NCERT Book Class 11 Physics Chapter 1 Units and Measurements

CHAPTER ONE

UNITS AND MEASUREMENT

1.1 INTRODUCTION
Measurement of any physical quantity involves comparison
with a certain basic, arbitrarily chosen, internationally
accepted reference standard called unit. The result of a
1.1 Introduction measurement of a physical quantity is expressed by a
1.2 The international system of number (or numerical measure) accompanied by a unit.
units Although the number of physical quantities appears to be
1.3 Significant figures very large, we need only a limited number of units for
expressing all the physical quantities, since they are inter-
1.4 Dimensions of physical related with one another. The units for the fundamental or
quantities base quantities are called fundamental or base units. The
1.5 Dimensional formulae and units of all other physical quantities can be expressed as
dimensional equations combinations of the base units. Such units obtained for the
1.6 Dimensional analysis and its derived quantities are called derived units. A complete set
applications of these units, both the base units and derived units, is
known as the system of units.
Summary
Exercises 1.2 THE INTERNATIONAL SYSTEM OF UNITS
In earlier time scientists of different countries were using
different systems of units for measurement. Three such
systems, the CGS, the FPS (or British) system and the MKS
system were in use extensively till recently.
The base units for length, mass and time in these systems
were as follows :
· In CGS system they were centimetre, gram and second
respectively.
· In FPS system they were foot, pound and second
respectively.
· In MKS system they were metre, kilogram and second
respectively.
The system of units which is at present internationally
accepted for measurement is the Système International
d’ Unit é s (French for International System of Units),
abbreviated as SI. The SI, with standard scheme of symbols,
units and abbreviations, developed by the Bureau
International des poids et mesures (The International Bureau
of Weights and Measures, BIPM) in 1971 were recently
revised by the General Conference on Weights and Measures
in November 2018. The scheme is now for international usage