This textbook chapter defines the SI base units for length, mass and time, explains metric prefixes and introduces dimensional analysis as a tool for checking equations. It is written in English with Arabic translations of key terms.
This is the opening chapter of a physics textbook covering the international system of measurement. It is intended for students beginning a physics or engineering course who need to understand standard units and their definitions.
The chapter first introduces the SI system agreed in 1960. It defines the meter as the distance light travels in a vacuum in 1/299,792,458 second, the kilogram as the mass of a specific platinum-iridium cylinder kept in Sèvres, France, and the second as 9,192,631,770 periods of radiation from a cesium-133 atom.
A section on prefixes explains how powers of ten are used with base units, giving examples such as millimeter, milligram and nanometer. A multiple-choice question tests the prefix for a micron.
The chapter then defines the concept of dimension as the physical nature of a quantity, denoted by square brackets. The three basic dimensions are length [L], mass [M] and time [T].
Dimensional analysis is presented as a technique to check the correctness of equations or to derive them. Dimensions can be added, subtracted, multiplied and divided, and both sides of an equation must have the same dimensions. The limitation is that dimensional analysis cannot give numerical factors.
A table lists the dimensions of common physical quantities with their SI units and symbols. The chapter provides the foundation for all subsequent physics calculations in the textbook.