Physics 101 Physics Metrics

Transcription

1 Physics 101 Physics Metrics Welcome to physics. No matter what your reasons for taking this course, you re going to learn a lot this year, about the world around you and about your own skills and abilities. Let s start by giving you an idea of what physics is and what you ll be studying. (text on screen) Physics is the branch of science that studies matter and energy and how they interact. During the first semester of this physics course we will study mechanics, which deals with objects and their motion. And that s where we ll start this lesson. We live in a universe that is in constant motion. The earth and other planets revolve around the sun. Galaxies rotate around other galaxies. The universe as a whole is constantly expanding. Here on earth jet airplanes can travel faster than the speed of sound. The motion of the wind powers the circular motion of wind turbines. Huge trains and ships carrying millions of tons of cargo hundreds of miles are also in motion. How is motion measured and what are the principles that govern the motion of objects, such as gymnasts or satellites in orbit high above the earth (Read objectives on screen) (student on screen) How would you describe the motion of this ball? You could use common, ordinary words to describe it, but that s not enough for physicists. Instead, we ll make measurements and try to come up with relationships between these quantities with graphs and mathematical equations. (text on screen) There are three fundamental physical quantities, which we can measure and then use to calculate anything else we need to know about the motion of the ball or any other object. See if you can come up with these three quantities. (students on screen) To give you a hint, watch what these students are doing. (green board) The chalkboard that appears on the screen tells you that it s time to take notes. Abbreviate where possible to ensure you have time to write all of the notes. You ll need the information to study for quizzes and the test. Are you ready? Let s get started! Mechanics is the study of motion. The three fundamental quantities used to study objects and their motions are: distance, mass, and time. If you said length instead of distance, that s OK. If you said weight instead of mass, that s not OK. You ll find that mass and weight are very different. Many of you may have suggested other quantities like speed, velocity,

2 area, density, or acceleration. Those are good suggestions, but all of those quantities can be calculated from the fundamental three. We ll see how later. (text on screen) For each of mechanic s fundamental quantities, you need to know the symbol and metric unit. These are often confused by students, so pay close attention to the difference. (green board) A symbol represents the quantity we are measuring in a table or chart or in a formula or equation. Symbols stand alone. They do not need to accompany a number. A unit is placed after the measured number. Example: to express 15 meters we place the letter m after the number 15. In physics, we use the MKS system, which is part of the metric system or system international, also referred to as SI. Teacher (numbers on screen) Look at the two examples in the notes you just took. What is the same in the two examples? Student Both contain a lower case m. Teacher How can you tell what the m represents? In the equation, E = mc 2, the m is a symbol representing what? Student Mass. Teacher Right. It is not attached to a number, so it stands alone. In 15m, the m is the what? Student It s a unit, meter. That s because it s after a number. Teacher Good. It s important to know the difference because lots of units and symbols use the same letters. (table on screen) Let s organize what you need to know about the fundamental quantities in mechanics. Make a table in your notes like the one on the screen. You will need four columns and four rows. Label the top cell of the first column Quantity, the second Symbol, the third cell MKS Unit, and the fourth cell Unit s Abbreviation. Be ready to fill in the remaining cells as they are discussed throughout this lesson. (table on screen) The first quantity is distance. We will use lower case d for distance. The MKS unit for distance is the meter, which is a little longer than a yard. The abbreviation for the meter is m.

3 (picture on screen) The simplest instruments used to measure distance are rulers, meter sticks, and tapes. But to measure larger distances, technology must be brought into the picture. For instance, odometers in your car use the rotation of the tires to measure distances traveled. The sound waves of foghorns were once used to measure the distance from a ship to an iceberg. Now they are used in sonar devices, sound navigation and ranging. The narrow focus of laser beams allows them to be used as measuring devices on construction sites or to provide a reference point for the realignment of automobile frames. Lasers like this one in California are used to measure the movement of the earth s crust along the San Andreas Fault (table on screen) The second quantity is mass. Mass is the amount of matter in an object. The symbol for mass is lower case m. In the MKS system, we use kilogram as the unit for mass. In chemistry, you used grams, but in physics we deal with larger objects with greater masses. A kilogram is two point two pounds. The abbreviation for kilogram is kg. There is a big difference between mass and weight, and in physics this difference is very important. When you are asked to find the mass of an object in the lab, you ll hear the say something like, Let s mass the object. It may sound funny to hear the word mass used as a verb, but you ll get used to it. We don t say weigh in physics unless we mean to find the weight of an object. We ll learn more about the difference between mass and weight in a later program. (table on screen) The third quantity is time. We use a script lower case t as the symbol for time so that it will not be confused with a plus sign. The second is the fundamental unit for measuring time. Its abbreviation is the lower case s, not sec which you may have used in the past. Look at your table, and find the metric units that begin with m, k and s of the MKS system of metric units. The letters, MKS, stand for meter, kilogram, second. That makes it easy to remember. (speed limit sign on screen) Earlier, you were asked to think of physical quantities we could use to describe the motion of the steel ball. You probably thought of speed or velocity. These are almost the same, so we ll deal with them together. Speed or velocity is called a derived physical quantity because it can be calculated or derived from two or more of the fundamental quantities. Let s use something that you already know to figure this out. Think of a speed limit sign. What goes after the 55? Of course, it s miles per hour. What fundamental physical quantities do miles and hours measure? I am sure you know that miles measure distance and hours measure time. Put them together and we have miles per hour. Since miles per hour measures distance per time, what MKS units would we use to measure speed or velocity?

