Why this chapter matters for UPSC: Units and measurement are a steady source of Prelims questions: what the SI base units are, which were redefined in 2019, what a light year measures, how distance differs from displacement. This chapter starts the topic with the most basic idea, that a measurement is a number plus a unit, and the three simplest kinds of motion. In NCERT's rationalised 2023-24 edition it is Chapter 7, "Motion and Measurement of Distances", with the text unchanged.

🧠 First Principles — Read This First

Paheli and Boojho's classmates travelled by train, bus, bullock cart, aeroplane and boat, and the teacher showed them news of small wheeled vehicles moving on the soil of Mars. Moving from place to place raises a question: how far?

NCERT's answer comes in three steps:

  1. Measurement means comparing an unknown quantity with a known fixed quantity, the unit. The result has two parts: a number and a unit ("12 foot-lengths").
  2. Body-based units (a foot, a handspan, a gilli or danda) differ from person to person, so we need standard units that do not change: today, the SI units, with the metre for length.
  3. Things around us are either at rest or in motion, and motion is a change in position with time. It can be rectilinear (along a straight line), circular (along a circle, at a fixed distance from a centre) or periodic (repeating after a fixed interval).

PART 1 — Quick Reference

Units of Length (NCERT)

UnitRelation
Metre (m)The SI unit of length
Centimetre (cm)1 m = 100 cm
Millimetre (mm)1 cm = 10 mm
Kilometre (km)1 km = 1000 m

Types of Motion (NCERT)

TypeWhat it isNCERT examples
RectilinearAlong a straight lineA vehicle on a straight road, soldiers in a march-past, a falling stone, sprinters in a 100 m race
CircularAlong a circular path; distance from the centre stays the sameA stone whirled on a thread; a point on a fan blade; the hands of a clock
PeriodicRepeats after a fixed interval of timeA pendulum; the needle of a sewing machine

An object can show more than one at once: a sewing machine's wheel moves in a circle while its needle moves periodically; a ball rolling on the ground moves in a straight line while it spins.

Measuring Devices

TaskSuitable device (NCERT)
Length of a pencil15 cm scale
ClothMetre rod (cloth merchant), tape (tailor)
Girth of a tree, size of the chestMeasuring tape
A curved lineA thread, then a scale

PART 2 — Concepts & Narrative

The Story of Transport

Long ago people moved on foot and carried goods on their backs; then they used animals. Boats began as hollowed logs and were later shaped like the streamlined animals that live in water. The wheel changed transport, its design improving over thousands of years. Until the beginning of the 19th century people depended on animals, boats and ships; the steam engine brought railways, then came motor cars, trucks and buses, motorised boats, and aeroplanes in the early 1900s. The 20th century added electric trains, the monorail, supersonic aircraft and spacecraft.

Why Standard Units?

Paheli and Boojho measure their shared desk with a gilli and a danda, then lose the set; the new set gives a different answer. Measuring the classroom with each student's foot, or a table with each handspan, gives different results too. A string marked in halves, quarters and eighths helps, but the real fix is a standard unit.

Explainer

Units through history (NCERT): a foot, the width of a finger and a step were used everywhere. The Indus Valley people must have used very good length measures, judging by their perfectly geometrical constructions. Ancient Egypt used the cubit (elbow to fingertip); a yard of cloth was measured from the outstretched arm's end to the chin; the Romans measured in paces; ancient India used the angul (finger) and mutthi (fist), and flower-sellers still measure garlands by the forearm.

In 1790 the French created a standard unit of measurement, the metric system. Scientists all over the world then accepted a set of standard units, now the International System of Units (SI).

Measuring Correctly

NCERT's rules for using a scale:

  1. Place the scale in contact with the object, along its length.
  2. If the end of the scale is broken, start from another full mark, say 1.0 cm, and subtract: from 1.0 cm to 14.3 cm the length is 13.3 cm.
  3. Keep your eye exactly in front of the point being read; from either side the reading changes.

Even with the same scale, different students' readings of a classmate's height may differ slightly: small errors in taking observations, which higher classes deal with.

Measuring a curved line: knot one end of a thread, lay the thread along the curve bit by bit, keeping it taut, mark where it reaches the end, then straighten it along a metre scale.

