Class 6 Science Chapter 5: Measurement of Length and Motion Notes

Measuring with body parts

The chapter begins with Deepa, an eleven-year-old in Haryana, who needs a new uniform. The shopkeeper measures two metres of cloth with a metal rod, the tailor uses a flexible tape, and her mother asks the tailor to add char angula, the width of four fingers. Deepa’s friends recall people measuring cloth with an arm’s length and fields with strides or feet.

The friends measure their classroom table in handspans (balisht) and get different answers: 13, slightly more or less than 13, between 13 and 14, and 14. The reason is that their handspans are of different sizes. A measurement has two parts, a number and a unit: in 13 handspans, 13 is the number and the handspan is the unit. Because body units differ from person to person, we need a unit that gives everyone the same answer.

India has a rich history of measurement. Units such as the angula (finger width), its multiples, the dhanusa and the yojana appear in ancient Indian literature and were used in making objects, buildings and towns; carpenters and tailors still use the angula. Objects with ruled markings, which could be scales, have been excavated from Harappan sites.

Standard units

When people began to travel, different local units caused confusion, so countries agreed on standard units: the International System of Units, or SI units. The SI unit of length is the metre, symbol m. A metre is divided into 100 centimetres (cm), and each centimetre into 10 millimetres (mm); 1 mm is the smallest length a 15-cm scale can measure. For long distances we use the kilometre (km).

1 km = 1000 m · 1 m = 100 cm · 1 cm = 10 mm

Some scales also show inches, where 1 inch = 2.54 cm; units such as the inch and the foot are still used by some people. Read more in standard units of length.

Measuring length correctly

Choose a suitable device: a 15-cm scale for a pencil, a metre scale or measuring tape for the height of a room, and a flexible tape, like a tailor’s tape, for the girth of a tree or the size of your chest. Place the scale in contact with the object, along its length, and keep your eye directly above the point you read. If the end of a scale is broken, start from any full mark, such as 1.0 cm, and subtract it: an object from 1.0 cm to 10.4 cm is 10.4 cm − 1.0 cm = 9.4 cm long. Visually challenged students use scales with raised markings they can feel.

Write the number, a space and the unit, such as 9.4 cm. Unit names and symbols are written in lowercase, symbols never take an ‘s’ for the plural, and no full stop follows a symbol except at the end of a sentence. To measure a curved line, such as string lights along arches, use a flexible tape, or lay a thread along the curve, straighten it and measure it on a metre scale. Read more in how to measure length correctly.

Position and the reference point

Deepa and her friends disagree about whether the school or the garden is closer, and all of them are right, because each measures from a different house. If they all measured from the same point, such as the bus stand, they would agree. A fixed object or point from which distance is stated is a reference point. On a road, kilometre stones that read ‘Delhi 70 km’ and then ‘Delhi 60 km’ give the position of a bus with Delhi as the reference point.

Motion and rest

An object is in motion if its position changes with respect to a reference point with time, and at rest if it does not. The reference point matters: passengers seated in a moving bus are at rest with respect to the bus, but in motion with respect to a building outside. The book asks you to think about a ship moving steadily in a straight line on a calm sea, with no windows: could you tell whether it is moving?

Types of motion

• Linear motion: along a straight line, such as an eraser dropped from a height, a heavy box pushed along the floor, or students in a march-past.
• Circular motion: along a circular path, such as an eraser whirled on a thread or a merry-go-round.
• Oscillatory motion: to and fro about a fixed position, such as a swing, an eraser hanging from a thread, or a metal strip pressed at one end and released.

If an object repeats its path after a fixed interval of time, its motion is periodic; circular and oscillatory motions are both periodic. Read more in types of motion.

Answers to every question are in the NCERT solutions for Chapter 5.