One of the first physicists was Aristotle. Aristotle classified motion into two classes. These classes were natural motion and violent motion. He thought that natural motion came from the nature of objects. He believed that every object in the universe had it's proper place and tried to get to that proper place. A rock was made of earth; therefore, it tried to reach earth, or fell to earth. Aristotle believed that heavier objects tried harder to reach their proper place; therefore, he believed that heavier objects fell faster than lighter objects.


Violent motion, according to Aristotle, occurred from pushing or pulling something. If you lift the rock, you impart violent motion onto it. Unnatural motion can only be imparted by an unnatural force. This concept is sometimes difficult to defend. For example, if a gun is fired, the bullet continues to move through the air without any visible force pushing it. Aristotle's explanation of this stated that the bullet parted the air in front of it, and then the air squeezed back behind it, thus pushing it forward.


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Copernicus was the first person to question Aristotle's views, which had reigned supreme for 2000 years. Copernicus believed that his astronomical observations proved that the earth moved around the sun. He did not publish this theory for two reasons: 1. This new theory went so far against what was being taught in his day that publishing the theory would brand him as a heretic, and 2. He himself had trouble with his theory, as he could not reconcile it with Aristotle's laws of motion. He finally published his work on the day he died, in 1543.


Galileo was the first to really discredit Aristotle's laws. He did this by postulating that heavier objects would hit the ground at the same time as lighter objects, when dropped from the same height. He proved this with a famous demonstration where he dropped two balls of different weights from the leaning tower of Pisa. The balls hit the ground at the same time. Galileo also came up with the idea of inertia. Rather than take Aristotle's view that and object must have a force acting on it to keep moving, Galileo said that an object would move continuously in a straight line as long as no outside forces acted on it.


Galileo proved this hypothethis through experiments with inclined planes. He placed two inclined planes facing each other. He saw that when a ball was rolled from a certain height down one incline plane it rolled up the other inclined plane to almost the same height. Galileo said that only friction prevented the ball from reaching the original height. He said that without friction, the ball would roll until it reached the same height that it started from. He then asked the question, "If I have a long horizontal plane, how far must the ball go to reach the same height? The obvious answer is forever-it will never reach its initial height."(from Galileo's Dialogues Concerning the Two New Sciences.). Galileo called this tendency for objects to keep moving in a straight line inertia. The concept of inertia finally discredited Aristotle's views of motion.


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Speed is the measure of how fast something is moving. It is the rate at which distance is covered. Speed=distance covered/time taken, or S=d/t. Since things do not always move at the same speed, this equation gives us the average speed. You can modify this equation so that it gives you distance covered. Distance Covered=average speed * time, or d=st.


Many people use the words "speed" and "velocity" interchangeably. However, there is a difference between them. If we not only describe how fast something is moving, but also in what direction it is moving, we are describing velocity. Therefore, an object moving in a curved path may have constant speed, but it cannot have constant velocity.


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Velocity is changes if either speed changes or direction changes. This change in velocity is called acceleration. Accelleration=change in Velocity/Time interval, or a=Vf-Vi/t. Vf, or final velocity, minus Vi, or initial velocity, gives the change in velocity. Acceleration happens whether we increase or decrease velocity.


The force of Gravity causes things to fall. When something is falling without friction, this is called free fall. Galileo discovered that all objects fall at the same rate, regardless of their mass. This rate is 9.8 m/s2. The term "meters per seconds squared" means that this rate is a rate of acceleration. By re-arranging the equation that we just learned, we see that final velocity equals initial velocity plus acceleration times time, or Vf=Vi+at. For free fall, we replace "a", acceleration, by "g", acceleration due to gravity, and get Vf=Vi+gt. This only holds true for an object falling straight down, in free fall.


What happens to an object that is thrown upward? It is still in free fall, because it has no friction (neglecting air resistance). Obviously what happens when something is thrown up is that it slows down going up, and then falls straight down. How quickly does it slow down? It slows down by the rate of acceleration due to gravity: 9.8 m/s2. An object thrown upward moves in a parabola.


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The speed of the object at every point of the parabola going up is the same as the speed of the object at every point of the parabola going down.



Galileo discovered a formula for how far an object falls. This formula is distance traveled= initial velocity times time plus 1/2 times acceleration times time squared, or d= Vit+1/2gt2. The "g" in this formula stands for the acceleration due to gravity, which is always a constant no matter what the mass of the object (remember Galileo's leaning tower of Pisa experiment?). This constant is 9.8 m/s2.