Science · Class 6 · Chapter 5

Measurement of Length and Motion

Aim: Build a clear, working understanding of this chapter's key ideas, connected to real NCERT examples, worked problems, and everyday situations.
  • Metre (m) — The SI unit of length.
  • Centimetre / Millimetre — 1/100 and 1/1000 of a metre.
  • Reference point — A fixed point used to describe position.
  • Motion — Position changes with time relative to a reference point.
  • Rest — Position stays the same relative to a reference point.
  • Linear motion — Motion along a straight line.
  • Circular motion — Motion along a circular path.
  • Oscillatory motion — To-and-fro motion about a fixed position.
40 min lesson

Chapter 5 · Concept 1 of 8

How Do We Measure?

  • People traditionally measured length using body parts — an arm, a foot, a handspan (balisht).
  • The problem — When Deepa and her friends measured the same table using their own handspans, they all got different numbers — because everyone's handspan is a different size.
  • Two parts to a measurement — A measurement always has a number and a unit — like “13 handspans.” 13 is the number, handspan is the unit.

Chapter 5 · Concept 2 of 8

Standard (SI) Units of Length

  • As people began to travel and trade between places, the world adopted one common standard: the International System (SI) of Units.
  • 1 kilometre (km) → = 1000 metres (m) → 1 metre = 100 cm → 1 cm = 10 mm

Chapter 5 · Concept 3 of 8

The Correct Way to Measure Length

  • Pick the right scale — A 15-cm scale for a pencil, a metre scale or tape for a room, a flexible tape for a tree's girth.
  • Keep contact — Place the scale in direct contact with the object, along its full length.
  • Eye level with the mark — Look straight down at the measurement mark — a tilted eye gives a wrong reading (parallax error).

Chapter 5 · Concept 4 of 8

Measuring with a Broken Scale

  • If a scale's zero mark or end is broken, use any other clear mark, then subtract that mark's reading from the reading at the other end.
  • Length = 10.4 cm − 1.0 cm = 9.4 cm — Subtract the starting reading from the ending reading to get the true length.

Chapter 5 · Concept 5 of 8

Measuring the Length of a Curved Line

  • Flexible tape — A flexible measuring tape can be laid directly along a curve — like string lights on an arch.
  • The thread trick — Lay a thread along the curve, mark its length, then straighten the thread and measure it with a metre scale.

Chapter 5 · Concept 6 of 8

Describing Position: The Reference Point

  • Whether "the garden is closer" is true depends on what point the distance is measured from. Comparing distances only makes sense when measured from the same fixed point.
  • Reference point — A fixed object or point with respect to which distance or position is described.
  • Kilometre stones — A road sign reading "Delhi 70 km," then later "Delhi 60 km," shows you're getting closer to Delhi — the reference point.

Chapter 5 · Concept 7 of 8

Motion and Rest

  • Motion — An object is in motion if its position changes with respect to a reference point, over time.
  • Rest — An object is at rest if its position does not change with respect to a reference point, over time. A bus passenger is at rest relative to the bus, but in motion relative to a building outside.

Chapter 5 · Concept 8 of 8

Types of Motion

  • Linear motion — A falling eraser, a pushed box
  • Circular motion — A merry-go-round, a whirled string
  • Oscillatory motion — A swing, a pendulum

Quick Recap

Check Your Understanding

  1. 1Match each length with the unit best suited to measure it: distance Delhi–Lucknow, thickness of a coin, length of an eraser, length of school ground.
  2. 2True or False? (i) A car moving on a straight road shows linear motion. (ii) An object changing position with respect to a reference point over time is in motion. (iii) 1 km = 100 cm.
  3. 3Which of these is NOT a standard unit of measuring length? (i) millimetre (ii) centimetre (iii) kilometre (iv) handspan