PART 1 MECHANICS

Chapter 1 ： GENERAL INTRODUCTION

( MEASUREMENT)

1.1 The Development of Science

1.1.1 Definition of physics

A science to study matter and energy

1.1.2 World of matter Up (in a unit of meter):

Human beings (100) a building (101) a park (102) a vicinity of a city (103) a river (104) a country (105) an ocean (106) the Earth (107) the orbit of the Moon (7.5108 m) the orbit of the Earth (1011) the Solar system (1012) the Galaxy of Milky Way (1021) known universe (1026, the Harper Radius)

Down:

Human beings our eyes (10–2) the diameter of a HIV virus (10–4) visible light (10–7) molecules (10–9) atoms (10–10) nucleus (10–14) protons (10–15) quarks (10–16 )

1.1.3 Areas of physics

Physics is composed of physical quantities and physical laws

• Mechanics• Thermodynamics• Electromagnetism • Quantum mechanics• Relativity

1.2 Science and Measurement

• Physics is an experimental science, which means that physical quantities are mostly measurable and most of

them have a unit. • Physical quantities are divided into:

Fundamental quantities are those that cannot be defined in terms of other quantities;

Derived quantities are those defined in terms of fundamental quantities.

In metric system or SI base units:

The fundamental physical quantities are:

Length (meter: m)

Time (second: s)

Mass (kilogram: kg)

Temperature (kelvin: K)

Amount of substance/number of particles (mole: mol)

Electric current (ampere: A)

Luminous intensity (candela: cd)

1.3 Length

The standard of 1 meter was originally defined as one ten-millionth of the distance from the north pole to the equator along the meridian line through Paris.

Now it is redefined as 1,650,763.73 wavelength of a particular orange-red light emitted by isotope krypton of Kr86 (Kr83.80).

1.4 Time

A second is defined to be 1/86,400 of a (mean solar) day.

Recently, a more precise standard, based on the cesium clock, is set up: 1 second equals to the duration of 9,192,631,770 periods of the cesium isotope Cs133 (Cs132.905) transition.

1.5 Mass

The mass standard is kilogram. A kilogram is a platinum-iridium cylinder. On the atomic scale we have a second standard of a kilogram mass as 1 atomic mass unit, 1u = 1.660×10-27 kg.

1.6 Dimensions and Units

We call the powers of L, T, and M the dimension of a physical quantity.

[A] = LpTqMr

For example: [velocity] = L1T-1M0

[acceleration] = L1T-2 [force] = [ma] = [m][a] = M1L1T-2

The dimension is helpful in:

(i) Unit exchange;

(ii) Check an unknown equation;

(iii) Find a new physical law.

Example 1: The force unit: newton change to another centimeter-gram-second system.

SOLUTION:

1 N = 1 kgm/s2

= 103 g102 cm/s2

= 105 gcm/s2

= 105 dyne

1 newton = 105 dyne

Example 2: From some experimental results, it is known that the speed of a free-falling object is proportional to the height it falls, h, and the gravitational acceleration constant, g. Therefore, the relation ship between them is estimated as

SOLUTION:

12

1

q

qp

2

1,

2

1 qp

hgkv

qpgkhv

Question:1. What do you think that we are using an English textbook to

study physics? What kind of difficulties might be meet? How are you going to overcome them?

2. What is the relationship of physics with mathematics, other

scientific branches, and technology?

Problems:

1. 5E; 2. 8P;

3. 14P; 4. 20P;

5. 24P.