Activities 5.3 and 5.4 — Friction — Exploring Forces

1. 14 marksCuriosity Grade 8, Chapter 5, page 67

   A ball rolling on flat ground stops on its own after some time, and a bicycle slows down when you stop pedalling. The chapter says that in these situations 'no force appears to be acting'. Explain how it nevertheless concludes that a force must be at work.

   Hint. Use the rule from section 5.2 in reverse.

   The reasoning, step by step.

   1. The ball's speed changes — it slows down and stops.
   2. From section 5.2: a force is essential to change the speed of an object, and none of these changes take place without the action of a force.
   3. Therefore a force must be acting on the ball, even though nothing visible is pushing or pulling it.
   4. So the chapter asks the right question — not is there a force, but which force is it: Is it possible that some force is indeed acting on them? Which force is that?

   ✦ This is the chapter's method at its clearest. The force is not discovered by seeing it. It is deduced from an effect that could not have happened without it, and only then is it looked for and named. Friction is invisible, which is precisely why it took an argument rather than an observation to establish.

   The clue in the everyday observation. If the road is rough, it stops sooner than on a smoother road. Whatever this force is, it depends on the surface — which already hints that it acts between the ball and the ground, and Activities 5.3 and 5.4 go on to confirm it.

   The same rolling ball appears again in exercise question 1 as the example matched to frictional force, and in exercise question 2(ii).

2. 23 marksCuriosity Grade 8, Chapter 5, page 67, Activity 5.3

   Describe Activity 5.3. Why does it ask you to repeat the push in the opposite direction?

   Hint. The repeat is testing something about the *direction* of the mystery force.

   The activity. Take an object with a flat base — an empty lunch box, a geometry box or a notebook — and place it on a table or the floor. Give it a gentle push and watch (Fig. 5.5). Does it stop after travelling some distance? Is there a force acting on it which brings it to rest? Then push it in the opposite direction and see whether it stops again.

   What you find. It stops both times, after sliding a certain distance.

   ✦ Why the second push matters. If the object stopped only when pushed one way, you might blame something about that particular direction — a slope in the table, a draught from a window. Pushing the other way and getting the same result rules that out, and shows something more specific: the force always opposes whichever way the object is moving. It reverses when the motion reverses.

   That is a property no fixed push could have, and it is written into the definition that follows: Friction always acts in a direction opposite to the direction in which the object is moving or trying to move.

   Why the object needs a flat base. So that it slides on its surface rather than rolling, and so that the two surfaces are properly in contact — which is where the force is going to turn out to come from.

3. 33 marksCuriosity Grade 8, Chapter 5, page 68

   Define friction. State its direction, and explain why it is classed as a contact force.

   Hint. Three separate things: what it is, which way it acts, and which family it belongs to.

   Definition. The force that comes into play when an object moves or tries to move over another surface is called the force of friction or simply friction.

   Direction. Friction always acts in a direction opposite to the direction in which the object is moving or trying to move.

   Why it is a contact force. The force of friction is a contact force since it arises due to two surfaces in contact. Separate the two surfaces and it disappears.

   ✦ The phrase 'or tries to move' is doing real work. Friction is not only for things already sliding. Push a heavy almirah gently and it does not budge — friction is acting even though nothing has moved, because the almirah is trying to move. Leaving those three words out of the definition costs marks and changes the meaning.

   Why the direction rule makes sense. The chapter reached it by experiment in Activity 5.3: push the object one way and it stops; push it the other way and it stops again. A force that resists motion whichever way you go must reverse when the motion reverses.

   Do not classify friction into named types — static, sliding, rolling. Those categories are not in this book.

4. 44 marksCuriosity Grade 8, Chapter 5, page 68, Fig. 5.6

   Why does friction arise? Explain using Fig. 5.6, and say what is surprising about the explanation.

   Hint. Look at what the figure shows about a surface that looks perfectly smooth.

   The explanation. Friction arises due to the irregularities in the two surfaces in contact. Even surfaces which appear smooth, have a large number of minute irregularities (Fig. 5.6). When placed in contact, the irregularities of two surfaces lock into each other and oppose any effort to move one surface over the other.

   What Fig. 5.6 shows. Two surfaces that look flat and smooth to the eye are drawn hugely magnified, and they turn out to be jagged — rows of tiny bumps and hollows. Where the two meet, the bumps of one sit in the hollows of the other. To slide one over the other, all those interlocked irregularities have to be dragged past each other, and that resistance is friction.

