NCERT Solutions Curiosity Chapter 8 .2 Slow or Fast · 8.3 Speed — In-text Questions

Book page 112 & 1138 Updated on2026-09-05

Q1.

For races covering the same distance, we can tell who was faster by measuring time. But how can we tell that when comparing races for different distances?

Answer

By comparing the distance each runner covers in unit time — that is, by comparing their speeds, not their times.

Time alone is useless when the distances differ. Compare these two runs:

Runner	Distance	Time	Distance in 1 second
Prerna	100 m	12.5 s	100 ÷ 12.5 = 8 m/s
Her friend	400 m	62.5 s	400 ÷ 62.5 = 6.4 m/s

The friend ran for a much longer time and covered a much longer distance, yet Prerna was the faster of the two — because she covered more distance in each second.

Why speed is the fair comparison: dividing distance by time strips away how long the race was. Whatever the length of the race, speed answers the same question — 'how much ground per second?' — so two completely different races can be placed side by side.

Q2.

What do we mean when we say something is moving fast or slow?

Answer

We mean a comparison of the distances moved in a given interval of time. Something is moving fast if it covers more distance in the same time than another object; it is slow if it covers less.

Bus in 1 h → 40 km
Cyclist in 1 h → 15 km
Same time, more distance ⇒ the bus is faster

'Fast' and 'slow' are therefore never absolute words. A cyclist is fast compared with a person walking and slow compared with a bus.

Why the time must be the same: if you compare 40 km in 1 h with 15 km in 20 min, you cannot say anything just by looking — 15 km in 20 min is really 45 km in an hour, so the cyclist would be the faster one. Only when the interval of time is the same does 'more distance' mean 'faster'.

Q3.

All the players begin from the starting line together but after sometime they are not running together (Fig. 8.10). How do you decide who is running faster amongst them?

Answer

The one who is ahead of the others at that instant is running faster. All the runners started together from the same starting line, so they have all been running for the same time. The one who is ahead has therefore covered more distance in the same time.

All three started together. Runner A is ahead, so A has covered the most distance in that time and is the fastest.

Why this works only here: because the starting instant was common to everybody. If the runners had started at different times, being ahead would prove nothing — you would have to work out each runner's speed instead.

Q4.

How can we determine the speed of an object?

Answer

It can be calculated if we know the total distance covered by the object and the time taken to cover it.

Speed = Total distance covered ÷ Total time taken

Worked out for Example 8.1 of the book — Swati's school is 3.6 km from her house and she takes 15 min by bicycle:

Distance = 3.6 km = 3.6 × 1000 m = 3600 m
Time = 15 min = 15 × 60 s = 900 s
Speed = 3600 m ÷ 900 s = 4 m/s

Why we change the units first: distance and time must be in units that match the unit we want for the answer. For an answer in m/s, put the distance in metre and the time in second; for km/h, put the distance in kilometre and the time in hour.

Tip: the same relation, rearranged, answers two more kinds of question — distance = speed × time and time = distance ÷ speed.

Q5.

What would be the unit of speed?

Answer

Since speed is distance ÷ time, its unit is a unit of length divided by a unit of time. The SI unit of speed is metre/second, written m/s.

If distance is in…	…and time is in…	…the unit of speed is	Used for
metre (m)	second (s)	m/s (the SI unit)	Runners, falling objects, physics problems
kilometre (km)	hour (h)	km/h	Buses, trains, speedometers

Converting: 1 m/s = 1 × 3600 m in 1 h = 3600 m/h = 3.6 km/h
So: m/s → km/h : multiply by 3.6 · km/h → m/s : divide by 3.6

Check it yourself: a train at 72 km/h moves at 72 ÷ 3.6 = 20 m/s, and a runner at 8 m/s runs at 8 × 3.6 = 28.8 km/h.

Was this helpful? Report an error