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NCERT Solutions Curiosity Chapter 5 .3 Correct Way of Measuring Length — Activity 5.1: Let us measure

Book page 865 Updated on2026-09-05

Q1.
Select some objects around you, such as a comb, a pen, a pencil, and an eraser to measure their lengths. Measure their lengths one by one using a metre scale and note down the measurements in Table 5.2.
Answer

Method. Lay each object flat on the table. Put the metre scale beside it, touching it and along its length, with the 0 mark exactly at one end. Bring your eye directly above the other end and read the mark there.

Here is a completed Table 5.2 with the kind of values you will get. Measure your own objects — your numbers will be a little different.

ObjectLength of the objectSame length in mm
Comb15.5 cm155 mm
Pen13.8 cm138 mm
Pencil17.2 cm172 mm
Eraser4.3 cm43 mm
Conversion used: 1 cm = 10 mm
15.5 cm × 10 mm/cm = 155 mm
4.3 cm × 10 mm/cm = 43 mm

Never forget the unit. Writing “15.5” alone tells nobody whether you mean centimetres, inches or metres. A result must always have two parts — a number and a unit.

Tip: Leave a space between the number and the unit — write 15.5 cm, not 15.5cm. Also write the symbols in small letters (cm, mm, m, km), never add an ‘s’ for the plural, and do not put a full stop after them.
How to read the last digit: your 15-cm scale has 10 small divisions in each centimetre, so the smallest length it can measure is 1 mm = 0.1 cm. That is why every reading is written to one decimal place in cm — 15.5 cm, not just 15 cm or 15.53 cm.
Q2.
Some of your friends in the class would have measured the length of the same objects. Compare the lengths measured by you with that of your friends. Are the measured lengths the same or slightly different? If not the same, discuss the possible reasons for the differences.
Answer

The lengths will be almost the same — usually agreeing to within 1 mm or 2 mm. This is a huge improvement on the handspan measurements of Section 5.1, where the answers differed by a whole unit.

ObjectYouFriend 1Friend 2Spread
Pencil17.2 cm17.1 cm17.3 cm0.2 cm = 2 mm
Eraser4.3 cm4.3 cm4.4 cm0.1 cm = 1 mm

Possible reasons for the small differences:

  1. Eye not directly above the mark — the parallax error of Fig. 5.5. This is the commonest cause.
  2. Scale not touching the object, or not laid along its length; a tilt always makes the reading larger.
  3. Zero not aligned — starting from the plastic edge of the scale instead of the 0 mark.
  4. Worn or broken end of an old scale, where the first millimetre is rubbed off.
  5. The end falls between two marks — one student rounds it up, another rounds it down. The scale cannot measure less than 1 mm, so this is a genuine limit.
  6. The object itself — a comb with a slightly bent tooth or a pencil that has been sharpened between the two measurements really has a different length.
The important conclusion: the differences are now small and random, not large and systematic. A standard unit fixed the big problem; careful technique shrinks what is left. No measurement is ever perfectly exact — every instrument has a smallest division below which it cannot go.
Try This: Measure the same pencil five times, lifting the scale each time, and write all five readings. Take the middle value. Repeating a measurement and taking the average is exactly what scientists do to reduce such errors.
Q3.
Why are some length measuring devices made up of flexible materials?
Answer

Because many things we need to measure are not straight. A stiff scale can only lie along a straight line, so it simply cannot follow a curved surface. A flexible tape bends and hugs the curve, and its markings stay the same length while it bends.

What is being measuredShapeInstrument that works
Length of a pencilStraight15-cm scale
Height of a roomStraight but longMetre scale or tape
Chest, waist, sleeve of a shirtCurved (round the body)Tailor's flexible tape
Girth of a tree trunkCurvedFlexible tape or a thread
Head size for a helmetCurvedFlexible tape

A flexible tape is also easy to roll up, so a 3 m or 30 m tape fits in a pocket, while a 3 m rigid rod would be impossible to carry to a Kabaddi ground.

The one condition: the material must bend but must not stretch. A tailor's tape is made of coated cloth or fibreglass, and a carpenter's tape of thin spring steel — both bend freely but keep their length. A rubber band would also bend, but it stretches, so every reading would be too small. That is exactly why Tasneem must not use stretchable rubber for her metre scale (Question 12).
Check it yourself: Wrap a thread once around your wrist, mark the point where it meets, straighten the thread on a 15-cm scale and read the length. You have just measured a curve with a straight scale.
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