Motion and Measurement of Distances - Correct Way of Measuring Length

Measurement is the comparison of an unknown quantity with some known quantity of the same kind. This known fixed quantity is called a unit.

For the sake of uniformity, scientists all over the world have accepted a set of standard units of measurement. The system of units now used is known as the International System of Units (SI units).

The SI unit of length is the metre ($m$). For measuring large distances, we use kilometres ($km$), and for smaller lengths, we use centimetres ($cm$) or millimetres ($mm$).

To measure the length of an object, the scale must be placed in contact with the object along its length.

If the zero mark of a scale is broken or not clear, we should use any other full mark of the scale, say $1.0 \text{ cm}$. Then, subtract the reading of this mark from the reading at the other end.

Correct position of the eye: Your eye must be exactly in front of the point where the measurement is to be taken. Looking from an angle leads to errors called parallax errors.

The length of a curved line cannot be measured directly using a metre scale. We use a thread to measure the length of a curved line and then measure the thread length on a scale.