化工热力学 基本量(时间、长度、质量、力)的定义 第一章1.3

Equation (1-1) provides a relationship among the four fundamental quantities, time, length, mass, and force, which m ust be valid regardless of ( 不管 , 无 论 ) the system of units employed. If th e units of time, length, and mass are de fined first as in Secs. 1-3 to 1-5, then th e selection of a unit of force determine s the value of gc through Eq. (1-1).
1-6 Force……………………………………….….difinition of Newton
In SI units the Newton is defined so a
s to make gc unity. Thus by Eq.(1-1)
1( N
)

1(Kg
)
1(m) gc
/(s)
2
and
gc 1(Kg)(m) /(N )(s)2
1-2 Fundamental Quantities……………………………….introduction
The universally recognized standard is the Int ernational System of Units ( 国际单位制 ) (a bbreviate SI, for System International) establ ished by the General Conference of Weights and Measures ( 国际计量大会 ) upon recom mendation by the International Committee of Weights and Measures ( 国际计量委员会 ) . Other systems of units, such as the English s ystem, are related to the SI units through fixe d conversion factors.
There are a number of fundamental or primi tive concepts, recognized through our sense p erceptions ( 通过感观认知 ) that form the ba sis for all physical measurements. Among the se are time, length, mass, force, and temperat ure, which may be regarded as primary dime
1-4 Length
The fundamental unit of length l is th e meter, symbol (m), defined in term s of ( 根据 ) the wavelength of orang e-red light from krypton 86 ( 氪 86). （ The English unit, foot (ft), is defi ned as 0.3048 (m). ）
nsions( 基本度量 ). For each of these there
must be established an arbitrary scale of mea sure ( 主观的度量标度 ), which is divided in to specific units of size ( 将其大小分成特定 单位 ) .
1-3 Time
1-3 Time
The fundamental unit of time is the seco nd, symbol (s), and until 1967 its definiti on was based on the earth’s motion. The present definition employs an atomic stan dard, and is based on an invariant ( 不变 的 ) property of cesium133 ( 铯 133).
1-6 Force
Definition of Newton Distinction between weight and mass Examples Questions
1-6 Force……………………………………….….difinition of Newton
The fundamental unit of force F is the Newton, symbol (N). It is the force which when applied to a one-kilogram mass will produce an acceleration of one meter per second per second.
1-2 Fundamental Quantities
1-2 Introduction 1-3 Time 1-4 Length 1-5 Mass 1-6 Force 1-7 Temperature
1-2 Fundamental Quantities……………………………….introduction
Example 1-2 A spring scale is calibrated ( 校 准 , 标定 ) so as to read kilograms mass at a location where the acceleration of gravity has its standard value of 9.8(m)/(s)2. If the s cale is used at a place where the acceleratio n of gravity is 9.818(m)/(s)2, what is the ma ss of an object which weighs 6.804(kg) acc ording to this scale?
1-6 Force……………………………………….….difinition of Newton
This definition is based on Newton’s se
cond law of motion, which expresses th
e proportionality between force F and t
1-6 Force……………………………………………….….….difference
Moreover, the use of a balance( 天平 ) t o compare masses is frequently called w eighting( 用天平比较质量大小时叫称 量 ). Thus, one must learn to recognize whether force or mass is meant when th e word weight is used in a casual or info rmal way ( 我们必须学会当随便与非 正式地使用重量这个词时，要能够判 别其意是指力或质量 ).
F ma
1-6 Force…………………………………………………..….difference
The term weight is properly used to indic ate the force of gravity on a body, whereas mass designates the quantity of matter in a body ( 重量一词是专门表示作用在物体 上的重力，而质量则表示物体的质量 ) . This distinction is not always made, howe ver, and standards of mass are often referr ed to as ( 看作 ) weights.
1-6 Force……………………………………….….difinition of Newton
The units of gc may be viewed in two w ays. If the Newton is regarded as an ind ependent unit, gc has the units just given . However, if the Newton is regarded m erely as an abbreviation for the composi te unit ( ㎏ )(m)/(s)2, then gc is consider ed unitless ( 无单位，无量纲 ), and Eq .(1-1) may be written:
1-5 Mass
1-5 Mass
The fundamental unit of mass m is the kil ogram, symbol( ㎏ ). The primary ( 原始 的 ) standard is a cylinder of platinum-iridi um alloy ( 铂铱合金 ) kept at the Internati onal Bureau of Weights and Measures at S evers, near Paris, France. In the English sy stem the pound mass (lbm) is defined as 0. 4535923( ㎏ ).
he product ( 乘积 ) of mass m and acce
leration a:
F

ma gc
(1-1)
where 1/gc is the proportiowhich
1-6 Force……………………………………….….difinition of Newton
1-5 Mass
It should be understood that mass is t he measure of quantity of matter. Whil e mass is related to weight, the two ter ms must be distinguished, for weight r efers to the force exerted by gravity on mass.( 质量与重量有关，这二个术 语必须加以区别，重量指的是作用 于质量的重力 )
1-5 Mass
At a particular location weight is prop ortional to ( 与…成正比 ) mass, but t he proportionality factor ( 比例因子 ) changes from place to place. Although the weight of an object varies from pla ce to place, its mass remains constant.
cceleration of gravity is 9.75(m)/(s)2? ( 在
重力加速度为 9.75(m)/(s)2 时 , 用 (kg)f
表示的质量为 5.443(kg)m 的物体的重力
是)
Solution
BFyEmgqac.(1-1mg)gc,
1-6 Force…………………………………………………….….example
1-6 Force…………………………………………………….….example
Example 1-1 What would be the weight or fo
rce gravity in (kg)f on an object having a ma
ss of 5.443(kg)m at a place where the local a