Gravity Gravity The gravitational force at the surface of the planet is the force that binds all bodies to earth, this force is one of the four forces recognized by physicists, and this kind of force is known as gravity it attracts every celestial object to earth, and though it is the most important of the forces essential for our lives, it is the least comprehended of them all Throughout ages scientists have tried to solve the mystery of gravity, and one of the first discoveries concerning gravity was made by Aristotle who concluded from his experiments that the downward movement of any body is that has weight had a proportional relationship between its quickness in motion and its size. However this theory was accepted for centuries, but after a series of experiments made by Galileo, Aristotles theory was proved to be incorrect, as Galileo said after a series of experimenting at the Pisa tower that bodies of different sizes fall with the same speed. Later on, the idea that the force is needed so as to change the motion of the body was discovered. After that a great scientist was to improve all the previously accepted theories, this scientist was Newton who was to make decisive advances in understanding gravity. In his first law Newton said that a body in state of rest or uniform motion in a straight line will keep on moving unless acted upon by a force, while in his second Newton expressed his first law in a more quantitative way as he said that force acting on a body is the rate of change of its momentum which can be put in a rule as F= ma Where (F) is the force acting on the body while (a) and (m) are the acceleration and inertial mass of the body respectively.

Newton also made the law of gravitation in which he expressed the gravitational force of attraction between any two bodies acting along the line joining them as Where (m) is the mass of the two bodies and (r) is the distance between them, while (G) is the proportionality constant known as the constant of gravitation, afterwards Newton made his very important assumption in which he showed that the inertial mass of a body is identical to its gravitational mass which implied that the gravitational force exerted on or by an object is directly proportional to its inertia. After that, scientists wanted to determine the value of the gravitational constant, in which the English scientist Henry Cavendish calculated as G= 6.754 x 10 Nm /Kg For hundreds of years, Newtons laws where considered as the basis of modern physics, and one of the things that proved Newtons theories was the discovery of the planet Neptune but there were also many important issues that were not discovered by Newton such as, that there is no way to describe mass except with reference to acceleration, another issue is that no force moves without acceleration, a third one is that when we mention acceleration we have to say with respect to what? . All the previously mentioned theories, were studied by Einstein who wondered why is the inertial mass proportional to the gravitational mass and after experimenting he made his theory of equivalence in which stated that if we had two systems, the first had acceleration with no gravitational field while the second has acceleration with no gravitational mass and while the other is at rest and has a gravitational field, the results would be equivalent, and by the year 1916 Einstein had completed the mathematical theory of gravitation which was the general theory of relativity. Einsteins theories opposed those of Newtons in at least two major and measurable issues that were the curvature of light and the shifting of the wavelength of the light. In conclusion, the issue was a subject of research done by the greatest scientists ever known as Newton and Einstein, and it will always be a subject of research, as think the gravitational mystery still might not be totally unraveled and I think that many other theories will be made to emphasize the force that keeps standing at that planet.

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