Key Points

Early measurements often used body parts like handspan, arm length, or strides, which vary from person to person. Traditional Indian units such as angula (finger width) were also used.

Due to variations in body part measurements, there was a need for universal units to ensure consistent measurements for the same length by different people.

Countries adopted a set of standard units known as the International System of Units, or SI units, to avoid confusion in measurements globally.

The SI unit of length is the metre, symbolized as $\text{m}$. It forms the basis for other length units in the metric system.

One metre ($\text{m}$) is divided into 100 equal centimetres ($\text{cm}$), and each centimetre is further divided into 10 equal millimetres ($\text{mm}$). Thus, $1 \text{ m} = 100 \text{ cm}$ and $1 \text{ cm} = 10 \text{ mm}$.

For measuring larger lengths, such as distances between cities, a kilometre ($\text{km}$) is used, where $1 \text{ km} = 1000 \text{ m}$.

To measure length accurately, the scale must be placed in direct contact with the object along its length, ensuring no gaps or misalignments.

When reading a scale, the eye should be positioned directly above the mark being read to avoid parallax errors, which can lead to incorrect measurements.

If a scale's zero mark is unclear or its end is broken, measurement can start from any clear full mark, and this initial reading must be subtracted from the final reading. For example, the length of an object could be $10.4 \text{ cm} - 1.0 \text{ cm} = 9.4 \text{ cm}$.

A measurement result consists of two parts: a number and a unit. For instance, a length of $13 \text{ handspans}$ means 13 is the number and handspan is the unit.

To measure the length of a curved line, a flexible measuring tape or a thread can be used. The thread is placed along the curve, straightened, and then measured with a metre scale.

A reference point is a fixed object or position used to describe the location or distance of another object. Distances are always stated with respect to a reference point.

An object is considered to be in motion if its position changes with respect to a chosen reference point over time.

An object is considered to be at rest if its position does not change with respect to a chosen reference point over time.

Linear motion occurs when an object moves along a straight-line path, such as a car moving on a straight road or a ball dropped from a height.

Circular motion is the movement of an object along a circular path, exemplified by a merry-go-round or an eraser whirled on a thread.

Oscillatory motion describes an object moving to and fro about a fixed central position, like a swing or a pendulum.

If an object repeats its path or motion after a fixed interval of time, its motion is classified as periodic. Both circular and oscillatory motions can be periodic.