Measurement_Notes

 2013 Yeow Kok Han Page 1 Measurement 1 Importance of Measurement Scientific knowledge is powerful because it is not just untested theory and hypotheses. Scientists demand that the theories be supported with empirical evidence or measurements. For example, Einstein’s General Rela tivity theory suggested that gravity can bend the path of light but our ultimate confidence in the theory is whether the bending can be measured and checked against the amount predicted by the theory. Measurement is about quantifying things and the abilit y to quantify things allows calculations, analyses and deductions which in turn lead to new knowledge or theories. Once a theory is well supported by empirical data, it then can be used for predicting outcomes or results. To understand what the big deal about prediction is, let’s just consider the construction of a high rise building which costs millions of dollars. An architect will carry out calculations to make sure the designed building can withstand the expected loading and maybe possible earthquake. T hose calculations are done using theoretical formulae that have been verified with prior measurements! 2 SI System of Quantities & Units For measurements to be useful, they need to be expressed in terms of appropriate units which are internationally accepted. * See Joint Committee for Guides in Metrology (JCGM), International Vocabulary of Metrology, Basic and General Concepts and Associated Terms (VIM), III ed., Pavillon de Breteuil : JCGM 200:2012. International System of Quantities* consists of International System of Units* consists of 7 base quantities which cannot be defined in terms of other quantities: length, mass, time, electric current, thermodynamic temperature, amount of substance, luminous intensity 7 base units corresponding to the base quantities: metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol) and candela (cd) (Note: candela is not required in syllabus) Derived quantities are defined in terms of the base quantities. e.g. Force maF  but t va   and t sv   so F is finally defined in terms of the base quantities length, mass and time. Derived units are defined in terms of the base units. The notation* [Q] means ‘unit of Q’: [F] = [ma] = [m] [a] = [m] [v/t] = [m] [s/t]/[t] = kg m s-2 The derived unit kg m s -2 is given a more convenient short form N for newton. In the SI system, the same set of equations is used to express derived quantities or units in terms of the base quantities or units. Note also that quantities are always defined in terms of quantities and units are always defined in terms of units i.e. quantities and units are different entities. Hence it would be wrong to define speed(a quantity) as ‘the distance(a quantity) travelled per second(a unit)’. There are 7 base quantities and corresponding base units in the SI