康涅狄格州大学：《普通物理》课程PPT教学课件（英文版）Lecture 2 Lectures available

Physics 121, Sections 9, 10, 11, and 12 Lecture 2 Announcements Lectures available on the web(short version) For over-enrollment please go to the physics office P107 aboratory sessions start next week Gotomywebsitewww.phys.uconn.edu/-rcote Syllabus+ homeworks lectures, etc WebAssign: ready Friday night gotowww.webassign.netandlogin username: first letter of first name plus last name e.g. John Fernando Lachance: lachance 》 institution: uconn > password: your People Soft ID #(with the initial0') Physics 121: Lecture 2, Pg 1

Physics 121: Lecture 2, Pg 1 Physics 121, Sections 9, 10, 11, and 12 Lecture 2 Announcements Lectures available on the web (short version) For over-enrollment please go to the Physics office P107 Laboratory sessions start next week Go to my web site www.phys.uconn.edu/~rcote Syllabus + homeworks + lectures, etc. WebAssign: ready Friday night … go to www.webassign.net and log in » username: first letter of first name plus last name » e.g. John Fernando Lachance: jlachance » institution: uconn » password: your PeopleSoft ID # (with the initial “0”)

Physics 121, Sections 9, 10, 11, and 12 Lecture 2 Today's Topics Chapter 1 Units significant digits Strategy to solve problems Chapter 2: Forces and vectors Types of forces Newton's Laws of motion Net force and vector addition Contact force and tension Physics 121: Lecture 2, Pg 2

Physics 121: Lecture 2, Pg 2 Physics 121, Sections 9, 10, 11, and 12 Lecture 2 Today’s Topics: Chapter 1: Units + significant digits Strategy to solve problems Chapter 2: Forces and vectors Types of forces Newton’s Laws of motion Net force and vector addition Contact force and tension

Significant Figures The number of digits that matter in a measurement or calculation When writing a number, all non-zero digits are significant Zeros may or may not be significant those used to position the decimal point are not significant those used to position powers of ten ordinals may or may not be significant in scientific notation all digits are significant Examples 1 sig fig 40 ambiguous, could be 1 or 2 sig figs 4.0×1012 sig figs 0.0031 2 sig figs 3.03 3 sig figs Physics 121: Lecture 2, Pg 3

Physics 121: Lecture 2, Pg 3 Significant Figures The number of digits that matter in a measurement or calculation. When writing a number, all non-zero digits are significant. Zeros may or may not be significant. those used to position the decimal point are not significant. those used to position powers of ten ordinals may or may not be significant. in scientific notation all digits are significant Examples: 2 1 sig fig 40 ambiguous, could be 1 or 2 sig figs 4.0 x 101 2 sig figs 0.0031 2 sig figs 3.03 3 sig figs

Significant Figures When multiplying or dividing, the answer should have the same number of significant figures as the least accurate of the quantities in the calculation When adding or subtracting, the number of digits to the right of the decimal point should equal that of the term in the sum or difference that has the smallest number of digits to the right of the decimal point Examples 2X3.1=6 3.1+0.004=3.1 4.0×101÷2.04×102=1.6X10 Physics 121: Lecture 2, Pg 4

Physics 121: Lecture 2, Pg 4 Significant Figures When multiplying or dividing, the answer should have the same number of significant figures as the least accurate of the quantities in the calculation. When adding or subtracting, the number of digits to the right of the decimal point should equal that of the term in the sum or difference that has the smallest number of digits to the right of the decimal point. Examples: 2 x 3.1 = 6 3.1 + 0.004 = 3.1 4.0 x 101  2.04 x 102 = 1.6 X 10-1

Adding or Subtracting When numbers are added or subtracted. the number of decimal places in the result equal the smallest number of decimal places of any term in the sum X=123:y=535 123.x0 5.35X 128.X Physics 121: Lecture 2, Pg 5

Physics 121: Lecture 2, Pg 5 Adding or Subtracting When numbers are added or subtracted, the number of decimal places in the result equal the smallest number of decimal places of any term in the sum. x = 123; y = 5.35 123.xxx + 5.35x 128.xxx

