Chapter 5 — Measurement of Length and Motion

Chapter 5 is about measurement of length and types of motion. It introduces SI units (metre, centimetre, millimetre, kilometre), the correct method of using a scale, measuring curved lengths, the concept of a reference point, and three types of motion: linear, circular, and oscillatory.

The SI unit of length is the metre, with the symbol m. Other units used are kilometre (1 km = 1000 m), centimetre (1 m = 100 cm), and millimetre (1 cm = 10 mm).

Body parts such as handspan, arm length, and stride differ from person to person. So the same object measured by different people gives different values. Standard units like the metre give the same result for everyone.

Start from any other clear mark on the scale (for example, the 1.0 cm mark). Place that mark at one end of the object, read the value at the other end, and subtract: Length = far-end reading minus starting mark. For example, if readings are 1.0 cm and 10.4 cm, the length is 10.4 − 1.0 = 9.4 cm.

A curved line can be measured using a flexible measuring tape or by laying a thread along the curve, marking the endpoints on the thread, then straightening the thread and measuring it with a metre scale.

A reference point is a fixed object or point with respect to which the position or distance of another object is stated. It is important because the same object can appear to be at rest or in motion depending on which reference point is chosen.

The three types are: (1) Linear motion — movement along a straight line, e.g., a falling eraser; (2) Circular motion — movement along a circular path, e.g., a merry-go-round; (3) Oscillatory motion — to-and-fro movement about a fixed position, e.g., a swing or pendulum.

In circular motion, the object moves along a complete circular path (e.g., a merry-go-round). In oscillatory motion, the object moves to and fro about a fixed position without completing a circle (e.g., a swing). Both are periodic — they repeat after a fixed time interval.

Yes, depending on the reference point. For example, a bus passenger is at rest when the bus is the reference point (their seat position doesn't change), but in motion when an outside building is the reference point (their distance from the building changes with time).

The scale must be placed in contact with the object along its length. The eye should be positioned directly above the tip of the object being measured — not at an angle — to avoid a parallax error that would give a shifted reading.

The chapter mentions angula (finger width), dhanusa, and yojana as ancient Indian units. Angula is still used by traditional craftspeople like carpenters and tailors. Objects with ruled markings that could be scales have also been found at Harappan Civilisation sites.

Yes — the NCERT Class 6 Science (Curiosity) textbook PDF, including Chapter 5 on Measurement of Length and Motion, is freely available on cbseprepmaster.com with no sign-up or account required.