Long and Short - Standard Units of Length

Introduction to Standard Units: To measure length accurately and consistently, we use standard units like centimeters ($cm$), meters ($m$), and kilometers ($km$). Unlike handspans or footsteps, standard units provide the same measurement for everyone.

Centimeter ($cm$): This unit is used for measuring small objects like a pencil, an eraser, or a mobile phone. On a standard $15\ cm$ plastic ruler, you can see small markings numbered $0$ to $15$. The distance between two consecutive numbers, such as $2$ and $3$, is exactly $1\ cm$. Small lines between these numbers represent even smaller units.

Meter ($m$): The meter is the standard unit of length used for measuring longer things like the length of a room, the height of a wall, or a piece of cloth. Visually, $1\ meter$ is roughly the width of a single door or the length of a long stride of an adult. It is made up of $100\ cm$.

Kilometer ($km$): We use kilometers to measure very long distances, such as the distance between two cities or the length of a highway. On a road, you will often see milestones—small stone pillars with a curved top—that show the distance to a place in $km$. One kilometer is equal to $1000\ meters$.

Measuring with a Ruler: To measure correctly, align one end of the object exactly with the $0$ mark on the ruler (not the edge of the ruler itself). The number that aligns with the other end of the object is its length. For example, if a toothpick starts at $0$ and ends at the $5$ mark, its length is $5\ cm$.

Estimation of Lengths: It is important to choose the right unit for the right object. For instance, the length of a fingernail is measured in $cm$, the length of a playground is measured in $m$, and the distance from Delhi to Mumbai is measured in $km$.

Conversion Basics: To change a larger unit to a smaller unit, we multiply. For example, to change $m$ to $cm$, we multiply by $100$. To change $km$ to $m$, we multiply by $1000$.