Base & Derived Quantities An Explanation

Base Units It is with Base Units that all the other units are derived. There are 7 Base units of the SI system. ▫Meter ▫Kilogram ▫Second ▫Ampere ▫Kelvin ▫Candela ▫Mole

The 7 Base Units of the SI System Unit NameUnit SymbolQuantity measured metermlength kilogramkgmass secondstime ampereAelectric current kelvinKtemperature molemolamount of a substance candelacdluminous intensity

Some of the SI base units are interdependent on each other in their definitions. For instance, the mole, mol, is defined as the number of carbon atoms in kg of Carbon-12. Therefore, the definition of the mole is dependent on the definition of the kilogram. Another example, the meter, m, is defined as the distance light travels in 1/ of a second. Therefore, the definition of a meter is dependent on the definition of the second.

Here is a diagram illustrating the SI Base units and their interdependence.

SI Derived Units Derived Units are units of measurement derived from the products and powers of the Base units. Example: Speed is a derived quantity. It is taken as the distance traveled divided by the time elapsed. Therefore, it is derived from the base units meter and second, to give meters per second. Speed=distance/time  meters/second

More examples of Derived Units The unit of force, the Newton, N, is derived from the equation: Unit Analysis:

Unit Analysis It is important that you understand how to conduct a unit analysis when checking your work on a mathematically rigorous physics problem. One technique is to simply reduce every quantity down to its base units. This way, you know that you are multiplying/dividing/etc. quantities of the same unit. Then, when you obtain your result, your units should simplify to your desired derived unit.

Derived quantities are composed of base units. It is very helpful to understand how a given quantity is derived. This will help you understand the nature of the unit and make unit conversions more apparent.

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