 I'm Professor Stephen Secula. Welcome to this introduction to the study of energy and forces and their effect on material bodies. This discipline is known as mechanics and this course is the first in the physics introductory sequence and begins a general exploration of physics itself. But first what is physics? Well physics quite basically is the study of energy, matter, space and time. So in a sense physics is the science that deals with the foundations of everything in the universe, all that is, all that ever was, and in principle everything that will be in the future in this thing we call the universe. Now physics itself is both a mathematical and a practical science. Now mathematics is the language that you and I will use to describe and even explain phenomena that we observe in the natural world. Mathematics is incredible. It allows us to write a reliable and self-consistent story that describes the cosmos and deeper than that it actually allows us to predict features of the cosmos that no human may yet have observed and yet as time and again we have learned if you can conduct some kind of search for the thing that's predicted, if you have a successful description of nature in the mathematics, you will often find those things that you didn't know existed but that the math told you should be there. Now this ties neatly into the fact that physics is also a practical science. It is practical in that mathematics can say whatever it likes, self-consistent or not, but evidence from the natural world for or against the claims of mathematical ideas, those are the final arbiters of what we know as a kind of scientific truth, a reliable, useful, reproducible assessment and a description of the natural world that is predictive in its abilities. Now together these twin tools, mathematical description and prediction and experimental investigation and assessment, they are the arsenal of ideas that have led us to a reliable and reproducible description of the principles of nature. These are what are referred to as the laws of physics. Now it turns out that there is a small set of such laws that are known to explain a great deal of the universe from the fact that when we walk and lose our footing and trip and we no longer have control of our balance, we fall toward the ground but not up toward the sky to the fact that the moon goes around the earth and together we as a pair go around the sun. It explains the reason why some matter appears solid, some liquid, some matter diffuse, some shiny, some bland, some tough and some brittle. Now these laws while successful are by no means a complete set but to find the complete set of laws we have to begin somewhere and so we will begin with what is known and start in this course from that foundation. Physics begins with observation and measurement. Now observation can be quite a simple thing or at least it can seem like it at first until you start to think about the details of reporting one's observations in a reproducible way. So for instance I might stand on the SMU campus and notice that some distance away from me there's a flagpole with a flag atop the pole. Now what if I want to quantify my distance from where I am to where the flagpole base is? That's the trick. How do you and I come up with a way of agreeing on exactly what that physical distance is between these two things? So that's one thing I might want to say what's the physical distance quantitatively from my present location to where the flagpole is. What if I want to say something instead about how the flag attached to the pole changes from one moment to the next to the next flapping back and forth and then back again? Well that's not just a spatial change that's a change in something else. I'm going to need some kind of system of units upon which you and I can agree and for that we need to think about two of the key players in physics as a study of the natural world space and time. Space if you will is a framework upon which matter can be placed. Think of space like a game board for instance perhaps you've played a game like chess or words with friends or go. You have pieces that you can place on the board and you can move them in certain ways in locations on the board. In this analogy the board to us is what we think of as space in the universe. It's a place where matter can be or not be. In our everyday life we experience space in three separate directions or dimensions as they are known. For instance I can step forward closer to the camera. I can step backward away from the camera. That's one dimension. I can move to my left. I can move to my right and recenter myself in the camera again. I can go up. I can come down. Now that gives us three ways of thinking about independent motion in space. Forward backward doesn't affect left right. Left right doesn't affect up down. Three dimensions and so space as we experience it in the day to day world is said to be three dimensional. Now time there's the trick. Time is the harder to understand of these twin concepts of space and time. Think about time this way. Time is too another kind of dimension or direction. Without a direction in time matter that's placed on the game board of space would simply do nothing. It would go nowhere. The universe would be static and unchanging without time. The universe would be a pretty boring place. Time adds the necessary dimension for changes in space to occur. In physics we can say that time is the displacement between events. For instance if the flap of the flag to one side of the flagpole is considered an event the flip of the flag to the other side of the flagpole might be the next event and so forth. Matter in motion. Events can be causally related to one another. The wind blows the flag flaps. Event one the wind blowing could be the cause of event two the flag flapping. But that needn't be the case. Two events can occur in the universe and actually have nothing to do with each other. Physics as a study of space and time can even give us the tools to understand causality cause and effect and whether or not event one and event two are related to each other or could be related to each other. Space and time together are not just some four-dimensional game board in which the events of the cosmos play out. Taken together they have profound consequences themselves as you will come to learn in the study of the universe through physics. However for the purposes of this introductory course it is merely sufficient to point out that they exist and that there is a standard system of units that we can use for measuring locations and distances in space and moments and displacements in time. For space this