Measurement is the process of comparing the unknown quantity means giving a number to something that is measurable, like length, mass, volume, etc.

Motion: When one object moves with respect to another, it is said to be in ‘motion’.  

History of transport: In ancient times, people had to walk, use boats or animals like horse to travel from one place to another. They used simple boat and raft made of tree trunks to travel across rivers. Later, invention of the wheel led to the development of several other modes of transport like cart, ship, train, car, and aero plane.

Measurement: Measurement is the process of finding the length, size, or quantity of a substance. A physical quantity (like length) to which to be measured with respect to some fixed quantity. A fixed quantity with respect to which a physical quantity is measured is called a unit. In early times, people used different body parts like hand span, cubit and fathom to measure length.

Foot, pace, and yard are some other units of length based on body parts.

Standard units of measurements: Units that have a fixed quantity and do not vary from person to person and place to place are called standard units. For example, the metric system, created by the French in 1790, is a standard set of units.

People in different countries may be using a different set of standard measurement units. For the sake of uniformity, scientist all over the world have adopted a common set of units. This system is called the international system of units or the SI units.

The SI unit of length is meter. Some common standard units of length are inch, millimeter, centimeter, and kilometer.

Depending on the size of the object, we need to measure; we have to choose an appropriate unit. For example, we use meters to measure the length of a piece of cloth, kilometer to measure the distance from one place to another. Centimeter (cm) and millimeter (mm) are used to measure shorter distances while kilometer (km) is used to measure longer distance.