Constructing Physics

Elementary Physics Course Notes

Adam Johnston
Department of Physics
Weber State University


Measuring Motion

Today's discussion started on the natural laws described by Aristotle (384-322 BC).  Often we call this "Aristotelian Physics"; although, it wasn't really "physics" in the same way that we think of physics today.  Aristotle was a philosopher, and was trying to figure out a simple set of rules that the universe abided by.

Basically, Aristotle's physics relied on the idea that different materials had different resting places, and that all materials naturally wanted to be at rest.  The four materials that we can interact with, according to Aristotle, were earth, water, air, and fire.  Things made of earth tended to fall, while items which are hot, such as steam, were said to be made from fire and would rise.  Water items would sit above earth materials, and air would rest above water but below fire.  Thus, the motion of any item could be predicted based upon what it was made from.  Many examples of this are demonstrated in class.

There are some modifications that were made to this idea in order to get a few other details pinned down.  Yet, there were still some problems.  Even though Aristotle's system is quite simple, understandable, and even works in many cases, it does not stand up to experiments and testing.  In his defense, Aristotle was not a scientist, but a philosopher, so he wasn't concerned about such nonsense.  However, when Galileo came along (in the late 1500's and early 1600's A.D.), he would actually put many of Aristotle's ideas to test via experiments.  This gave Galileo the lofty title of "the father of science."  By putting this on his resumé (just kidding), he was able to get a nice academic position in Padua, Italy; though such a title didn't save him from the Inquisition a few years later.

Galileo and his (and our) experiments with motion were the topics for next lecture.

Everyone was so thrilled with how easy Aristotle made physics, so they were a little dismayed that Aristotle didn't have the entire picture in focus.

There are only a few things we can measure: position, time, and mass.  Today we want to talk about motion, so how do we get there?  Motion is a change in position during some change in time.  Thus, we need to measure the position of an object and the time at which an object is in a given position, and then watch how these values change.  In order to do this, students were given stopwatches, and each stopwatch was assigned a particular position on the track.  After watching a cart travel across the track, the position and time data were recorded.  However, all of this just looked like a bunch of numbers (even though we could tell that the cart increased its position as time increase -- definitely a form of motion).  We decided to graph the data to make sense of it.  The graph showed that the motion was constant, and we realized that the slope of this graph told us something about what direction and how fast the cart was moving.  This was demonstrated with a motion detector which made graphs for us in real time.

By looking at the graph, it seems that if the track were longer, the cart would have continued doing very constant, uniform motion.  This is different than what Aristotle would have said, but jives quite well with the experiments of Galileo.  Galileo described motion in a similar way: being uniform unless something acted upon the moving object.  Galileo also experiment with balls rolling down hills and back up ramps.  He noted that a ball would always try to return to the same height from which it started; so, a ball rolling along a level plane could go forever, since it never reached that initial height of the hill.  This is the idea leads us to the law of inertia, which we'll talk about next week, along with the other laws of motion.