This chapter explains how we measure length and how we describe motion. It begins with the everyday problem that measuring with body parts such as a handspan (balisht), foot, or angula gives different results for different people, so we need standard units that do not vary. The world solved this by adopting the International System of Units (SI), in which the SI unit of length is the metre (m); one metre is divided into 100 centimetres, each centimetre into 10 millimetres, and a kilometre equals 1000 metres (1 km = 1000 m, 1 m = 100 cm, 1 cm = 10 mm). The chapter teaches the correct way to measure: placing the scale in contact with the object along its length, keeping the eye directly above the point being read to avoid error, and, if the zero end is broken, measuring from a full mark and subtracting. Flexible measuring tapes and threads are used for curved lines and round objects. The chapter then turns to position and motion. A position is always stated with respect to a fixed point called a reference point, illustrated by kilometre stones that show distance from a city. An object is in motion if its position changes with respect to a reference point with time, and at rest if it does not; whether something is moving can depend on the chosen reference point, as a bus passenger is at rest relative to the bus but in motion relative to a building outside. Finally, the chapter classifies types of motion: linear motion (along a straight line), circular motion (along a circular path), and oscillatory motion (to and fro about a fixed position). Both circular and oscillatory motion are periodic because they repeat after a fixed interval of time.
Need for standard unitsSI units of lengthCorrect way of measuring lengthReference point, rest, and motionTypes of motion: linear, circular, oscillatory
Key terms
Standard unit
A fixed unit of measurement, such as the metre, that gives the same result no matter who measures the same length.
SI unit of length
The metre (m); 1 km = 1000 m, 1 m = 100 cm, and 1 cm = 10 mm.
Reference point
A fixed object or point with respect to which the position of an object is stated.
Motion and rest
An object is in motion if its position changes with respect to a reference point with time, and at rest if its position does not change.
Linear motion
Motion of an object along a straight line.
Circular and oscillatory motion
Circular motion is along a circular path, while oscillatory motion is a to-and-fro movement about a fixed position; both are periodic.
Extra questions & answers
Activity 5.1: Let us measure
Activity 5.2: Let us explore
Activity 5.3: Let us explore
Activity 5.4: Let us investigate
Activity 5.5: Let us investigate
Activity 5.6: Let us investigate
Activity 5.7: Let us identify
Let us enhance our learning
Learning further
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Why is a handspan not a reliable unit for measuring length?
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