unit of measure is the meter and for time the unit of measure is the second. I mentioned earlier that space and time are a game board for matter. Let's talk about matter for a moment. Now matter as we study it on earth in laboratories and even the matter we can see out in the cosmos with instruments like telescopes they all seem to be made from atoms. The indelible fingerprint of atomic matter is just about everywhere we look and see light coming to us from another place in the universe. Now you are going to learn more about atoms as you continue to study physics but you probably first encountered atoms in chemistry class. Remember all those things you were forced to memorize about atoms, what were their weights, masses, what were the number of electrons each kind of atom had, how many protons did an atom have, how many neutrons did an isotope of an atom have, what energy can you get from an atom by liberating an electron from it or what energy might be released from an electron falling into a locked orbit in an atom. These are things that you commit to memory when you first study chemistry but it is physics and specifically it is the laws of physics at the smallest scales that tell us exactly why all of those things are the way that they are. Physics again gives us deep answers as to questions about factual observations and where they come from in the natural world. Matter like space and time can itself be subdivided into little units, little chunks. The atom is one such chunk for instance. There is a standard unit of material substance that we will utilize a lot in this course and that is the kilogram. So with this brief introduction in mind let's take a slightly deeper dive into these units of space and time and matter. In doing so we will learn the standards for these quantities and something of the history of measurement and where the definitions of the modern standards of measurement come from. This altogether is an activity that is key to completing a full understanding of the cosmos at least as can be achieved today. So let's take a deeper look at measurement and units of measurement. To make measurements humans have agreed on an international standard of a system of units of measurement centered around the meter for distance, the kilogram for mass, and the second for time. This is known as the System Internationale or SI and in English we would simply refer to this as the international system of units. Now let's consider the standard definition for the unit of length which is the meter. This is probably an instrument that's familiar to most of you in one form or another. It's a stick that has been graded in various ways in order to mark off units of distance and the full length of this stick from end to end is supposed to be one meter. Now the definition of the meter is something which has evolved over time. In fact at one point in the history of this particular unit of measurement there were competing definitions of the meter. But it was eventually agreed upon that the meter would be considered one ten millionth of the distance from the north pole of the earth to the south pole of the earth along a line on the surface of the earth that ran through Paris, France. Nowadays we have a much more reliable way of defining the meter and for a long time that had been something which was another object. It was a platinum iridium rod and it was robust against temperature and pressure and it was considered the standard unit representing the meter against which all other things that claim to be a meter had to be set. But that rod did in fact have problems. It was only accurate to about 0.0001 meter or about one ten thousandth of a meter and today we have a much better way of defining the meter and that is to use the fastest thing we know of in the universe and that is light. Light regardless of the motion of its source of emission or the motion of the detector that you use to find it moves at a fixed speed in empty space. This is known as the speed of light. And that speed is in fact constant no matter what state of motion the emitter or the observer are in. It's an interesting fact that we will eventually get to in physics. The meter today is defined as the distance that light will travel in 1, 299,792,458th of one second. Now of course that definition depends on the definition of time and I mentioned that that in the international system of units is the second. We're used to measuring time very easily. We all have devices like this in our pockets or most of us do at least. We have some way of keeping record of the passage of time, the moment between one event and the next. But you have to have events in order to keep time and so what sequence of events are you going to use to define the second? Well originally the second was defined as one 86,400th of a solar day. But the problem with that particular definition is that you all have to agree upon what it means to have one solar day. This is an astronomical definition and in fact that definition of the solar day evolved as our knowledge of planetary motion evolved. It's better of course to find something that all observers can not only get access to but then measure the same property of and get the same answer. And so these days in fact as of 1997 the standard of time was altered to use the behavior of an atom of cesium 133, one of the atomic elements. And in fact the second is defined as the duration required for a cesium 133 atom to execute 9,192,631,770 transitions between two well-defined energy levels in the cesium atom. That cycling between those energy levels is something that is insensitive to temperature and pressure and your changing definition of a solar day as planetary motion evolves over the history of the planet in the solar system. This is much more reliable. Now I mentioned earlier that space and time are the game board upon which the chess pieces or the go pieces of matter are arranged and through which they move. Matter as we study it on earth thinking about material objects like this little chunk of metal here. We study this in laboratories, we can observe matter beyond the earth, the moon, our sun, other planets in our solar system, other stars in the universe, other galaxies containing stars just like our own and everywhere we look we see the thumbprint of atoms in the light that we can study from those objects. So it seems that the whole universe of visible information that we have accessible to us is composed of atoms and we learn about atoms really for the first time in a course like chemistry. In chemistry it seems often a very rote exercise in memorizing the properties of atoms, their masses or atomic