NCERT Solutions Curiosity Chapter 5 Interdisciplinary projects — Discover, design, and debate

How to run it. Cut a few tiny bits of paper and keep them on a dry table. Take two materials at a time, rub them together briskly for about half a minute, then hold each one just above the paper bits without touching them. Note whether the bits jump up. Do not touch the rubbed part with your hand or with any metal, and do the experiment on a dry day — on a damp day the charge leaks away and nothing works.

Record it like this (a sample table, with the kind of result you can expect):

Writing it up as a research paper. Use the sections a real paper uses: Question (which materials get charged on rubbing?), Method (exactly what you did, so someone else can repeat it), Observations (the table), Analysis (which pairs worked and which did not), Conclusion, and Limitations (humidity, how long you rubbed, size of the paper bits).

What your story has to get right. Gravity is the force that keeps everything pressed to the ground and gives every object its weight. Take it away and (a) nothing falls, (b) nothing has weight, though every object still has its mass, (c) everything not tied down drifts, (d) water does not pour and does not stay in a glass, (e) you cannot walk, because walking depends on friction, and friction between your shoes and the floor needs your weight to press them together.

Sample story — The morning gravity took the day off

1. Panel 1: Meera wakes up and finds her bed sheet, her pillow and herself hanging in mid-air near the ceiling fan.
2. Panel 2: She pours tea for her grandmother. The tea leaves the kettle in a wobbling silver ball and floats across the kitchen.
3. Panel 3: She tries to run to school and only spins in place — her shoes cannot press on the floor, so there is no friction to push against.
4. Panel 4: In the classroom the spring balance reads zero for every object. The teacher points out that the pencil box still has the same 200 g of matter in it — only the pull has gone.
5. Panel 5: A cricket ball, once hit, never comes down. It sails away over the village and does not stop.
6. Panel 6: Gravity returns at four o'clock. Everything comes down at once, and Meera decides that a force she used to complain about while cycling uphill is the one holding her whole world together.

The conclusion the discussion should reach: friction is neither simply good nor simply bad. It is a necessity wherever we need a grip, and a problem wherever we want easy motion — and often both at once in the same machine.

How to run the discussion: divide the class into two groups, give each ten minutes to collect examples from home, the road and the playground, then let them argue. Insist that every example names the two surfaces in contact — that is what turns an opinion into a scientific point.

Making and calibrating it. Hang a light spring from a rigid stand, fix a pointer to its lower end and a paper scale behind it. Mark the pointer's position with nothing hanging — that is your zero. Now hang standard masses one at a time (100 g, 200 g, 300 g …), marking the pointer's new position each time. Join the marks and you have a calibrated scale.

The pattern you will find. The marks come out equally spaced — each extra 100 g stretches the spring by the same extra amount. And when you divide weight by mass, the answer is nearly the same for every object.

Building it (Fig. 5.18). Take a glass jar with a plastic or cardboard lid. Push a straw through a hole in the lid. Pass a copper wire down through the straw, wind the top end into a flat spiral, and bend the lower end into a small hook inside the jar. Hang two thin strips of aluminium foil side by side from the hook so that they touch each other and hang freely. Close the lid.

Testing it. Rub a plastic scale with polythene and touch it to the copper spiral. The two foil strips fly apart. Charge has passed down the wire to both strips; both strips receive the same kind of charge, and like charges repel — so the strips push each other away. Touch the spiral with your finger and they fall back together, because the charge escapes through you.