Bài giảng vật lý đại cương chương 1

Phys-211 Instructor: Telephone: Nguyen Quoc Thinh 01685036392 E-Mail: nguyenquocthinh@hus.edu.vn Office: 213 T1 (temporary) Office Hours: By appointment Textbook: Physics for scientists and engineers, Serway, 4th edition World wide web: http://user.hus.edu.vn/nguyenquocthinh Study Suggested Study Procedure Read the assigned topics/materials before coming to the class/lab Attend the class, take good notes, and actively participate in all the activities in the class Reread the topics/materials Doing lots of homework problems is the best way to well in the class As you each problem, think of what strategy you are using to solve the problem Evaluation Lecture and exercise section grades are combined Assignments Midterm Exam Exam 20% 30% 50% The Branches of Physics Physics Physics attempts to use a small number of basic concepts, equations, equations and assumptions to describe the physical world These physics principles can then be used to make predictions about a broad range of phenomena Physics discoveries often turn out to have unexpected practical applications, and advances in technology can in turn lead to new physics discoveries Theories and Experiments The goal of physics is to develop theories based on experiments A theory is a “guess” expressed mathematically, about how a system works The theory makes predictions about how a system should work Experiments check the theories’ predictions Every theory is a work in progress Chapter The Scientific Method There is no single procedure that scientists follow in their work However, there are certain steps common to all good scientific investigations These steps are called the scientific method Chapter Models Physics uses models that describe phenomena A model is a pattern, plan, representation, or description designed to show the structure or workings of an object, system, or concept A set of particles or interacting components considered to be a distinct physical entity for the purpose of study is called a system Chapter Hypotheses Models help scientists develop hypotheses A hypothesis is an explanation that is based on prior scientific research or observations and that can be tested The process of simplifying and modeling a situation can help you determine the relevant variables and identify a hypothesis for testing Chapter Hypotheses, continued Galileo modeled the behavior of falling objects in order to develop a hypothesis about how objects fall If heavier objects fell faster than slower ones,would two bricks of different masses tied together fall slower (b) or faster (c) than the heavy brick alone (a)? Because of this contradiction, Galileo hypothesized instead that all objects fall at the same rate, as in (d) Example A certain corner of a room is selected as the origin of a rectangular coordinate system If a fly is crawling on an adjacent wall at a point having coordinates (2.0, 1.0), where the units are meters, what is the distance of the fly from the corner of the room? Example A high fountain of water is located at the center of a circular pool as shown in the figure below Not wishing to get his feet wet, a student walks around the pool and measures its circumference to be 15.0 m Next, the student stands at the edge of the pool and uses a protractor to gauge the angle of elevation at the bottom of the fountain to be 55.0° How high is the fountain? Example A surveyor measures the distance across a straight river by the following method: Starting directly across from a tree on the opposite bank, he walks 100 m along the riverbank to establish a baseline Then he sights across to the tree The angle from his baseline to the tree is 35.0° How wide is the river? Chapter Accuracy and Precision Accuracy is a description of how close a measurement is to the correct or accepted value of the quantity measured Precision is the degree of exactness of a measurement A numeric measure of confidence in a measurement or result is known as uncertainty A lower uncertainty indicates greater confidence Uncertainty in Measurements There is uncertainty in every measurement, this uncertainty carries over through the calculations need a technique to account for this uncertainty We will use rules for significant figures to approximate the uncertainty in results of calculations Significant Figures A significant figure is one that is reliably known All non-zero digits are significant Zeros are significant when between other non-zero digits after the decimal point and another significant figure can be clarified by using scientific notation Chapter Rules for Determining Significant Zeros Example How many significant digits are in each of the following: a.) 0.007 b.) 1.09 c.) 100 d.) 0.8090 Operations with Significant Figures When adding or subtracting, round the result to the smallest number of decimal places of any term in the sum If the last digit to be dropped is less than 5, drop the digit If the last digit dropped is greater than or equal to 5, raise the last retained digit by Operations with Significant Figures, cont When multiplying or dividing two or more quantities, the number of significant figures in the final result is the same as the number of significant figures in the least accurate of the factors being combined Rules for Calculating with Significant Figures Example A fisherman catches two striped bass The smaller of the two has a measured length of 93.46 cm (two decimal places, four significant figures), and the larger fish has a measured length of 135.3 cm (one decimal place, four significant figures) What is the total length of fish caught for the day? Example Using your calculator, find, in scientific notation with appropriate rounding, (a) the value of (2.437 × 104)(6.5211 × 109)/(5.37 × 104) and (b) (3.14159 × 102)(27.01 × 104)/(1 234 × 106) Derived Units Derived units are combinations of the base units (mks) Area is measured in m2 Volume is measured in m3 Speed is measured in m/s [...]... that can “cancel” each other See the inside of the front cover for an extensive list of conversion factors Example: 2.54 cm 15 .0 in × = 38 .1 cm 1 in Converting Units Example 1 Convert the following: a 25m to cm b 345m to km c 550cm to km d 0.3 m/s to km/hr (important) Chapter 1 Mathematics and Physics Tables, graphs, and equations can make data easier to understand For example, consider an experiment... units kg · m/s2 What are the SI units of the proportionality constant G? Prefixes Prefixes correspond to powers of 10 Each prefix has a specific name Each prefix has a specific abbreviation Chapter 1 SI Prefixes In SI, units are combined with prefixes that symbolize certain powers of 10 The most common prefixes and their symbols are shown in the table Conversions When units are not consistent, you... International Bureau of Weights and Measures in France Time Units seconds, s in all three systems 9 ,19 2,6 31, 700 times the period of oscillation of radiation from the cesium atom Fundamental Quantities and Their Dimension Length [L] Mass [M] Time [T] other physical quantities can be constructed from these three Chapter 1 Dimensions and Units Measurements of physical quantities must be expressed in units that...Chapter 1 Controlled Experiments A hypothesis must be tested in a controlled experiment A controlled experiment tests only one factor at a time by using a comparison of a control group with an experimental group Units To communicate the result of a measurement for a quantity, a unit must be defined Defining units allows everyone to relate to the same fundamental amount Chapter 1 Numbers as Measurements... ball falls A convenient way to organize the data is to form a table, as shown on the next slide Chapter 1 Data from Dropped-Ball Experiment A clear trend can be seen in the data The more time that passes after each ball is dropped, the farther the ball falls Hard to imagine and analyze Chapter 1 Graph from Dropped-Ball Experiment One method for analyzing the data is to construct a graph of the distance... subtract, multiply, divide Both sides of equation must have the same dimensions Dimensional Analysis, cont Cannot give numerical factors: this is its limitation Dimensions of some common quantities Example 1 The following equation was given by a student during an examination: v = v 0 + at 2 Do a dimensional analysis and explain why the equation can’t be correct ν has dimensions L T a has dimensions L 2 T... typically expressed in units of meters per second (m/s) Systems of Measurement Standardized systems agreed upon by some authority, usually a governmental body SI Système International agreed to in 19 60 by an international committee main system used in this course also called mks for the first letters in the units of the fundamental quantities Systems of Measurements, cont cgs – Gaussian system named... construct a graph of the distance the balls have fallen versus the elapsed time since they were released a The shape of the graph provides information about the relationship between time and distance Chapter 1 Equation from Dropped-Ball Experiment We can use the following equation to describe the relationship between the variables in the dropped-ball experiment: (change in position in meters) = 4.9 × (time