Order-of Magnitude Calculations Sometimes it is necessary to know a quantity only within a factor of 10 This is know as an order of magnitude For example that is the total mass of everyone in this class? mass of a person m- 75 kg Number of people n 475 la~7575kg=5625kg~6103kg Physics 121: Lecture 2, Pg 6

Physics 121: Lecture 2, Pg 6 Order-of Magnitude Calculations Sometimes it is necessary to know a quantity only within a factor of 10 This is know as an order of magnitude For example that is the total mass of everyone in this class? mass of a person m ~ 75 kg Number of people n ~ 75 mTotal ~ 75 ´ 75 kg = 5625 kg ~ 6 ´ 103 kg

Problem solution method Five steps: 1) Focus the Problem draw a picture- what are we asking for? 2)Describe the physics what physics ideas are applicable what are the relevant variables known and unknown 3) Plan the solution what are the relevant physics equations 4)Execute the plan solve in terms of variables solve in terms of numbers 5) Evaluate the answer are the dimensions and units correct? do the num bers make sense? Physics 121: Lecture 2, Pg 7

Physics 121: Lecture 2, Pg 7 Problem Solution Method: Five Steps: 1) Focus the Problem - draw a picture – what are we asking for? 2) Describe the physics - what physics ideas are applicable - what are the relevant variables known and unknown 3) Plan the solution - what are the relevant physics equations 4) Execute the plan - solve in terms of variables - solve in terms of numbers 5) Evaluate the answer - are the dimensions and units correct? - do the numbers make sense?

Chap 2: Forces and vectors In classical mechanics Need to study interactions between objects Described by forces We have an idea of what a force is from everyday life Physicist must be precise A force is that which causes a body to accelerate (See Newton's Second Law) A Force is a push or a pull A Force has magnitude& direction(vector) F Physics 121: Lecture 2, Pg 8

Physics 121: Lecture 2, Pg 8 Chap.2: Forces and vectors In classical mechanics Need to study interactions between objects Described by forces We have an idea of what a force is from everyday life. Physicist must be precise. A force is that which causes a body to accelerate. (See Newton’s Second Law) A Force is a push or a pull. A Force has magnitude & direction (vector). F

Fundamental Forces Example of Forces Hooke' s law for ideal spring: F=-kX Units of a force are 1 N 1 kg m/s 2 Fundamental Forces Gravity(more later) >) For motion of planets, etc Strong and weak nuclear forces(not here !) >)Explains behavior of nucleus in atoms Electromagnetic force(next semester in PHY122) >)Relevant for electric systems, chemical properties, etc Physics 121: Lecture 2, Pg 9

Physics 121: Lecture 2, Pg 9 Fundamental Forces Example of Forces Hooke’s law for ideal spring: F = -k x Units of a force are 1 N= 1 kg m/s2 Fundamental Forces Gravity (more later) »For motion of planets, etc. Strong and weak nuclear forces (not here !) »Explains behavior of nucleus in atoms Electromagnetic force (next semester in PHY122) »Relevant for electric systems, chemical properties, etc

The laws of motion Isaac Newton (1642-1727) published Principia Mathematica in 1687. In this work, he proposed three " laws"of motion Law 1: An object subject to no external forces is at rest or moves with a constant velocity if viewed from an inertial reference frame Law 2: For any object, FNET =2 F=ma Law 3: Forces occur in pairs: FAB =-FBA (For every action there is an equal and opposite reaction More in following chapters Physics 121: Lecture 2, Pg 10

Physics 121: Lecture 2, Pg 10 The Laws of Motion Isaac Newton (1642 - 1727) published Principia Mathematica in 1687. In this work, he proposed three “laws” of motion: Law 1: An object subject to no external forces is at rest or moves with a constant velocity if viewed from an inertial reference frame. Law 2: For any object, FNET =  F = ma Law 3: Forces occur in pairs: FA ,B = - FB ,A (For every action there is an equal and opposite reaction.) More in following chapters