Here we are, in jolly old Italy. The time is about 1600, and as I promised we are going to meet the man with the name that makes us all laugh. "Galileo Galilee" Ah ha , there he is. "Hello Mr. Galilee. Could I ask you a few questions, please?" .

"Sure sir, only just don't poke my eye with your *enchanted* hat."

"Okay. I wanted to know, what do you think about motion?"

"Well I have a lot to say but I'll try to make it quick. First, I think, unlike Aristotle, that motion to the Earth does not depend on the weight of an object. Rather, I believe all objects fall to the ground at the same speed. Let me show you an experiment I did on top of the leaning Tower of Pisa."

This is what Galileo would have seen at Pisa - two balls with unequal masses both fall in the same time. (There was little wind on the day of Galileo's experiment, so friction was minimal)

"So you see, the speed at which an object falls at has nothing to do with the mass of the object. All objects that are released from the same starting point reach the ground in the same time. My theory unlike Aristotle unifies the concept of what I call force and mass."

"WOW! That's an amazing observation Galileo, but if you're right why then does a feather not fall in the same time as say a rock??"

"Well my friend, you see I have asked myself the same question, and I came up with an answer that I think you would find particularly interesting. I think that all objects have an opposing force to motion - a force that retards the motion. This is the force that slows down the feather - the flat and thin surface of the feather is pushed up by the retarding forces in the air. The rock, because it is more sleek, is less prone to these effects, and hence falls to the ground in less time. My theory though is that if the feather and the rock were placed in a vacuum (absence of all matter) they would fall in the same time - it is the air that slows down the feather not the Earth."

"Very true Galileo, now I have also heard that you have had some theories about motion in general - why objects don't stop immediately as soon as you remove an applied force - as Aristotle thought."

"Quite true, you see I played a mind game with myself a while ago - I saw that when you roll a ball down a ramp such that it rises up the path in its path, that the ball essentially rolls up the plane to its original height. But in time, the ball slows down, not reaching the original height it had to begin with I wondered, and asked myself what slows down the ball - well, like the feather, I figured there must be some form of resistance on the ball slowing it down, only it was not the air this time, but the surface of the ball and the plane. I called this force friction. It explains why when you push a cart the cart slows down to a stop when you stop pushing - the friction opposes the motion of the cart, hence slowing it down. Back to the plane, if the angle of the second ramp were to decrease, the ball would 'try' reach a height original to the release height. Now this model assumes there is no opposing force - friction. I thought about this model some more and realized that if the angle of the second ramp was made to approach 0, the ball would keep rolling in a straight line in constant speed.

The above depicts the thought experiment Galileo "thought of" (Now doesn't that make sence!)

Note how the package slows down, and does not move at a constant (uniform) speed. The package is affected by the retarding force of friction

"Very good points Mr. Galilee, thank you for your time, and I hope things go well for you."

"Well thank you Mr. Wizard type of person sir, I hope you get a better hat. Good day!"

Well to give you a summary of our conversation with Galileo, he essentially stated that all objects when released from the same point, reach the ground in the same time, providing a minimization of air resistance. In fact, the hypothesis Galileo had was right, as when the same experiment with the rock and the feather was performed in a vacuum, some decades later, both fell to the ground in the same time, as there was no air resistance on the feather or the rock. In case you don't know a vacuum is when there is no matter in a volume. In case this concept isn't very clear, don't worry, as our conversation with Newton will help to clear this up. In addition to this, Galileo spoke of his famous thought experiment, that led to the conclusion that all objects want to move in a straight line at a constant speed. This property of an object wanting to maintain this property was called inertia. The final point Galileo spoke of in our conversation was that all objects have an opposing force called friction that slows the object down.

We will talk to Galileo again, but for our next lesson in Motion, we will hear from the man who was thought as the father of physics - Sir Isaac Newton. So to voyage to his neck of the woods, click on the time machine.

According to Galileo, the motion of a falling object is independent of mass. Meaning two objects of unequal masses will fall to the ground when dropped from a set height in equal times

The reason why objects did not always fall in the samwe time was because of a retarding force called friction.