Length and timePhysical quantitiesThe foundation of physics rests upon physical quantities in terms of which the laws of physics are expressed. Among these are mass, length, time velocity, force, density, temperature, electric current, and numerous others.Physical quantities are often divided into two categories: base quantities and derived quantities.Derived quantities are those whose definitions are based on other physical quantities. Velocity acceleration and force etc. are usually viewed as derived quantities.Base quantities are not defined in terms of other physical quantities.The base quantities are the minimum number of those physical quantities in terms of which other physical quantities can be defined.Typical examples of base quantities are length, mass and time.LengthLength is the distance between two points. S.I. unit of the length is meter (m).Note: Length is a scalar quantity.Length-MeasurementLength can be measured using micrometers, vernier calipers, rulers and measuring tapes.Length to be measuredExampleMeasuring instrumentLong length (Several meters)Length of the class roomMeasuring tapeMedium lengthLength of tableMeter ruleShort lengthExternal or internal diameter of the test tubeVernier calliperVery short lengthDiameter of the wireMicrometer screwgaugeMicrometerMicrometer is used for more accurate measurements of the small lengths. e.g measuring the diameter of a wire or thickness of a small plate.