4 We use meters per second in the MKS system. (green board) Derived physical quantities can be determined by measuring two or more fundamental physical quantities and calculating. Derived MKS units are combinations of two or more fundamental units: meter, kilogram, second. In a minute, you are going to think of some examples of derived quantities. You are not responsible for knowing all of these, but you should be able to fill in the MKS units if you are given the formula. Also, know one example of a derived quantity and its unit. Pick the easiest one to remember. Now start by copying this table and then fill it in as the teacher writes. (table on screen) Let s start with velocity, which is almost the same as speed. What is the formula or equation for determining velocity? Let me give you a hint. In the USA, what units do we use to measure speed? You know that we measure speed in miles per hour. Even though miles per hour aren t in metric units, it helps us figure it out. Miles and hours measure what fundamental physical quantities? Miles measure distance and hours measure time. So velocity equals d divided by t. Write that down under formula. Now, fill in the MKS units for distance and time, and we have the derived unit, meters per second. That s the derived MKS unit for speed or velocity. Let s try another one. Let s try area. How do you calculate area? You multiply length, times width. Write the formula. Length and width are distances. So when put the MKS units for distance in the formula, you get the MKS derived unit for area, which is meter squared. So far, you should have filled in the information on velocity and area in your table. It should look like this. Now take area one step farther and try to fill in the formula and MKS unit for volume. (Pause) We use capital V as the symbol for volume. Since volume is length, times width, times thickness, the unit will be meter, times meter, times meter, or meter cubed. If you said meter cubed, you are correct. (cartoon on screen) Here s a harder one: density. Do you remember the formula from chemistry?

5 Its mass divided by volume. Write that formula on the chart. Density is capital D. What would be the MKS unit? Mass is kilogram, and volume is meter cubed, so this is kilogram per meter cubed. Is this Physics Fact or Fiction? You tell us! The English system of measurement is still used in England. (pause) Fact or Fiction? It s fiction! The metric system of measurements, now called the SI or Systeme Internationale, is used all over the world except in the United States. We still use the outdated English System. But the English don t even use it. Why is the English System so bad? Think about it. (text on screen) The English System uses ounces for both weight and Volume. The ounce means something different for each. And math is difficult in the English System. Try to convert 15 miles to inches in you head. These are the facts you would have to use. The math is not easy in the English System. ( FYI on screen - no audio) Now that we ve decided how bad the English System is, let s see how easy the metric system is to use. Most metric units are based on powers of ten so that converting from one to another is a simple matter of multiplying or dividing by 10 a certain number of times. The only exception is the units for time. When the metric system was first proposed in the late 1700 s, an effort was made to redefine other time intervals in terms of multiples of 10 seconds. The day would be divided into 10 hours, the hours into 100 minutes and the minutes into 100 seconds. However, decimal days were never officially adopted by any county, so the numbers on our clocks and calendars remain as exceptions to the metric system. Before we get down to details of the metric system, let s see how much you already know. Your teacher will give you this quiz, and then we ll come back and go over the answers. (Pause Tape Here.) Student 1) A decimeter is 10 centimeters. One square decimeter is one decimeter on a side. Most people would agree that the answer is b. A bathing suit made of one decimeter squared of fabric would be indecent. Student 2) Since a meter is over three feet, the person would be over seven feet tall. So the answer is b, a center for the Hawks. 3) It takes 1000 milligrams to make one gram. So 100 milligrams is one-tenth of a gram. You might remember from chemistry that one milliliter of water has a mass of one gram. So 100 mg would be point one milliliter, just two drops. That s pretty light, so a, an English pea is our best answer!