Rest and Motion

List things around you: a school bag, a mosquito, a table, a horse, a moving train, a wall clock. A bird is not in the same place after some time; the table is. Spread sugar on a white sheet, mark an ant's position every few seconds and join the marks: motion is a change in the position of an object with time. NCERT Class IX adds that we describe position from a reference point: a school "2 km north of the railway station" uses the station as the reference.

Key Term

Distance and displacement (NCERT Class IX): distance is the actual length of the path travelled; displacement is the shortest distance from the initial to the final position, in a direction. An object that goes 60 km from O to A and then 25 km back to B has covered a distance of 85 km but a displacement of 35 km. Displacement can be zero even when distance is not (a full lap of a track).

Speed (NCERT Class VII): speed = total distance covered ÷ total time taken (the average speed). The basic unit of time is the second (s), and of speed metre per second (m/s); km/h is common for vehicles. An object moving along a straight line at constant speed is in uniform motion. A vehicle's speedometer shows speed in km/h; its odometer records the distance travelled.

The pendulum (NCERT Class VII): one oscillation is a full to-and-fro movement of the bob; the time it takes is the time period, which stays the same for a given pendulum. NCERT tells the story of Galileo (1564-1642) noticing a church lamp swinging in time with his pulse.

Measuring Time (NCERT Class VII)

Our ancestors noticed that many events in nature repeat at definite intervals: the time from one sunrise to the next was called a day; a month was measured from one new moon to the next; a year by the Earth's journey round the Sun. Before pendulum clocks became popular, people measured time with sundials, water clocks and sand clocks, designed differently in different parts of the world. Pendulum clocks use the constant time period of a pendulum; most modern clocks and watches use electric circuits with quartz crystals and are much more accurate.

Key Term

Fastest speeds some animals can reach (NCERT Class VII, Table 9.4): falcon 320 km/h, cheetah 112 km/h, rabbit 56 km/h, blue fish 40-46 km/h, human 40 km/h, squirrel 19 km/h, domestic mouse 11 km/h, giant tortoise 0.27 km/h, snail 0.05 km/h. To convert km/h to m/s, multiply by 1000 and divide by 3600: 112 km/h is about 31 m/s.

Measuring the Very Large (NCERT Class VIII)

The Sun is nearly 150 million km from the Earth; the next nearest star, Proxima Centauri, about 40,000,000,000,000 km. Such distances are expressed in light years: the distance light travels in one year, at about 300,000 km per second. On that scale the Sun is about 8 light minutes away and Alpha Centauri about 4.3 light years. A light year is a unit of distance, not time.

The SI and Its 2019 Revision

UPSC Connect

The International System of Units (BIPM, The International System of Units, 9th edition, 2019):

  • The Metre Convention was signed in Paris on 20 May 1875 by seventeen States; it set up the International Bureau of Weights and Measures (BIPM).
  • The 11th General Conference on Weights and Measures (CGPM), 1960, formally defined and established the SI.
  • Seven base units: metre (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance) and candela (luminous intensity).
  • The 26th CGPM (2018) revised the SI to rest on seven fixed defining constants, with effect from 20 May 2019. It brought redefinitions of the kilogram, the ampere, the kelvin and the mole. The metre had already been defined through the speed of light since 1983 ("the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second"), and the second through caesium, so those definitions were only reworded.
  • The defining constants: speed of light c = 299 792 458 m/s; caesium frequency 9 192 631 770 Hz; Planck constant h = 6.626 070 15 × 10⁻³⁴ J s (kilogram); elementary charge e (ampere); Boltzmann constant k (kelvin); Avogadro constant N_A = 6.022 140 76 × 10²³ mol⁻¹ (mole); luminous efficacy K_cd = 683 lm/W at 540 × 10¹² Hz (candela).
  • Celsius and kelvin: a temperature in °C is its difference from 273.15 K; one degree Celsius equals one kelvin in size.

PART 3 — UPSC Integration

The chapter teaches the habit behind all of science and technology: say what you measured, in what unit, and how carefully. That habit runs from the Indus Valley's precise geometry to the SI's 2019 revision, which ended the era of physical artefacts such as the platinum-iridium kilogram by tying every unit to a constant of nature. Motion vocabulary (reference point, distance, displacement, speed, period) recurs in physics questions, in astronomy (light years, astronomical distances) and in space missions. Prelims questions are mostly about definitions and units; the classic traps are a light year as time, distance versus displacement, and which units changed in 2019.

