A gravitational force, in the simplest terms, is the attractive force between two separate bodies. Gravity is related to mass: the greater an object’s mass, the greater the gravitational force it exerts on other objects. It is one of the essential forces that account for the structure of the universe. Although it is one of the most easily observable cosmic forces, the principles of gravity are still not fully understood by modern science.Gravity is popularly associated with Sir Isaac Newton, the 17th century English scientist, but scientists had been studying gravity for centuries before Newton. Galileo, some decades earlier, had demonstrated that objects fall at the same rate, regardless of their weight. The fall of an apple inspired Newton to study gravitational force, although the apple did not actually hit his head and bring instant understanding; he spent more than 20 years on the subject. He realized gravitational force also held the moon in orbit around the Earth and was able to prove this by using the moon’s orbit to check his calculations