NCERT Solutions Curiosity Chapter 5 .3 Correct Way of Measuring Length — In-text Questions
Book page 84 & 855 Updated on2026-09-05
Q1.
What is the correct way to place the scale?
Answer
Place the scale in contact with the object, along its length, with the marked edge of the scale touching the object and the zero mark exactly against one end of it (Fig. 5.4a).
Method of placing the scale (as in Fig. 5.4). In (a) the object rests on the scale, along its length, starting at 0. In (b) it is held above the scale and tilted, so the reading is too large.
The three rules:
Contact — the object must actually touch the scale. A gap between them makes you read the mark from a slant.
Along the length — the scale must lie in the same direction as the object being measured, not crossing it at an angle. A slanted scale always gives a length that is too large.
Zero at one end — line up the 0 mark with one end of the object and read the mark at the other end.
Why a tilt makes it too large: the slanting line between the two ends of the object is longer than the straight one, just as a slanting path across a field is longer than walking straight across. So every degree of tilt adds a little extra to your reading.
Tip: Many plastic scales have a thin white margin before the 0 mark. Use the 0 mark, never the physical edge of the plastic.
Q2.
What is the correct position of the eye while reading the scale?
Answer
The eye must be directly above the mark being read — position B in Fig. 5.5. From the slanting positions A and C the same pencil tip appears to fall against a different mark, and the reading is wrong.
Correct position of the eye is ‘B’. From A the tip seems to fall to the right of the true mark, from C it seems to fall to the left.
What goes wrong at A and C: the pencil lies a little above the plane of the markings. When you look from a slant, your line of sight travels on past the tip and meets the scale at a different mark. This error is called parallax.
Eye at A (to the left) → line of sight continues to the right → reading is too large
Eye at B (directly above) → line of sight is vertical → reading is correct
Eye at C (to the right) → line of sight continues to the left → reading is too small
Why only B works: only a vertical line of sight joins the tip and the mark that is truly beneath it. Any other line passes through the tip and lands somewhere else, and the error grows the more you lean.
Check it yourself: Hold a pencil on a scale and read its tip with your head straight above it. Now, without moving the pencil, move your head 20 cm to the left and read again. The reading changes even though nothing was touched — that is parallax.
Q3.
How to measure the length if the ends of the scale are broken?
Answer
A broken scale can still be used. Do not try to guess where 0 was. Instead, line up one end of the object with any clear full mark — say 1.0 cm — read the mark at the other end, and subtract.
Correct method of placing a scale with a broken end (as in Fig. 5.6). One end of the object is at 1.0 cm and the other at 10.4 cm.
Reading at the second end = 10.4 cm
Reading at the first end = 1.0 cm
Length of the object = 10.4 cm − 1.0 cm = 9.4 cm
In millimetres: 9.4 cm × 10 mm/cm = 94 mm
Why subtraction works: the numbers on a scale are distances measured from the 0 mark. So 10.4 cm and 1.0 cm are both distances from the same starting point, and the gap between them is simply the difference. The broken piece never enters the calculation.
Tip: Always start from a full centimetre mark such as 1.0 cm or 2.0 cm, not from 1.3 cm. Subtracting a whole number is quicker and you are far less likely to slip.
Q4.
How do visually challenged students measure lengths?
Answer
They use scales whose markings are raised, so that the divisions can be felt with the fingertips instead of being seen.
Tactile (embossed) scales — the centimetre and millimetre lines stand up above the surface, and the numbers are printed in Braille beside them.
Notched metal rules — small notches or studs are cut at every centimetre, with a deeper notch at every fifth or tenth mark, so the fingers can count quickly.
Click measuring tapes — the tape gives a small click or catch at every centimetre as it is pulled out.
Talking instruments — modern digital tapes and laser distance meters announce the reading aloud.
The unit does not change. A tactile scale measures in the same centimetres and millimetres as an ordinary scale; only the way the reading reaches the person is different — through touch or sound instead of sight.
The idea behind it: a measuring instrument has two jobs — to hold a set of fixed marks, and to pass those marks on to the user. Braille and raised markings change only the second job, so the measurement stays exactly as accurate.
Did you know? Braille itself is a system of raised dots, invented by Louis Braille when he was only fifteen. Braille scales, protractors and even geometry kits are made in India for schools.