Exam Strategy

Prelims traps:

  • A measurement = number + unit.
  • 1 m = 100 cm; 1 cm = 10 mm; 1 km = 1000 m.
  • Metric system: France, 1790 (NCERT). SI: 11th CGPM, 1960. Metre Convention: 20 May 1875.
  • 2019 revision: kilogram, ampere, kelvin and mole were redefined; the metre (1983) and second were already based on constants.
  • Rectilinear (straight), circular (fixed distance from a centre), periodic (repeats at fixed intervals); a pendulum is periodic.
  • Distance ≥ displacement; displacement can be zero.
  • Light year = distance; Sun ≈ 8 light minutes; Alpha Centauri ≈ 4.3 light years (NCERT VIII).
  • Speedometer (speed) vs odometer (distance).
  • 0 °C = 273.15 K.

Mains angle: Explain why standard units of measurement are needed and how the 2019 revision of the SI changed the definition of the kilogram.

Practice Questions

Practice (UPSC-pattern, not past papers).

Prelims:

  1. The SI unit of length is the:
    (a) Centimetre
    (b) Metre
    (c) Kilometre
    (d) Foot

  2. The to-and-fro motion of a simple pendulum is an example of:
    (a) Rectilinear motion
    (b) Circular motion
    (c) Periodic motion
    (d) Random motion

  3. A scale's zero mark is broken. One end of an object is at the 1.0 cm mark and the other at 14.3 cm. The object's length is:
    (a) 14.3 cm
    (b) 13.3 cm
    (c) 15.3 cm
    (d) 12.3 cm

  4. Which of the following SI base units were redefined by the revision that took effect on 20 May 2019?

    1. Kilogram
    2. Metre
    3. Kelvin
    4. Mole
      Select the correct answer using the code below:
      (a) 1 and 2 only
      (b) 1, 3 and 4 only
      (c) 2, 3 and 4 only
      (d) 1, 2, 3 and 4

  5. A car goes 60 km east and then 25 km back west along the same road. Its distance travelled and displacement are:
    (a) 85 km and 85 km
    (b) 85 km and 35 km
    (c) 35 km and 85 km
    (d) 60 km and 25 km

  6. A light year is a unit of:
    (a) Time
    (b) Distance
    (c) Speed
    (d) Brightness

  7. The metric system of measurement was created in:
    (a) 1875, by the Metre Convention
    (b) 1790, by the French
    (c) 1960, by the 11th CGPM
    (d) 2019, by the 26th CGPM

Mains (UPSC-pattern):

  1. "A measurement is meaningless without a standard unit." Explain with examples from NCERT and the history of the SI. (150 words)
  2. Distinguish between distance and displacement, and between rectilinear, circular and periodic motion, with examples. (150 words)

📦 Revision Capsule

Revision Capsule

Hard Facts

  • Transport: foot → animals → boats → wheel → steam engine (19th century) → cars → aeroplanes (early 1900s) → spacecraft
  • Measurement = number + unit; standard units avoid person-to-person differences
  • Old units: cubit (Egypt), yard, pace (Romans), angul and mutthi (India)
  • Metric system: France 1790; SI: 1960; Metre Convention: 20 May 1875
  • 1 m = 100 cm; 1 cm = 10 mm; 1 km = 1000 m
  • Rectilinear, circular, periodic motion
  • Speed = distance ÷ time; m/s; time period of a pendulum is constant
  • 2019 SI revision: kg, A, K, mol redefined; metre via light since 1983; 0 °C = 273.15 K
  • Light year: distance; Sun ~8 light minutes; Alpha Centauri ~4.3 light years

Core Concepts

  • Motion is change in position with time, judged from a reference point
  • Correct measurement needs the right device, contact, a full starting mark and the eye in front
  • Every SI unit now rests on a fixed constant of nature

Confused Pairs

  • Distance vs displacement
  • Rectilinear vs circular vs periodic motion
  • Speedometer vs odometer
  • Light year (distance) vs year (time)
  • Metric system (1790) vs SI (1960) vs 2019 revision

PYQ Pattern

  • No question in BharatNotes' PYQ bank tests this chapter directly. Expect Prelims general science on SI units and their revision, types of motion, and astronomical distances.

Sources

Sources