Sir Isaac Newton discovered many laws dealing with math and physics. One particular law, the law of gravitation, dealt with the heavens and planets. This law was derived and at first though that it could only apply to outer space, but is shown that we can use it elsewhere also. Newton's law of gravitation is applied to many circumstances, and helps us derive other very useful equations. His law is very versatile and powerful because it opens gateways to other laws and equations.
             In 1666, 45 years after Johannes Kepler discovered his laws about planetary motion; Isaac Newton discovered an interesting fact about planets. (Zitzewitz p160) Newton discovered that the net force on a planet must vary inversely with the square of the distance between two planets from the centers. (Zitzewitz p160) This is expressed in algebraic form as F~1/d2. (Zitzewitz p160) ~ is substituting for the symbol of "is proportional to". Also, Newton discovered that the force acted in the direction of a line connecting the centers. (Zitzewitz p160) But Newton was unable at this time to measure the magnitude of the force. (Zitzewitz p160)
             The falling apple made Newton think of the problem of the motion of the planets. (Zitzewitz p161) Newton recognized that the Earth attracted to the apple, which caused it to fall directly downward. (Zitzewitz p161) This could mean gravity attracts the planets to the sun. (Zitzewitz p161) Newton recognized the force on the apple must be proportional to the mass of the apple. (Zitzewitz p161) In addition, according to Newton's third law of motion, which states for every action there is an equal reaction in the opposite direction, the apple would also attract to the Earth. (Zitzewitz p161) Therefore the force of attraction must be proportional to the mass of the Earth. (Zitzewitz p161)
             Newton was confident the laws that governed the motion on Earth would work anywhere that he assumed ...