Before the metric system was invented, many measurements were based on body parts, such as feet and hands. Why do you think this might have been an inaccurate way of measuring?
Basic Units
By the 1960s, many scientists all over the world had agreed to use the same system of measuring physical quantities, called the metric system. This international system paved the way for the SI units of measurement that are widely used today.
The full SI system has seven basic units of measurement. We will look at five of them. The quantities and their units of measurement are listed in the table below.
Physical quantity Symbol SI unit of measurement (and symbol) Length Time Mass
l t
Temperature Electric current
Fig. 15.1.3 Basic SI units
Problems can occur when people do not agree on a common system of units.
Researching units
Do some research to find out the answers to the following questions:
1. What are the two other SI units not listed in the table above? 2. Where did imperial units of measurement come from? 3. What makes a unit a basic unit? 4. Why are there 24 hours in a day?
‘Derive’
The word ‘derive’ means to get something from a source.
Derived Units Some other physical quantities are measured in derived units.
Derived units are units that are calculated by combining basic units in a specific way.
For example, the unit for speed is metres per second. It is derived by dividing the basic unit of distance (metres) by the basic unit of time (seconds).
Speed = Distance Time = Metres
Seconds = metres per second, or m/s
Other common derived units are density, power, voltage and resistance. We will learn more about these units in later chapters.
