Discover, Design and Debate — Projects — Exploring Forces

1. 15 marksCuriosity Grade 8, Chapter 5, page 79, project 1

   Collect objects of different materials — plastic, wool, silk, rubber, polythene sheet, paper, metals. Rub one material with another and check whether it then attracts small pieces of paper, that is, whether it gets charged. Record your observations systematically and write a research paper.

   Hint. Charging takes a *pair* of materials, so your record has to be a table of pairs, not a list of materials.

   Why this has to be recorded as a table of pairs. The chapter says charges build up when two objects of certain materials are rubbed together — so 'plastic gets charged' is not a result. Plastic rubbed with what? Both objects get charged, and which pairs work is exactly what you are finding out.

   Build a grid with your materials down the side and across the top, and fill in each cell:

   Rubbed with →	Wool	Silk	Polythene	Paper
   Plastic scale
   Rubber
   Metal rod

   In each cell record whether the paper pieces were attracted, and roughly how strongly — 'many pieces lifted', 'a few', 'none'.

   ✦ Method notes that decide whether the experiment works at all.

   • Rub vigorously, as Activity 5.6 says. A gentle rub often produces nothing.
   • Do not touch the rubbed part with your hand or with metal, or the charge escapes and the test silently fails.
   • Use the same size of paper pieces and the same rubbing time each trial, so the cells can be compared.
   • Repeat each pair, because a single failure may mean a bad rub rather than a real result.
   • A dry day works far better than a humid one. Note the weather in your report — it may explain differences between your results and a classmate's.

   Report the failures. The pairs that produced nothing are as much a result as the pairs that worked, and it is the failures that show charging is a property of the pair, not of a single material. A paper that lists only successes has thrown away half its data.

   If your metal rod never seems to charge however you rub it, say so and describe how you held it. Explaining why is beyond this chapter, but the observation is worth recording accurately.

2. 24 marksCuriosity Grade 8, Chapter 5, page 79, project 2

   Imagine a scenario where gravity disappears. Develop a story and present it as a cartoon strip.

   Hint. Work out the consequences from the chapter first, then build the story on them.

   Work out the physics before you draw anything. A story is only good if the consequences follow from the chapter, so start with a list:

   What gravity does now	What happens without it
   Pulls everything towards the Earth	Nothing falls — dropped objects, spilt water, dust all stay where they are
   Holds you on the ground	A single step sends you drifting upwards
   Gives objects weight	Every spring balance reads zero, though mass is unchanged
   Makes a thrown ball come back	It keeps going, and never returns
   Keeps water in a glass, a river in its bed, the sea in its basin	Water forms drifting blobs
   Makes the swing at the start of this chapter swing at all	It hangs still — or drifts

   Story ideas that use these rather than decorate them: a cricket match where a well-struck ball simply never comes down; a kitchen at breakfast; a class trying to hold a running race; a village well from which the water rises out on its own.

   ✦ The most interesting things to get right are the ones people forget.

   • Mass does not disappear. A cupboard is weightless but still hard to stop once moving. That is a much better story beat than everything simply floating.
   • Friction still exists. It is a contact force and has nothing to do with gravity — though if nothing is pressed against the ground any more, there is far less of it in practice.
   • The other forces still work. Magnets still attract, charged balloons still repel. A character could pull themselves along a corridor using a magnet and an iron rail.

   A note on honesty in fiction. This is deliberately impossible — the chapter says gravitational force is always attractive and gives no way of switching it off. Enjoy the invention, but keep the consequences faithful to the physics, which is the point of the exercise. Label it as a story, not as a prediction.

3. 34 marksCuriosity Grade 8, Chapter 5, page 79, project 3

   Organise a class discussion on the topic: 'Friction — a necessity or a problem?' Make a note of the discussion and state where friction is a necessity and when it is a problem.

   Hint. A good debate needs both sides argued properly, and a conclusion that does not simply pick one.

   Motion for the house: friction is a necessity.

   • Walking is impossible without it — the foot would slide backwards instead of pushing you forward. Exercise 7 is this argument in miniature.
   • Brakes work by friction. A bicycle with frictionless brakes cannot be stopped.
   • Holding anything — a pen, a glass, a cricket ball — depends on friction between your fingers and the object.
   • Writing leaves a mark only because of friction between the pencil and the paper.
   • Knots, nails, matchsticks, a ladder against a wall all need surfaces to grip.

   Motion against: friction is a problem.

   • It opposes motion, so vehicles and machines need extra effort and fuel.
   • Rubbing surfaces wear out and have to be replaced.
   • Air and water friction resist aeroplanes, ships and high-speed trains, which is why they are designed with specific shapes to reduce it.
   • Riding into the wind or on a rough road is harder — the chapter's opening story.

   ✦ How to close the discussion honestly. Do not let it end with a vote for one side. Friction is neither good nor bad in itself; it helps or hinders depending on what you are trying to do, and often both at once in the same machine. The friction that slows a cyclist on the flat is the friction that lets the same cyclist brake safely at the bottom of the hill.

   The better question to end on is not should there be friction? but how much do we want, and where? — which is exactly what engineers decide when they polish one surface and roughen another on the same vehicle.

   What to write down. Keep a two-column note during the discussion, and add a third column for examples raised that belong in both columns. Those are the ones worth reporting.

