< Classical Mechanics

 Lecture 1 Part 1 Part 2 Part 3

Physical Quantities and Units

Classical Mechanics

Physical quantity is a property of an object that can be quantified by measurement and is expressed by a numerical magnitude (real number) and a unit. These are written as $xy$ where $x$ is the magnitude and $y$ the unit. As an example, a pear weighting 0.15 kg has a magnitude of 0.15 and is specified by the unit of kilograms.
o The SI system (International System of Units) started in France hundred years ago due to the need for a consistent metric system that could be applied worldwide. The following quantities are known as the base quantities, which are the ones of each other quantities can be derived/expressed.
- Length, usually denoted as $a,b,c,d,x,d,y,z,r,l,h$, with an SI unit of metre and SI symbol of $m$;
- Mass, usually denoted as $m$, with an SI unit of kilogram and SI symbol of $kg$;
- Time, usually denoted as $t, \tau$, with an SI unit of seconds and SI symbol of $s$;
- Temperature, usually denoted as $T, \theta$, with an SI unit of Kelvin and SI symbol of $K$;
- Amount of substance, usually denoted as $n$, with an SI unit of mole and SI symbol of $mol$;
- Luminous intensity, usually denoted as $l_v$, with an SI unit of candela and SI symbol of $cd$;
- Electric current, usually denoted as $i,l$, with and SI unit of ampere and SI symbol of $A$.
o A general derived unit, is the one whose derivation is based on the base units specified above. Some examples are the Area $(m^2)$, the Volume $(m^3)$, the Joule $(kg m^2 s^2 \rightarrow J)$ and many others.
o In cases where large quantities are to be measured, the metric system uses prefixes in front of the base units to simplify the scale of what is being measured(i.e distance from Athens to London would be measured in kilometres instead of metres). It is important to note that putting prefixes in front of a base unit does not change it from being a base unit.
Watts is a derived unit.

1. True
2. False
 Part 1 Part 2 Part 3 Lecture 2 