These are the properties of a body which can be measured, for example, position, mass, charge, temperature, etc
Physical Quantities are expressed as a number plus and unit. A number on its own has no meaning - what is faster, a centimeter per second or furlong per fortnight?
Physical Quantities are expressed as a number plus a unit. The preferred units are the SI Units
The following prefixes are commonly used:
| Prefix | Magnitude | Symbol |
|---|---|---|
| atto | 10-18 | a |
| femto | 10-15 | f |
| pico | 10-12 | p |
| nano | 10-9 | n |
| micro | 10-6 | μ |
| milli | 10-3 | m |
| centi | 10-2 | c |
| deci | 10-1 | d |
| deca | 10 | da |
| hecto | 102 | h |
| kilo | 103 | k |
| mega | 106 | M |
| giga | 109 | G |
| tera | 1012 | T |
| peta | 1015 | P |
| exa | 1018 | E |
The SI base units are as follows:
| Quantity | Unit | Symbol |
|---|---|---|
| Mass | kilogram | kg |
| Time | second | s |
| Length | meter | m |
| Electric Current | Ampere | A |
| Thermodynamic Temperature | Kelvin | K |
| Amount of Substance | mole | mol |
| Luminous Intensity | Candela | cal |
Note that the below definitions have been updated. These have only been included as they were the correct definitions of the SI base units at the time the book that is being followed in compiling these note was published. See Up-to-date SI Unit definitions
| Quantity | Unit | Definition |
|---|---|---|
| Mass | kilogram | Mass of the International Prototype Kilogram |
| Time | second | The duration of 9192631770 periods of the radiation corresponding to the transition between to hyperfine levels of the ground state of a caesium-133 atom |
| Length | meter | The distance light travels in a vacuum in 1/299792458 seconds |
| Electric Current | ampere | The constant current which, if maintained in two straight parallel conductors of infinite length and negligible cross sectional area when placed one meter apart in a vacuum would produce a force of 2e10-7 newtons per meter of length between the two conductors |
| Thermodynamic Temperature | kelvin | The fraction 1/273.16 of the thermodynamic triple point of water |
| Amount of Substance | mol | The amount of substance of a system which contains as many elementary entities as there are atoms in 0,0012 kilograms of carbon-12 |
It can be seen from the above that their definitions have interdependency - they can only be defined in a particular order
The base units above can be combined to create derived units for all measurable physical quantities
Derived units can always be expressed in terms of both base units and base quantities.
Newtons are equivalent to kgms-2 and [M L T-2], where M = mass, L = length and T = time