1. The History of Gravity Aristotle, Galileo, Newton and Einstein

2. Gravity

3. Aristotle had the view that all objects have a natural place in the universe: that heavy objects like rocks wanted to be at rest on the Earth and that light objects like smoke wanted to be at rest in the sky and the stars wanted to remain in the heavens. He thought that a body was in its natural state when it was at rest, and for the body to move in a straight line at a constant speed an external agent was needed to continually propel it, otherwise it would stop moving. Aristotle thought that all bodies move towards their natural place. For some objects, Aristotle claimed the natural place to be the center of the earth, wherefore they fall towards it. For other objects, the natural place is the heavenly spheres, wherefore gases, steam for example, move away from the center of the earth and towards heaven and to the moon. Aristotle Meta-physics

4. Aristotle thought that a body was in its natural state when it was at rest, and for the body to move in a straight line at a constant speed an external agent was needed to continually propel it, otherwise it would stop moving. Galileo, however, realized that a force is necessary to change the velocity of a body, i.e., acceleration, but no force is needed to maintain its velocity. This insight leads to Newton's First Law —no force means no acceleration, and hence the body will maintain its velocity. Galileo was the first scientist to actually do a scientific analysis of gravity, attempting to explain how it operated, and he actually calculated the acceleration due to gravity using an inclined plane and some heavy balls. Much of what Newton did was based on expanding on Galileo’s work over a century earlier. He said that what he had accomplished, as much as it was exclaimed as genius was from standing on the shoulders of giants. Several centuries later…

5. Galileo and Newton Until Galileo no one had attempted to mathematically describe gravity. Also people still saw gravity as something that applied on earth alone—Aristotle. Newton came up with a universal law which calculates the force of attraction between any two bodies anywhere in the universe, which we now call gravity. To the right.  This revolutionized physics. It stood for over 250 years… until Einstein, who attempted to describe WHY objects of mass are “attracted” by gravity to one another. But what led Newton to his ideas about gravity?

6. Galileo Newton's first law is a restatement of the law of inertia which Galileo had already described, and Newton gave credit to Galileo. So we need to go back before Isaac Newton. Galileo first demonstrated that two objects fall at the same rate regardless of their mass or weight… This referred to as the equivalence principle – a cornerstone of physics. Galileo had an assistant drop two cannon balls of different weight from the Leaning Tower of Pisa. They hit at the same time.

7. Newton Enter Isaac Newton a century later. The story goes that he was puzzling over gravity one day, when he saw an apple fall from a tree. Whether or not that actually happened, we do know that he eventually came up with his famous 3 laws of motion. But more universal, more basic than that was his…

8. What really happened with the apple? Newton (born at 1643) told at least 4 people that he was inspired by an apple tree for his theory, although this did not happen instantly and probably not by an apple falling on his head. What really happened probably goes something like this: Newton, after noticing an apple fall from a tree, began to think about it, and reasoned something like the following: 1. The apple is accelerated, since its velocity changes from zero as it was hanging on the tree and moves toward the ground. 2. Thus, by his 2nd Law which he’d devised by this time, there must be a force that acts on the apple to cause this acceleration. Let's call this force "gravity", and the associated acceleration the "acceleration due to gravity.” 3. Now imagine the apple tree is twice as high. Again, the apple is accelerated toward the ground, so this suggests that this force that we call gravity reaches to the top of the tallest apple tree.