NCERT Solutions Curiosity Chapter 8 Interdisciplinary projects — Exploratory Projects

Book page 119 & 120 Updated on2026-09-05

Q1.

Construct a floating bowl-type water clock. Experiment by using bowls of different sizes and making holes of different sizes in them so that the sinking time of the bowl can be close to 24 minutes.

Answer

What to build: a copy of the ancient Ghatika-yantra — a bowl with a fine hole at the bottom, floated on water in a bucket. Water enters slowly, the bowl fills and finally sinks. The bowl is lifted out and floated again, and each sinking marks one ghati.

Method

Take a large tub or bucket of water and a light metal or plastic bowl (a katori works well).
Make one fine hole exactly at the centre of the bottom with a pin or a thin nail.
Float the bowl gently on the water so that no water enters over the rim. Start a watch at that moment.
Note the time at which the bowl sinks. Record it in a table.
Repeat with a bigger hole, a smaller hole and bowls of different sizes.

Trial	Bowl	Hole made with	Sinking time	What to do next
1	Small katori	Thin nail (large hole)	2 min	Sinks far too fast — use a smaller hole
2	Small katori	Drawing pin	9 min	Still fast — use a bigger bowl
3	Large bowl	Drawing pin	21 min	Very close — try a slightly finer hole
4	Large bowl	Sewing needle	≈ 24 min	Target reached — this is one ghati

Why the sinking time changes the way it does: a bigger bowl needs more water to sink, so it takes longer. A bigger hole lets water in faster, so it sinks sooner. To lengthen the time, make the bowl bigger or the hole smaller — and change only one of the two at a time, so you know which change did what.

Did you know? 24 minutes is exactly 1/60 of a day (24 h = 1440 min; 1440 ÷ 60 = 24). That is why the ancient Indian day had 60 ghatis. Every sinking was announced with drums, conch shells or a gong.

Q2.

Design an activity for measuring the pulse rate (number of times the pulse of a person beats in 1 minute) of your friends. Think of an activity where you can use your pulse to measure time and develop a story over that idea.

Answer

Part 1 — Measuring the pulse rate

Place the first two fingers (not the thumb) of your right hand on the inside of a friend's left wrist, below the base of the thumb, and press very gently until you feel the throbbing.
Count the beats for 30 s with a watch and multiply by 2. Repeat three times and take the average.
Record every friend's reading, first while sitting quietly, then after 20 skips or a short run.

Name	Beats in 30 s (at rest)	Pulse rate (per min)	Pulse rate after exercise
Sample: Meera	39	78	116
Sample: Arun	36	72	124

Part 2 — Using the pulse as a clock. If your pulse rate is 72 per minute, then one beat ≈ 60 ÷ 72 = 0.83 s. Use it to time things where a stopwatch is not allowed: how long a paper plane stays in the air, how many beats a friend takes to run 50 m, how many beats a pendulum needs for 10 oscillations.

Sample story: 'The Boy Who Carried a Clock in His Wrist'. Kabir forgets his watch on the day of the school sports. His friend is about to run the 100 m and there is no one to time her. Kabir remembers this chapter, puts two fingers on his wrist and counts — 10 beats from the whistle to the finish line. Later, at home, he measures his pulse rate as 72 per minute, works out 10 × 0.83 = 8.3 s, and realises his friend has beaten the school record. But he also learns why Galileo's method was replaced: when he was excited at the race his own heart was beating faster, so his 'clock' was running fast.

Why the pulse is a poor clock, scientifically: its rate changes with excitement, exercise, illness and age, so the 'unit' itself keeps changing. A good clock needs a process that repeats at a rate which does not depend on what is happening around it — which is exactly the advantage a pendulum has over a heartbeat.

Q3.

What might be the reasons for the slight differences in the time periods of a pendulum of a given length in different readings taken in Activity 8.2. Think of ways to control those and repeat the activity to check if the difference in readings is reduced.

Answer

Possible reason for the difference	How to control it
Reaction time — our hand starts and stops the watch a little late	Time 20 or 30 oscillations instead of 10 and divide; start the watch at the count 'zero' when the bob passes the mean position, where it moves fastest and the instant is easiest to judge
Miscounting the oscillations	Count only the passes through the mean position in one direction; have a second student count aloud
Push while releasing the bob, or too wide a swing	Hold the bob, let the thread become taut, and release without any push; keep the swing small
Slack or stretching thread, or a slipping knot — the length keeps changing	Use a thin, strong, non-elastic thread; clamp it firmly; measure the length up to the centre of the bob each time
A shaky support, or the bob swinging in a circle instead of a plane	Fix the support to a heavy table or a rigid stand; release the bob in one plane only
Draughts of air from a fan or window	Switch off the fan and close the window while taking readings
Least count of the watch (a wall clock reads only to 1 s)	Use a stopwatch or a mobile phone stopwatch that reads to 0.01 s

Repeat and compare. Take five readings with the old method and five with the controls in place, and put them side by side:

Before: 19.6, 20.4, 19.8, 20.5, 20.1 s for 10 oscillations → time period 1.96 to 2.05 s, spread = 0.09 s
After (30 oscillations, stopwatch, fan off): 60.1, 60.0, 60.2, 60.1, 59.9 s → time period 2.003 to 2.007 s, spread = 0.004 s

Why the spread shrinks: a fixed error of about 0.2 s in starting and stopping is spread over 30 oscillations instead of 10, so its effect on one oscillation becomes three times smaller. Removing draughts and a shaky support takes away the causes that made the pendulum itself behave differently from trial to trial.

Q4.

Visit a playground with a few swings. Measure the time taken by a swing for 10 oscillations and calculate its time period. Repeat it a few times with children of different weights to find out if its time period is almost the same. Repeat this with swings of different lengths. Find out how the time period changes with increasing length of the swings. Is the swing also an example of a pendulum?

Answer

Yes — a swing is a large pendulum. The chains are the thread, the seat with the child on it is the bob, and the frame overhead is the rigid support.

What to do: give the swing one gentle push, wait for the motion to settle, then time 10 oscillations (each oscillation = out and back to the same side) and divide by 10.

Swing	Child on the swing	Time for 10 oscillations	Time period
Long swing (about 2.5 m)	Light child	31.6 s	3.16 s
Long swing (about 2.5 m)	Heavier child	31.7 s	3.17 s
Long swing (about 2.5 m)	Two children together	31.5 s	3.15 s
Medium swing (about 1.6 m)	Light child	25.4 s	2.54 s
Short swing (about 1.0 m)	Light child	20.0 s	2.00 s

Findings

With children of different weights on the same swing, the time period stays almost the same — just as the bob's mass made no difference in Activity 8.2.
With swings of different lengths, the time period clearly changes: the longer the swing, the greater the time period. A tall swing swings lazily; a short one swings quickly.

Why a swing is not a perfect pendulum: the 'bob' is a child, not a small heavy ball, and its centre keeps shifting as the child leans back and forward — which is exactly how children make a swing go higher by themselves. For clean readings, ask the rider to sit still.

Safety tip: stand well clear of the swinging seat, and do the timing from the side, never from the front.

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