped an object that weighed ten times as much as another, the light object did not take ten times as long to hit the ground. It took exactly the same time.

He proved that the objects’ time of descent was independent of their mass (excluding the limited effect of air resistance). He thus proved that objects fall at the same rate and accelerate as they fall. Aristotle was obviously wrong. Galileo also realized that gravity not only causes a body to fall, but also determines the motion of rising bodies and, furthermore, that gravity extends to the centre of the Earth, so the Earth is affected by gravity as well. He had the brilliant idea of slowing vertical motion by measuring the movement of balls rolling down inclined planes, and determined the correct mathematical law for acceleration.

The next revolution in the understanding of gravity came with Isaac Newton's mathematical formulation of a law of universal gravitation, which he published in Principia Mathematica in 1687. Through experiments, supposedly the fall of an apple, Newton described the gravitational force held on the Moon in its orbit, as well as falling objects. He devised the inverse square law of attraction, and his three laws of motion. He defined the law of universal gravitation.

The next person to see farther was, of course, Albert Einstein, who, in 1915, revolutionized the view of gravity by developing a geometrical view where gravity is not viewed as a force, but rather as a manifestation of curved space-time. Massive objects have a lot of momentum, and are difficult to stop or turn around. That is described by inertial mass. Massive objects also create a strong gravitational force. That is described by gravitational mass.



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Bibliography


The following websites were our sources of information:

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