4. 45 marksCuriosity Grade 8, Chapter 5, page 79, project 4

   Make your own spring balance with your teacher's help and calibrate it using standard weights. Measure the weights of different objects and calculate the ratio of weight to mass for each. Do you observe a pattern?

   Hint. The last part is the real question — and the chapter's own table already contains the answer.

   Making it. A spring hung from a fixed support, a hook or pan at the lower end, and a pointer moving against a card. That is the design given in the chapter's A step further box: a spring fixed at one end, with a hook attached at the other end.

   Calibrating it. Mark the pointer's position with nothing hanging — that is your zero. Hang a standard weight and mark the new position; repeat with larger standard weights, marking each. Then work out the smallest division exactly as Activity 5.11 does: find the weight between two big marks, count the small divisions between them, and divide.

   Measuring and tabulating.

   Object	Mass m (kg)	Weight w (N)	Ratio w/m (N/kg)
   Pencil box
   Stone
   Water bottle

   ✦ The pattern you should find — and where to check it. The ratio comes out roughly the same for every object, whatever it is made of. The chapter's own planet table already predicts this: on Earth a 1 kg object weighs 10 N, so the ratio is about 10 N/kg, and Fig. 5.13's spring balance is marked accordingly, with 1000 g lined up against 10 N.

   That single fact is why a spring balance can carry a mass scale at all. If the ratio differed from object to object, no gram scale could be printed beside the newton scale.

   Report your ratios honestly, including the scatter. A home-made spring balance is a rough instrument, and values from 8 to 12 N/kg would be a perfectly good result. Rounding them all to 10 to make the pattern look neat would be dishonest, and would hide the more useful lesson — that the pattern is visible despite the scatter, which is what makes it convincing.

   What the chapter does not ask you to do: give this ratio a symbol or call it anything. The quantity has a name and a standard value, but neither is in this book.

5. 54 marksCuriosity Grade 8, Chapter 5, page 79, project 5

   An electroscope is a device that can determine whether an object is electrically charged. Make your own electroscope (Fig. 5.18) with your teacher's help, test it, and explore what else you could use it for.

   Hint. Look at the parts in Fig. 5.18 and work out which one actually tells you the answer.

   The parts, as labelled in Fig. 5.18: a jar with a lid; a straw pushed through the lid; a copper wire, coiled at the top and running down through the straw; and a strip of aluminium foil hanging from the wire inside the jar.

   Which part gives you the answer. The aluminium foil. Bring a charged object near the coil at the top, and the foil moves — that movement is the reading. Everything else exists to make that movement possible and visible: the wire carries the effect down, the straw holds the wire without letting the charge run away into the lid, and the jar keeps draughts from moving the foil and fooling you.

   Testing it. Charge a plastic scale by rubbing it with polythene, as in Activity 5.6, and bring it near the top coil. The foil should move. Take the scale away and the foil should settle back. Then try an object you have not rubbed — the foil should stay still, and that negative result is what shows the device is responding to charge rather than to your hand approaching.

   ✦ What else you could use it for. The chapter asks you to explore, so treat these as questions to test, not answers to report:

   • Which of the material pairs from project 1 charge best? The electroscope is a more sensitive detector than paper pieces, and may pick up charging that the paper test missed entirely.
   • How long does a charge last on a rubbed object? Test it every minute and see.
   • Does humidity matter? Try the same object on a dry day and a damp one.
   • Does rubbing longer or harder move the foil further?

   Why the jar matters more than it looks. Without it, a passing draught moves the foil and you record a charge that was never there. Enclosing the moving part is what turns a curiosity into an instrument, and it is worth saying so in your report.

   The chapter does not explain how an electroscope works, and you are not expected to. Build it, test it carefully, and describe what it does.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity, Textbook of Science for Grade 8, Chapter 5 'Exploring Forces', book pages 62-79 (hecu105.pdf, 18 pages, Reprint 2026-27), downloaded from ncert.nic.in and read page by page. Every activity number, figure number, quantity and quoted sentence below was checked against that PDF. Fig. 5.17 (exercise 10) was MEASURED, not eyeballed: the page was rendered at 1200 dpi and the three cylinders and their waterlines located by colour segmentation. Taking the waterline at the ellipse's mid-height gives submerged fractions of about 82%, 59% and 32% for objects 1, 2 and 3; taking it at the ellipse's top edge gives 69%, 46% and 19%. Both methods give the same strict ordering 1 > 2 > 3, so object 1 displaces the most water, has the largest buoyant force and therefore the largest weight - answer (ii), w1 > w2 > w3. Fig. 5.13's scale was also read directly: NEWTONS 0 to 10 N alongside GRAMS 0 to 1000 g, so 1000 g lines up with 10 N, consistent with the planet table on page 75. Three deliberate restraints on what is claimed. (1) The chapter is entirely qualitative and contains no formula. Nothing here uses F = ma, W = mg, F = Gm1m2/r^2, a value of g, or Newton's laws of motion - none of which is in this book. Balanced forces are named once and explicitly deferred by the chapter to higher grades, and this file defers them too. (2) The swing question in Probe and ponder is answered by distinguishing weight from the seat's upward push, with an explicit note that the chapter does not explain it and that the full account needs later ideas. (3) Buoyancy is explained purely by comparing two forces, as the chapter does, because density is not defined until a later chapter of the same book; the chapter's own phrase 'less dense than water' is quoted only where the book itself uses it.. Questions are referenced from the NCERT textbook for identification.