weights, the number of electrons a specific atom has, the number of neutrons, a specific isotope of an atom has, the exact energies available to the electrons in a specific atom. These all seem like features of the universe we have to commit to memory. But in fact physics gives us the toolkit to understand where all of those facts are set. Physics explains those energy levels, it explains the numbers of electrons and why and how they're arranged in different atoms the way they are. It is the laws of physics that help us to understand those tiniest bits of matter we call atoms. Now mass, which is the final unit of the international system of units that I will discuss today, is standard measure is a kilogram. So what is a kilogram? Well, originally a kilogram was defined as a volume of water and the mass associated with that volume of water. And what was the volume of water? Well, you could imagine a cube 10 centimeters by 10 centimeters by 10 centimeters in dimension, make that entirely out of water and that would represent the kilogram. Now water is a fickle thing. It expands and contracts as temperature and pressure changes. A little bit of it splashes out, you've just changed your definition of the kilogram and you may not even notice that those water molecules have left, they can evaporate. Water is a difficult thing to work with, albeit fundamental to life as we know it. So instead, similar to the definition of the meter, in the 1800s it was decided to use a platinum iridium block and that block was very carefully isolated and protected to keep material from flaking off, at least as much as possible. And to this day, that platinum iridium block is considered the standard definition of the kilogram. All kilograms that you have available to you, and so for instance, this block of metal that I hold here in my hand, which is supposed to represent one kilogram, will have to be ideally calibrated against that single block of platinum iridium at the International Standards Bureau. Now of course, that's perhaps not the most stable definition of mass. In fact, it's entirely possible that atoms of platinum and iridium are lost from that block over time and its mass can change. After all, mass really is a measure of the sum total of the masses of the atoms that make up the material. Every block of material is the sum of its parts and those parts are atoms. And they can flake off this little block of metal without me even noticing. And that makes atoms a very difficult thing to work with. Now this lets me lead finally into the last subject in this introductory lecture and that is measurement. For me as an experimental scientist, measurement is my bread and butter. It's the foundation of everything that I do and I have to not only be good at doing it, but also understand that nobody is perfect at measurement and in fact some measurements have a limit to how perfect they can ever be. These are features given to us by nature and you will learn more about that in physics later. We can consider a measurement and begin to understand the challenges involved in making reproducible observations of the natural world. As I said, this is supposed to be a kilogram and in fact, if I take away these smaller weights, this single block of metal here is supposed to be 500 grams or one half of a kilogram, a kilogram being a thousand grams. That's what's stamped on this block. I have here a holder, a little metal hanger and I can slide the metal block onto it, very convenient. And I have here a scale which is supposed to tell me the mass that is placed on this tray on your left. Now the way that this works is a balancing act. There are weights associated with this arm of the balance. They can slide around. When I place a new weight over here on the tray, the balance will go out of equilibrium. It will start to move. It will experience a force. That force will cause a motion and eventually that motion should settle down to an equilibrium point. If I've done a good job of making the claim that this is in fact half a kilogram of mass, when I place it on this little tray here, the whole device will begin to move. And over here on your right, there's a little arrow and when the arrow points to zero, it means that the weight in the rest of the scale balances the weight here. And it can in practice tell us the mass of this block which is supposed to be 500 grams. So let's take a close look at some of these numbers. This mass is supposed to be 500 grams. Here, however, I have reached equilibrium where the arrow on the balance is pointing just about at zero. And you'll notice that I have a 500 gram mass over here. But in order to balance out the mass I put on the tray, I had to add a little extra mass over toward the right of this scale. I had to add a little bit more than 500 grams to get this system to balance. So who is correct? Is it the manufacturer of this mass who stamped 500 grams on it? Or is it the manufacturer of this balance who claimed that this should be able to somehow accurately measure the mass that's placed on the tray? Now I should say that I went through a little bit of an exercise before this. You might say, well, you put some extra mass on here. This is 500 grams, but this hanger adds mass. In fact, it's 50 grams according to the manufacturer. When I place this on the tray and I let this balance out, when the motion settles down, I will see that I do not have 550 grams measured on the balance. I have 500 and a tiny bit. We're far from 550 grams. I already factored in the mass of this when I set up the scale. So again, who's right? The manufacturer who stamped the mass or the manufacturer who made the balance? This is the challenge to the experimental scientist. If we are to understand nature, we must not only agree on a system of measurement, distance and time and mass, the number of atoms in a material, but we must also understand that our methods for assessing the natural world have limitations. They have uncertainties and we must understand where they come from. We must assess them as best we can. We must report them faithfully and that is measurement. It is not just a number, 500 grams. It is 500 grams plus the limits of my knowledge of that statement. That is what differentiates science from all other ways of knowing. It's not just about learning something. It's about learning the limitations of learning and quantifying them and doing better and better as you make more measurements and mature your methods. So welcome to Physics 1303. I'm looking forward to a very exciting semester with you as we explore the cosmos from these most basic foundations of observation, measurement and units.