Long and Short - Measuring Distance

Units of length include Centimeters ($cm$), Meters ($m$), and Kilometers ($km$). We use $cm$ for small objects like a pencil, $m$ for medium distances like the length of a room, and $km$ for long distances like the gap between two cities. Imagine a small paperclip as $1 cm$ long, a tall doorway as $2 m$ high, and a long highway stretching for many $km$.

Measuring with a ruler requires starting from the '$0$' mark. If a ruler is broken at the start, you can measure from the '$1 cm$' mark and subtract $1$ from the final reading. Visualize a pencil placed against a ruler: if it starts at $0$ and ends at the $12$ mark, its length is exactly $12 cm$.

Relationship between units is based on multiples of $10$ and $100$. There are $100$ centimeters in $1$ meter and $1000$ meters in $1$ kilometer. You can visualize a $1 m$ stick being divided into $100$ small equal parts, where each part is $1 cm$.

Conversion from a larger unit to a smaller unit involves multiplication. To change $m$ into $cm$, we multiply by $100$. To change $km$ into $m$, we multiply by $1000$. For example, a $5 km$ race is the same as $5 \times 1000 = 5000 m$.

Conversion from a smaller unit to a larger unit involves division. To change $cm$ into $m$, we divide by $100$. To change $m$ into $km$, we divide by $1000$. If you have $400 cm$ of ribbon, it is exactly $400 \div 100 = 4 m$ long.

The boundary or perimeter is the total length around a closed shape. Visualize walking along the four sides of a square garden; the total distance you walk is the sum of all four side lengths. This is measured in the same units as the sides ($m$ or $cm$).

Estimation is used to guess lengths before measuring. For example, looking at a chalkboard and guessing it is about $3 m$ long, or looking at a finger and guessing it is $5 cm$ long. It helps in understanding the scale of objects visually.