Measurement of Length and Motion – Class 6 Science Chapter 5 Notes & NCERT PDF

Class 6ScienceChapter 5NCERT book: Curiosity - Science Class 6

Notes and a simple summary of Chapter 5 of the NCERT Class 6 Science book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 5: Measurement of Length and Motion

Introduction

Deepa, an eleven-year-old girl from Haryana, needs a new school uniform because she has grown taller. At the cloth shop the shopkeeper measures two metres of cloth with a metal measuring rod, and the tailor measures Deepa with a flexible measuring tape. Her mother asks the tailor to add char angula (four finger-widths) to the length. Deepa and her friends Anish, Hardeep, Padma and Tasneem start wondering: how do we measure, and why do we need standard units?

This chapter builds two big ideas: measuring length correctly and describing position and motion using a reference point.

Key Concepts

5.1 How do we measure?

The friends measure their classroom table using their handspans (called balisht). Their answers differ — Padma gets 13, Hardeep 14, Deepa "between 13 and 14" — because everyone's handspan is a different size.

A measurement has two parts: a number and a unit. In "13 handspans", 13 is the number and handspan is the unit.

Body-part units (handspan, foot, arm length, stride, fist, finger) vary from person to person, so they are unreliable.

India has a rich history of measurement. Ancient units include angula (finger width), dhanusa and yojana. Carpenters and tailors still use angula, and ruled objects that could be scales have been excavated from Harappan sites.

5.2 Standard units

Travel and trade created confusion because different regions used different units. Countries agreed on the International System of Units (SI units).

The SI unit of length is the metre (symbol m).

1 km = 1000 m

1 m = 100 cm

1 cm = 10 mm (so 1 mm = 0.1 cm)

1 inch = 2.54 cm (older unit still used by some people)

On a 15-cm scale, each centimetre is divided into 10 small parts; 1 mm is the smallest length you can measure with it.

Writing units correctly: units and symbols are written in lowercase (km, m, cm, mm), never add 's' for plural ("5 cm", not "5 cms"), no full stop after the symbol, and always leave a space between number and unit.

Worked example (Exercise 5): Distance from school to home = 1.5 km.

1.5 km = 1.5 × 1000 m = 1500 m.

Worked example: A friend is 1.42 m tall → 142 cm → 1420 mm.

5.3 Correct way of measuring length

Choose an appropriate instrument: 15-cm scale for a pencil, metre scale or tape for a room, flexible tailor's tape for the girth of a tree or chest.

Place the scale in contact with the object, along its length.

Keep your eye directly above the point being read (position B in Fig. 5.5), not to the left or right — otherwise the reading is wrong.

Broken scale: start from any full mark (say 1.0 cm) and subtract. If the other end reads 10.4 cm, length = 10.4 cm − 1.0 cm = 9.4 cm.

Visually challenged students use scales with raised markings they can feel.

5.4 Measuring a curved line

Anish's family needs string lights for the curved arches of their verandah. For a curved line, use a flexible tape or lay a thread along the curve, straighten it and measure it on a metre scale.

5.5 Describing position

The friends argue whether the garden or the school is closer. Everyone is right — they are measuring from their own houses. If all measure from the bus stand, their answers agree.

When distance is stated with respect to a fixed object or point, that point is called a reference point.

Padma, travelling to Delhi, reads kilometre stones: "Delhi 70 km", then "Delhi 60 km". Here Delhi is the reference point, and her position changes with time — she is moving.

5.6 Moving things

An object is in motion if its position changes with respect to the reference point with time. If its position does not change with time, it is at rest.

Deepa in the bus: with the bus as reference, the seated passengers are at rest; with a building outside as reference, they are in motion. Rest and motion depend on the reference point.

5.7 Types of motion

Linear motion – along a straight line: a falling eraser, march-past in a parade, a box pushed on the floor, a car on a straight road.

Circular motion – along a circular path: an eraser tied to a thread and whirled, a merry-go-round.

Oscillatory motion – to-and-fro about a fixed position: a swing, a hanging eraser pulled aside and released, a metal strip pressed on a table edge.

Periodic motion – repeats after a fixed interval of time. Circular and oscillatory motions are both periodic.

Activities in the Chapter

Handspan measurement (Table 5.1): measure a table with handspans → different results → need for standard units.