6 4) One way to approach this problem would be to remember that one kilometer equals 1,000 meters, and a meter is a little longer than a yard. So zero point one kilometer is about 100 yards. The answer is b. 5) A centimeter is about the width of your little finger. It takes two point five four centimeters to make one inch. So a 20 centimeter nose would belong on Pinocchio, answer b. 6) 32 degrees Celsius is equal to 90 degrees Fahrenheit. If it s winter, you must be in Tahiti. The answer is c. : 7) Here s a good way to remember ordinary temperatures on the Celsius scale. Zero is cold, 10 is cool, 20 is warm, and 30 is hot. So two degrees Celsius would be cold. In fact, it equals 36 degrees Fahrenheit. You are a penguin, answer a. 8) An area of 200 meters squared could be 10 meters by 20 meters. That s about 10 yards by 20 yards or 30 feet by 60 feet. Too little for Turner Field and too big for a living room but just about right for a tennis court. The answer is c. 9) A liter is a little more than a quart. You should be familiar with a liter bottle of cola. Since it takes four quarts to make a gallon, 40 liters would fill a 10 gallon aquarium. The answer is c. 10) If you ve ever done any running, you already know that a 10k race is about six miles. So one kilometer is about six-tenths of a mile. Using this fact, you can convert 89 kilometers per hour to 55 miles per hour. That s the speed limit on most country roads. The answer is d. 11) Since one decimeter is 10 centimeters, a cube with a volume of one decimeter to the third power would be 10 centimeters on each side. Its Volume would be 10 centimeters times 10 centimeters times 10 centimeters, or 1000 cubic centimeters. This is the same as 1000 milliliters, which equals one liter. The answer is a. 12) The metric prefix, capital M is mega. One megasecond is equal to one million seconds. That kiss would last for over 11 days. It would have to be a record, wouldn t it? How did you do? Some of the questions were pretty easy if you re at all familiar with metric prefixes. Others required you to do some conversions. Let s start with metric prefixes. One reason for the metric system s success is the fact that it is based on powers of 10. That means that the math will be pretty easy if you know your metric facts. Make a chart like this one to organize your metric facts. You will be responsible for the six metric prefixes most commonly used in physics. It is very important that you learn the facts in this form: One large unit equals (what) smaller units? If you stick to this format, you won t have to worry about fractions. Your facts will always be in whole numbers. (table on screen)

7 Our first prefix is mega. Remember the one mega second kiss? When you think of mega, think big really big! We use a capital M for mega. Don t make this look like a big lower case m. Use straight lines to draw the letter. We ll be using meters for our metric facts, but the same prefixes can be used with gram, liters, or seconds. Using the format, One large unit equals (what) smaller units, the one will go with the larger unit, mega meter. So here s the fact: One mega meter equals one million meters. The next prefix is kilo. Kilo also sounds big. You wouldn t name a Chihuahua Killer. You d save that name for a really big dog. Also, you may have heard on the news that police have confiscated a kilo of cocaine. That s a large quantity. So here are the facts about kilo. We use a lower case k for the abbreviation. And the fact is that one kilometer equals 1000 meters. (picture on screen) The next prefix you need to know and use is deci. Just like the words, teenie and tiny and mini, prefixes with the ie sound are small. The abbreviation for deci is the lower case d. And the fact is that one meter equals 10 decimeters. Remember that we are using one large unit equals so many smaller units, and a decimeter is smaller than a meter. You should be familiar with the prefix, centi. See if you can fill in the information on centi in your chart. Use what you know about cents in a dollar and years in a century to help. The abbreviation for centi is lower case c. One meter equals 100 centimeters. Milli is our next prefix. We use lower case m for milli. The term may be misleading, because it has nothing to do with a million. Instead, one meter equals 1,000 millimeters. (picture on screen) The smallest unit we ll use is micro. Doesn t micro sound smaller than mini? Since the lower case m is already used for milli, we have to go to the Greek alphabet to find a prefix abbreviation. We use the Greek letter mu. It looks like a u with a tail on the left. The fact is that one meter equals one million micrometers. You need to be very comfortable with these metric prefixes and facts before we start using the facts to convert from one unit to another. So let s do some practicing. We used the meter as our base unit for the metric facts in your table, but we ll add other base units now. Remember that the prefixes mean the same thing, whether we re talking about meters, grams, seconds or even dollars. Your teacher will cut off the tape and give you some statements with one or two blanks. Copy each statement, and fill in the blanks to make a statement of fact.. When there are two blanks, decide which unit is larger. Remember that our metric facts are always in the form, One large unit equals so many smaller ones. So the number one will always go with the larger unit, even if it is written last. Your teacher will give you time to finish. Then we ll go over the answers. (text on screen) Local teachers, problem sets will be left on the screen for a few seconds only. It is recommended that you cut off the tape and put these problems on the board or the overhead projector to make it easier to read. They are

8 in the facilitator s guide under unit 1, video problem set number 1. (text on screen) Here are the correct answers. a. One kilogram equals 1000 grams b. One second equals 10 deciseconds c milliliters equals One liter d. One dollar equals 100 centidollars e. 10 decigrams equals One gram f. One million seconds equals One megasecond g. One liter equals 100 centiliters h. One meter equals 1000 millimeters i. One million micrograms equals One gram Your teacher will give you some more practice and then you ll have a quiz before we start using these facts. Before we close this program, let me tell you how important units are. Numbers without a unit have no meaning. For example, if I ask you how much money you have and you say, I have two should I be jealous, ask you for a loan, or give you some money so that you can buy lunch? Think about that! At the end of each lesson, you ll get a chance to show what you know after watching the video and participating in the lesson. We ll give you several multiple-choice questions. Jot down your answers. Your local teacher will go over the answers with you. So get ready to..show WHAT YOU KNOW!! (Read multiple choice questions on screen.) Hope you did well. If not, you have some extra studying to do, and I don t mean a few deciseconds! See you next time!