《基础物理学》电子教案 哈里德 HR1.ppt

* Chapter 1 Measurement In this chapter we will explore the following concepts: 1. Measurement of a physical parameter 2. Units, Systems of units 3. Basic Units in Mechanics 4. Changing units 5. Significant figures (1-1) In Physics we carry out experiments in which we measure physical parameters. We then try to deduce the relationship between the measured quantities. We usually express this relationship in the form of a mathematical equation which we call the “physical law” that describes the phenomenon under study. A familiar example is Ohm’s law. The experiment in this case consists of measuring the electric voltage difference V applied across a conductor and the resulting electric current I that flows through the conductor If we plot I versus V we get a straight line. This is expressed in the form: The equation is known as: “Ohm’s Law” R is known as the “resistance” of the conductor (1-2) Assume that you step on your bathroom scale and that it reads 120 The number alone is meaningless. It must be accompanied by the units 120 lb is a very different reading from 120 kg! Conclusion: For every physical parameter we will need the appropriate units i.e. a standard by which we carry out the measurement by comparison to the standard. Does this mean that we have to define units for all parameters? The answer is no. In mechanics we need to define only three parameters: These parameters are: Length , Time, and Mass They are known as: base quantities Note: For the rest of the non-mechanical parameters we need to define only one more unit, that of the electric current In this book we use the International System of Units (SI) In this system the units for the