Keep the Curiosity Alive — Chapter Exercises — Exploring Forces

1. 15 marksCuriosity Grade 8, Chapter 5, page 77, exercise 1

   Match the items in Column A with those in Column B.

   Column A (Type of force): (i) Muscular force, (ii) Magnetic force, (iii) Frictional force, (iv) Gravitational force, (v) Electrostatic force

   Column B (Example): (a) A cricket ball stopping on its own just before touching the boundary line, (b) A child lifting a school bag, (c) A fruit falling from a tree, (d) Balloon rubbed on woollen cloth attracting hair strands, (e) A compass needle pointing North

   Hint. For each example, ask which two objects are interacting.

   The matching:

   Column A		Column B	Why
   (i) Muscular force	→	(b) A child lifting a school bag	The force comes from the child's muscles contracting and elongating — a contact force
   (ii) Magnetic force	→	(e) A compass needle pointing North	The needle is a tiny magnet, turned by the Earth's magnetic field without anything touching it
   (iii) Frictional force	→	(a) A cricket ball stopping on its own before the boundary	Friction between the ball and the ground acts opposite to the motion and brings it to rest
   (iv) Gravitational force	→	(c) A fruit falling from a tree	The Earth attracts the fruit towards itself
   (v) Electrostatic force	→	(d) Balloon rubbed on woollen cloth attracting hair strands	Rubbing charges the balloon, and a charged object attracts uncharged objects

   Answer in short: (i)–(b), (ii)–(e), (iii)–(a), (iv)–(c), (v)–(d).

   ✦ The phrase on its own in (a) is the clue that makes it friction. Nobody stops the ball — yet it stops, so by the chapter's rule a force must be acting even though nothing visible is doing it. That is precisely how the chapter introduced friction on page 67.

   And (e) is worth a second look. A compass needle is not being attracted by a magnet you can see; the second object is the Earth, which behaves like a giant magnet — a fact from Chapter 4 of this book. Two of the five examples have the Earth as one of the interacting objects, in two completely different roles: as a magnet in (e), and as a gravitating body in (c).

2. 23 marksCuriosity Grade 8, Chapter 5, page 78, exercise 2

   State whether True or False.

   (i) A force is always required to change the speed of motion of an object.

   (ii) Due to friction, the speed of the ball rolling on a flat ground increases.

   (iii) There is no force between two charged objects placed at a small distance apart.

   Hint. Each is settled by one sentence of the chapter.

   (i) True. The chapter states it twice. A force is essential to change the speed of an object, and when the characters ask whether every change of speed, direction or shape means a force is acting, the answer given is Yes, none of these take place without the action of force. This is the rule that lets you infer friction from a ball that slows down by itself.

   (ii) False. Friction does the opposite. Friction always acts in a direction opposite to the direction in which the object is moving, so it decreases the speed of a rolling ball until it stops. The whole discussion on page 67 begins from exactly this observation — a ball rolling on flat ground stops on its own.

   (iii) False. Electrostatic force is a non-contact force: This force comes into play even when the objects are not in contact. Two charged objects a small distance apart exert a force on each other — repelling if their charges are alike, attracting if unlike. Activity 5.7 shows it directly, with two hanging balloons that never touch.

   ✦ Notice the shape of this question. Statement (i) is the chapter's rule stated correctly. Statements (ii) and (iii) each take a correct idea and reverse it — friction increasing speed instead of decreasing it, and force absent across a gap instead of present. When a True/False statement mentions a direction or a negative, check that part first.

3. 34 marksCuriosity Grade 8, Chapter 5, page 78, exercise 3

   Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why?

   Hint. Both were rubbed the same way, with the same cloth.

   What happens: they move apart — they repel each other.

   Why, step by step:

   1. Rubbing charges an object. When two objects of certain materials are rubbed together, electrical charges build up on their surfaces.
   2. Both balloons were charged in the same way, with the same woollen cloth, so they have acquired similar charges.
   3. Similar (like) charges repel each other.
   4. So the two balloons push each other apart, without ever touching — the electrostatic force is a non-contact force.

   This is exactly Activity 5.7: We observe that the balloons move away from each other as if they are repelling each other.

   ✦ The contrast that completes the answer. Bring the woollen cloth near one of the rubbed balloons and they attract. The cloth was the rubbing object, and both the rubbing object and the rubbed object get charged but they acquire opposite kind of charges — so the cloth carries the opposite charge, and opposite kind (unlike) of charges attract each other.

   Mentioning this second case makes the answer much stronger, because it shows the repulsion is not something balloons simply do — it depends on what charge the other object carries.

   Practical note: do not touch the rubbed balloons with your fingers, or the charge leaks away and nothing happens.

4. 44 marksCuriosity Grade 8, Chapter 5, page 78, exercise 4

   When you drop a coin in a glass of water, it sinks, but when you place a bigger wooden block in water, it floats. Explain.

   Hint. The word 'bigger' is in the question to mislead you. Compare each object with the water it pushes aside.

   The two forces on anything placed in water. When an object is placed in a liquid, the gravitational force due to the Earth acts on it downwards. But a buoyant force is applied on it by the liquid in the upward direction. Which one wins decides everything.

   	Coin	Wooden block
   Water pushed aside	Only as much as the small coin's own volume	A larger volume, but the block is light for its size
   Weight of that water	Less than the coin's weight	Equal to the block's weight, once it has sunk in far enough
   Comparison of forces	Gravitational force more than buoyant force	The two forces are equal
   Result	Sinks	Floats

   In the chapter's words: If the gravitational force is more than the buoyant force, the object sinks, but if the two forces are equal, the object floats. Archimedes' Principle adds that the upward force is equal to the weight of the liquid it displaces.