   ✦ What is surprising: no surface is actually smooth. A polished table, a sheet of glass, a marble floor — all of them are rough when you look closely enough. 'Smooth' and 'rough' are not two kinds of surface but two ends of a scale, and that is why friction is never zero, only smaller or larger.

   Two consequences the chapter draws immediately:

   • Rougher surfaces have bigger irregularities, so they lock together more firmly and friction is greater — which is what Activity 5.4 measures.
   • Ice and polished floors have very few irregularities, so friction is small there and we slip — which is exercise question 7.
5. 54 marksCuriosity Grade 8, Chapter 5, page 68, Activity 5.4

   Describe Activity 5.4 and state what it establishes. Why must the same object be used throughout?

   Hint. One thing changes between trials; make sure you can say what everything else was.

   The activity. Repeat Activity 5.3, but place the same object on different surfaces in turn — glass, cloth, wood, ceramic tile, and sand. Push it each time and see how far it travels before stopping. Does the object stop at the same distance on all surfaces?

   What you find. No. For different surfaces, the object stops after moving different distances so we can say that the force of friction depends upon the nature of the surfaces in contact. Friction is greater on rough surfaces.

   Reading the distances. A shorter distance means the object was stopped more quickly, so the friction was greater. Expect the shortest run on cloth and sand and the longest on glass or a polished tile.

   ✦ Why the same object matters — and the same push. You are trying to find out what the surface does. If you changed the object as well, or shoved it harder on one surface than another, you would not know which change produced the different distance. The surface is the one thing that is allowed to vary. Give as nearly equal a push as you can each time, and try each surface more than once.

   This activity is the key to exercise question 6, which asks how to make a ball stop before or after a given point when it is always released from the same place. Since you cannot change the release point, the surface is what you change.

6. 63 marksCuriosity Grade 8, Chapter 5, page 68, A step further

   Does friction act only between solid surfaces? What follows for the design of aeroplanes, ships and high-speed trains?

   Hint. Sonali and Ragini already met this force in the opening story without naming it.

   No — liquids and gases exert friction too. Air, water, and other liquids also exert force of friction on the objects moving through them.

   The design consequence. Hence the objects, such as aeroplanes, ships, boats, and high-speed trains are designed with specific shapes to reduce the force of friction due to the air or water around them. Their smooth, tapering shapes let air or water flow past with less resistance, so less of the engine's effort is wasted in overcoming friction.

   ✦ You have already met this force in the chapter's first paragraph. "Oh no! The wind is pushing me hard!" said Ragini. Riding into the wind is hard because the air resists the cyclist's motion through it. A cyclist crouching low over the handlebars, and a racing cyclist's pointed helmet, are the same design idea as an aeroplane's shape.

   Fish and birds arrived at it first. The tapering shape of a fish and the smooth form of a bird in flight do exactly the same job in water and air.

   The chapter does not use the word streamlined here, and does not go into how much friction is reduced. Describe the shapes and their purpose, and stop there.

7. 74 marksCuriosity Grade 8, Chapter 5, pages 66-68

   Is friction useful or a nuisance? Argue both sides, as the chapter's project asks you to do.

   Hint. Try to imagine walking across a room with no friction at all.

   Where friction is essential:

   Situation	Why friction is needed
   Walking	Your foot pushes back on the ground and friction stops it sliding, which is what moves you forward
   Brakes	A bicycle or vehicle is slowed by friction between the brake and the wheel, and between the tyres and the road
   Holding things	Friction between your fingers and a glass, a pen or a cricket ball is what stops it slipping out
   Writing	A pencil leaves a mark because of friction between the lead and the paper
   Tying a knot, driving a nail, striking a match	All depend on surfaces gripping each other

   Where friction is a problem:

   Situation	Why friction is unwanted
   Vehicles and machines	Friction opposes motion, so extra effort and fuel is needed to overcome it
   Moving parts wearing out	Surfaces rubbing together are gradually worn away
   Aeroplanes, ships, trains	Friction from air and water resists them, which is why their shapes are designed to reduce it
   Cycling into the wind or on a rough road	Exactly the opening story — harder pedalling for the same speed

   ✦ The honest conclusion is that the question has no single answer, and that is the point of setting it as a class debate. Friction is not good or bad in itself; it helps or hinders depending on what you are trying to do. The very same friction between tyre and road that a cyclist works against on the flat is what lets that cyclist brake safely at the bottom of the hill.

   A useful way to close the debate: ask not should there be friction? but how much friction do we want here, and in which direction?

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.