Gravitation is the force of attraction between any two objects having masses. It is responsible for keeping the earth and other planets in their orbits. Galileo recorded observations of the planets with naked eye. These observations were analyzed by his assistant Kepler, who gave the three laws describing the motion of planets in the solar system. These laws are law of orbits, law of areas and law of periods and are collectively known as Kepler’s laws of planetary motion. These laws enabled Newton to propose his universal law of gravitation, according to which the gravitational force of attraction between any two bodies is directly proportional to the product of the masses and inversely proportional to the square of the distance between them. The value of gravitational constant appearing in the universal law of gravitation was first determined by Henry Cavendish. The acceleration experienced by the mass due to the gravitational force of the earth is called acceleration due to gravity of the earth. It changes as the mass is taken above or below the earth’s surface. As the gravitational force is conservative, therefore, a potential energy function can be defined. The potential energy associated with the force of gravitation is called gravitational potential energy. The gravitational pull of the earth brings the objects thrown upwards back to the earth. However, if any object is thrown with high initial speeds, then it doesn’t fall back to earth. This is known as its escape velocity, which is defined as the speed at which the kinetic energy of an object becomes equal to the magnitude of its gravitational potential energy. Earth satellites are objects which revolve around the earth. They are used in fields like telecommunication, geophysics and meteorology. Artificial satellites launched can either be geostationary or polar. Every object inside a satellite is in the state of free fall. People inside it don’t experience gravity. This phenomenon of an object being in free fall is called weightlessness.
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