Suppose we all measure the length of the table again, but this time using a metre scale. Will our results still be different? Name of the Student Number of Handspans Anish Slightly more than 13 Padma 13 Tasneem Slightly less than 13 Deepa Between 13 and 14 Hardeep 14 Table 5.1, page 81 — the l

No — this time everyone will get essentially the same answer , because the metre scale carries a standard unit . One centimetre on Deepa's scale is exactly one centimetre on Hardeep's scale, whereas one handspan was different for each of them. If the table is, say, 1.24 m long, then every one of the five friends should get: Length = 1.24 m = 1.24 × 100 cm = 124 cm (using 1 m = 100 cm) = 124 × 10 mm = 1240 mm (using 1 cm = 10 mm) But — and this is what Padma's friend replies in the book — tiny differences of one or two millimetres can still appear , and they have nothing to do with the unit. They come from careless handling of the scale: keeping the scale away from the table instead of touching it; not laying the scale along the length of the table; looking at the mark from the side instead of from directly above; losing or gaining a millimetre each time the scale is lifted and shifted along a long table. That is why the very next thing to learn is the correct way of using a scale . Why it happens: a standard unit removes the large disagreement (13 versus 14). Careful technique removes the small disagreement (124.0 cm versus 124.2 cm). You need both to get a trustworthy measurement. Tip: For a table longer than your scale, mark the point where the scale ends with a pencil, then start the next measurement exactly from that mark. Adding up 100 cm + 24 cm gives 124 cm.

NCERT Solutions for Class 6th Science Chapter 5 .2 Standard Units — In-text Questions

Book page 83 & 845 Updated on2026-09-19

Q1.

Would it be convenient to use the unit metre to measure larger lengths, such as the length of a railway track between two cities, or to measure smaller lengths, such as the thickness of a page of a book?

Answer

No, it would not be convenient in either case. The metre is the right size for everyday objects such as a room or a piece of cloth, but it is far too small for a railway line and far too big for a page.

Case 1 — a railway track between two cities. The Delhi–Mumbai railway line is about 1384 km long.

1 km = 1000 m (conversion factor)
1384 km = 1384 × 1000 m = 13,84,000 m

A number with seven digits is clumsy to say, to write and to compare. Written as 1384 km it is instantly understood — which is why we use the kilometre for long distances.

Case 2 — the thickness of a page of a book. One page is about 0.1 mm thick.

1 m = 100 cm and 1 cm = 10 mm
∴ 1 m = 100 × 10 mm = 1000 mm
So 1 mm = 1/1000 m = 0.001 m
Thickness of a page ≈ 0.1 mm = 0.1 × 0.001 m = 0.0001 m

A measurement written as 0.0001 m is hard to picture and easy to get wrong by a zero. Written as 0.1 mm it is simple — which is why we use the millimetre for very small lengths.

To measure	Convenient unit	Typical value
Railway track between two cities	kilometre (km)	≈ 1384 km (Delhi–Mumbai)
Length of a cloth, height of a room	metre (m)	2 m, 3 m
Length of a pencil	centimetre (cm)	≈ 17 cm
Thickness of a page, of a coin	millimetre (mm)	0.1 mm, 2 mm

Why it happens: a good unit is one that gives a small, easy number for the thing being measured. That is the only reason km, m, cm and mm all exist — the length itself never changes, only the way we write it. You would not measure milk in drops, or a river in drops either.

Q2.

Suppose we all measure the length of the table again, but this time using a metre scale. Will our results still be different?

Name of the Student	Number of Handspans
Anish	Slightly more than 13
Padma	13
Tasneem	Slightly less than 13
Deepa	Between 13 and 14
Hardeep	14

Table 5.1, page 81 — the length of the same table, measured by five children in their own handspans.

Answer

No — this time everyone will get essentially the same answer, because the metre scale carries a standard unit. One centimetre on Deepa's scale is exactly one centimetre on Hardeep's scale, whereas one handspan was different for each of them.

If the table is, say, 1.24 m long, then every one of the five friends should get:

Length = 1.24 m
= 1.24 × 100 cm = 124 cm (using 1 m = 100 cm)
= 124 × 10 mm = 1240 mm (using 1 cm = 10 mm)

But — and this is what Padma's friend replies in the book — tiny differences of one or two millimetres can still appear, and they have nothing to do with the unit. They come from careless handling of the scale:

keeping the scale away from the table instead of touching it;
not laying the scale along the length of the table;
looking at the mark from the side instead of from directly above;
losing or gaining a millimetre each time the scale is lifted and shifted along a long table.

That is why the very next thing to learn is the correct way of using a scale.

Why it happens: a standard unit removes the large disagreement (13 versus 14). Careful technique removes the small disagreement (124.0 cm versus 124.2 cm). You need both to get a trustworthy measurement.

Tip: For a table longer than your scale, mark the point where the scale ends with a pencil, then start the next measurement exactly from that mark. Adding up 100 cm + 24 cm gives 124 cm.

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