NCERT Solutions Curiosity Chapter 5 .2 Standard Units — In-text Questions

Book page 83 & 845 Updated on2026-09-05

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?

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.

Was this helpful? Report an error