Measurement - Measurement of Length

Standard Units of Length: Length is the distance between two points. In the ICSE Grade 4 curriculum, we primarily use Kilometres ($km$) for long distances like between cities, Metres ($m$) for heights of buildings or lengths of cloth, and Centimetres ($cm$) or Millimetres ($mm$) for small objects like pencils or notebooks. Imagine a long highway stretching out for $km$ and a small wooden ruler for $cm$.

Measuring with a Ruler: A standard $15 cm$ ruler is used for measuring small objects. Each centimetre is divided into $10$ equal parts called millimetres. When measuring, you must always align the '0' mark of the ruler with the start of the object. Visualize the edge of your pencil tip starting exactly at the $0$ line and ending at a specific number on the ruler.

The Base Unit (Metre): The Metre ($m$) is the standard unit of length. One metre is approximately the distance from the floor to the waist of an average adult, or the width of a standard doorway. It takes $100$ centimetre markings on a measuring tape to make exactly $1$ metre.

Conversion of Units (Big to Small): To change a larger unit to a smaller unit, we always multiply. For example, since $1 m = 100 cm$, to convert $5 m$ to $cm$, you multiply $5$ by $100$. Think of this as breaking down a large block into many smaller pieces; the number of pieces increases.

Conversion of Units (Small to Big): To change a smaller unit to a larger unit, we divide. For example, to convert $2000 m$ to $km$, you divide $2000$ by $1000$. Visualize gathering $1000$ individual $1$-metre sticks and bundling them together to create a single $1$ kilometre block.

Addition and Subtraction of Length: When performing operations, always align the units in separate columns (e.g., a column for $m$ and a column for $cm$). If the sum in the $cm$ column reaches $100$ or more, you must carry over to the $m$ column because $100 cm = 1 m$.

Understanding Kilometres: Kilometres are used for measuring very long distances. $1 km$ is equal to $1000 m$. If you imagine a standard running track, which is usually $400 m$ long, two and a half laps around that track would roughly equal $1 km$.