Activity 5.1 Let us measure: measure comb, pen, pencil, eraser with a scale; compare with friends and discuss why readings differ slightly (placement, eye position, scale).

Activity 5.2 Let us explore: list five moving and five resting objects with justification → motion means change of position with time.

Activity 5.3: drop an eraser → linear motion.

Activity 5.4: whirl an eraser tied to a thread → circular motion.

Activity 5.5: swing a hanging eraser → oscillatory motion.

Activity 5.6: vibrate a 50 cm metal strip held on a table → oscillatory motion.

Activity 5.7: classify motions in a children's park (swing, slide, merry-go-round, see-saw).

Key Terms

Measurement (माप) – comparing an unknown quantity with a known unit

Unit (मात्रक) – the standard quantity used for comparison

SI units (अंतर्राष्ट्रीय मात्रक प्रणाली) – internationally agreed system of units

Metre (मीटर) – SI unit of length, symbol m

Centimetre / Millimetre / Kilometre (सेंटीमीटर / मिलीमीटर / किलोमीटर)

Handspan (बालिश्त), Angula (अंगुल) – traditional body-based units

Reference point (संदर्भ बिंदु) – fixed point from which distance is stated

Motion (गति) / Rest (विराम)

Linear motion (रैखिक गति)

Circular motion (वृत्तीय गति)

Oscillatory motion (दोलन गति)

Periodic motion (आवर्ती गति)

Real-life Connections

Tailors use flexible tapes; carpenters still use angula.

Kilometre stones on highways use a city as the reference point.

Sports courts (like the Kabaddi court) are marked starting from a reference point.

A Jones Counter fixed to a bicycle wheel measures road-race distances: distance = circumference of wheel × number of turns.

Common Misconceptions

"Different handspan counts mean someone measured wrongly." — No, the unit itself differs from person to person.

"Writing 5 cms is fine." — Unit symbols never take an 's'.

"An object is either moving or not." — Motion is always relative to a reference point.

"A broken scale is useless." — Start from any clear full mark and subtract.

"Stretchable rubber can be used to make a metre scale." — No; the length would change each time (Exercise 12).

Keep the Curiosity Alive (Exercise Hints)

Matching units: Delhi–Lucknow → km; thickness of a coin → mm; eraser → cm; school ground → m.

True/False: 1 km = 100 cm is false (1 km = 1,00,000 cm).

Not a standard unit: handspan.

Rollercoaster (Ex. 11): straight portions → linear; curved loop portions → circular-type motion.

Thickness of one page: measure the thickness of 100 pages together and divide by 100.

💡 Key Learning Points

✓Body-part units like handspan vary between people, so standard SI units are needed; the SI unit of length is the metre (m).
✓Convert confidently: 1 km = 1000 m, 1 m = 100 cm, 1 cm = 10 mm.
✓Measure correctly: right instrument, scale along the object, eye directly above the reading, subtract when the zero mark is broken.
✓Position and motion are described with respect to a reference point; an object moves if its position changes with time.
✓Classify motion as linear, circular or oscillatory, and recognise circular and oscillatory motion as periodic.

👨‍🏫 Teaching Tips

→Repeat the handspan activity: have 5 students measure the same desk and write results on the board to create the need for a standard unit.
→Give each group a deliberately 'broken' scale (tape over the first 1 cm) and ask them to measure pencils correctly by subtraction.
→Demonstrate parallax: let students read a pencil length from positions A, B and C and compare readings.
→Measure curved objects (bottle base, bangle, leaf edge) with thread and a metre scale.
→Draw a map of the school with the gate as reference point and ask students to describe positions of rooms from it.
→Take the class to the playground or park and classify motions of swings, slides, merry-go-rounds and see-saws.
→Play a unit-conversion card game (Exercise 13) — pairs match '2.5 km' with '2500 m' etc.
→Share the story of angula and Harappan scales to connect science with Indian heritage.

📋 Assessment Questions

Can the student explain why handspan measurements of the same table differ?
Can the student convert 1.5 km to metres and 142 cm to metres and millimetres?
Can the student measure an object correctly with a broken scale and write the answer with the correct unit format?
Can the student explain why a bus passenger is at rest for one observer and in motion for another?
Can the student give two examples each of linear, circular and oscillatory motion?

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