Chapter 1 Introduction

Transcription

1 Chapter 1 Introduction 1.1 What is phsics? Phsics deals with the behavior and composition of matter and its interactions at the most fundamental level. 1 Classical Phsics Classical phsics: ( ) 1. Classical mechanics: The stud of the motion of particles and fluids. 2. Thermodnamics: The stud of temperature, heat transfer, and the properties of aggregations of man particles. 3. Electromagnetism: Electricit, magnetism, electromagnetic waves, and optics. 2

2 Modern Phsics Modern phsics: ( now) 1. Special Relativit : theor of the behavior of particles moving at high speeds. It led to a radical revision of our ideas of space, time, and energ. 2. Quantum Mechanics: theor of submicroscopic world of the atoms. It also require a profound upheaval in our vision of how nature operates. 3. General Relativit: theor that relates the force of gravit to the geometrical properties of space. 3 The basic interactions (I) 1. The gravitational interaction produces an attractive forces between all particles. 2. The electromagnetic interaction between electric charges is manifested in chemical reactions, light, radio and TV signals, friction, and all other forces we eperience ever da. 3. The strong interaction between quarks and most other sub-nuclear particles holds particles within the nucleus. 4. The weak interaction between quarks and leptons is associated with radioactivit. Clearl, the dream of phsicists is to discover a single fundamental interaction from which all forces can be derived. 4

3 The basic interactions (II) Concepts, Models, and Theories Concepts: concept is an idea or a phsical quantit that is used to anale natural phenomena. (operational definitions) Laws and Principles: law is a mathematical relationship; a principle is a ver general statement about how nature operates. Models: model is a convenient analog or representation of a phsical sstem and can be useful even if it is incomplete or incorrect. Theories: theor uses a combined principles, a model, and initial assumptions to deduce specific consequences or laws (alwas tentative). 6

4 1.3 Units: Sstème International The value of an phsical quantit must be epressed in terms of some standard or unit. MKS unit sstem: ll phsical quantities can be epressed in terms of three fundamental quantities: Mass (kg), length (m), and time (s). It is convenient to define additional base units: the kelvin (K) for temperature, the ampere () for electric current, and the candela (cd) for luminous intensit. How about the gaussian unit sstem? Units: Derived Units 8

5 1.3 Units: Conversion of Units Eample: The wind speed measured at the ridge is 25 km/h and the recommended fling condition of paraglider is below 7 m/s for the intermediate pilot. Shall this pilot take off? (This is a common condition at the green ba.) Solution: 25 km / h 25 km 1000 m 1h = ( )( )( ) h 1km 3600 s = 6.94 m / s 7 m / s I will suggest this pilot not fling for the moment. 9 Standard bbreviations for Units 10

6 The Greek lphabet Power of Ten Notation and Significant Figures Numerical values obtained from the measurement alwas have some uncertaint. Significant figures indicate the precision of data. 12

7 1.5 Order of Magnitude Use to guesstimate the sie of something onl within a factor of ten. Eample 1.1: n engineer is designing a pacemaker for a cardiac patients. For a 20-ear-old woman, how man times should the device have to beat for her to have normal life epectanc? Solution: We require several estimates. (a)ssuming that she lives to 80 ears, the device must last at least 60 ears. (b)how man heart beat per minute? Sa, 80 beat per minute. (c)how man minutes per ear? = m/ The total heart beats For the safet reason, I will design a pacemaker having beats guarantee before breaking down Dimensional nalsis Each derived unit in mechanics can be reduced to factors of the base units mass (M), length (L), and time (T). If one ignores the unit sstem, that is, whether it is SI or ritish, then the factors are called dimension. For eample: Newton s second law F=ma Force: kg m/s 2 = MLT -2 cceleration: m/s 2 = LT -2 Left-hand side equals to the right-hand side. n equation must be dimensionall consistent. It provides a quick check. 14

8 1.7 Reference Frames and Coordinate Sstem The position of a bod has meaning onl in relation to a frame of reference, which is something phsical, such as a tabletop, a room, a ship, or the earth itself. The position is specified with respected to a coordinate sstem that consists of a set of aes, each of which specifies a direction in space. Three commonl emploed coordinate sstems are Cartesian (,, ), Clindrical (r,, ), and Spherical (r,, ). 15 Historical note: The Geocentric Theor Versus the Heliocentric Theor Even the great minds are confused before concepts get sorted out. Phsics is the end products of the labors of brilliant minds. 16

9 r = Chapter 2 Vectors: summar (I) scalar is specified b a number and its unit. It has magnitude but no direction. It obes the rules of ordinar algebra. vector has magnitude and direction. It obes the laws of vector algebra. In the tail-to-tip method of vector addition, the tail of each vector is placed at the tip of the preceding one. In three dimensions a vector ma be epressed in unit vector notation: and its magnitude is iˆ + ˆj + kˆ = r R R Chapter 2 Vectors: summar (II) The vector equation is equivalent to three equations: r = + = r + R = + R = + The scalar (dot) product of two vectors is r r = cosθ = + + The vector (cross) product of two vectors is r r = nˆ sinθ where the direction of n is given b the right-hand rule. 18