   ✦ Why 'bigger' is a trap. The block being larger is the reason many students give for it floating, and it is the wrong reason — a large stone sinks and a small matchstick floats. Size by itself decides nothing. What matters is whether the object can push aside water weighing as much as itself, and a wooden block can while a coin cannot.

   The question is deliberately worded to make you compare the two objects with each other. Do not. Compare each object with the water it displaces — that is the only comparison the rule is about, and once you make it, both cases fall out at once.

5. 55 marksCuriosity Grade 8, Chapter 5, page 78, exercise 5

   If a ball is thrown upwards, it slows down, stops momentarily, and then falls back to the ground. Name the forces acting on the ball and specify their directions during (i) its upward motion, (ii) its downward motion, (iii) at its topmost position.

   Hint. One force acts throughout, and it never changes direction.

   The answer expected at this level: the gravitational force, acting vertically downwards, at all three stages.

   Stage	Force	Direction	What it produces
   (i) Going up	Gravitational force (the ball's weight)	Downwards, towards the Earth	Opposes the motion, so the speed of the object goes on decreasing
   (ii) Coming down	Gravitational force	Downwards	Now acts along the motion, so the speed goes on increasing
   (iii) At the top	Gravitational force	Downwards	The ball is momentarily at rest, and this force is what brings it back down

   ✦ Stage (iii) is where the marks are lost. The commonest wrong answer is 'no force acts at the topmost point, because the ball is at rest.' If that were true the ball would simply stay up there. The ball stops; the force does not. Indeed it is precisely because the force is still acting that the ball cannot remain at the top for more than an instant.

   One force, three different-looking effects. Notice that a single steady downward pull slows the ball, then changes its direction of motion, then speeds it up — three of the four effects listed in section 5.2, with nothing about the force changing at any point.

   An honest addition, if you want to go further. The chapter's A step further box on page 68 says that air, water, and other liquids also exert force of friction on the objects moving through them. So strictly there is also a small frictional force from the air, always opposing the motion — acting downwards while the ball rises and upwards while it falls, and effectively absent at the top where the ball is momentarily still. Gravity is the answer the question wants; air friction is a real second force you may mention if you name it correctly.

6. 65 marksCuriosity Grade 8, Chapter 5, page 78, exercise 6

   A ball released from point P moves down an inclined plane and then along a horizontal surface, stopping at point A (Fig. 5.16). Suggest a way to make the ball, released from the same point P, stop (i) before point A, (ii) after crossing point A.

   Hint. You are not allowed to change where it starts. What else is there to change?

   What you may not change. The ball is released from the same point P every time, so you cannot give it a harder start or release it from higher up. That leaves the surface — and Activity 5.4 showed exactly what changing the surface does.

   (i) To make it stop before A — increase the friction.

   Cover the horizontal part with something rough: a piece of cloth, a mat, or a sprinkling of sand. Friction depends upon the nature of the surfaces in contact, and friction is greater on rough surfaces. A greater opposing force stops the ball sooner, so it comes to rest short of A.

   (ii) To make it cross A — reduce the friction.

   Make the horizontal part smoother: lay down a sheet of glass or a polished ceramic tile. With fewer irregularities to lock into each other, friction is smaller, the ball is slowed less, and it travels further before stopping — past A.

   ✦ Why the answer must be about the surface and not about the ball. Everything else is fixed by the question. The same ball, released from the same point, arrives at the bottom of the slope in the same state each time. The only thing left that can change how far it then travels is what it is travelling on.

   Activity 5.4 is the evidence, not a guess. There the same object was pushed across glass, cloth, wood, ceramic tile, and sand and stopped after different distances every time. This exercise simply asks you to use that result in reverse: choose the surface to get the distance you want.

   A shorter distance means more friction, a longer distance less. Getting that relationship the right way round is the whole question.

7. 73 marksCuriosity Grade 8, Chapter 5, page 78, exercise 7

   Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.

   Hint. Start from why we do *not* slip on ordinary ground.

   Because there is very little friction between our feet and such surfaces.

   Why walking needs friction at all. When you walk, your foot pushes backwards against the ground. Friction is what stops the foot from simply sliding backwards, and the result is that you are pushed forward instead. Without friction the push achieves nothing except sliding your foot out from under you.

   Why ice and polished floors are different. Friction arises due to the irregularities in the two surfaces in contact ... the irregularities of two surfaces lock into each other and oppose any effort to move one surface over the other. A very smooth surface has far fewer irregularities to lock into, so friction is greater on rough surfaces and correspondingly small on ice or polished stone. With too little friction the foot slides, and you lose your balance.

   ✦ The same reason a wet floor is dangerous, which is the second Probe-and-ponder question of this chapter. A film of water gets between the two surfaces and keeps the irregularities from meeting properly, so a floor that grips perfectly well when dry becomes slippery when wet.

   And it is why the remedies work. Rough-soled shoes, a mat at the doorway, sand or ash spread on an icy path — every one of them puts back the irregularities that were missing.

   This is friction seen as a necessity rather than a nuisance, which is the debate the chapter's third project asks you to hold.

8. 83 marksCuriosity Grade 8, Chapter 5, page 78, exercise 8

   Is any force being applied to an object in a non-uniform motion?

   Hint. What does 'non-uniform' tell you about the object's speed?

   Yes, a force must be acting on it.

   The reasoning.

   1. In non-uniform motion the object's speed is not constant — it covers unequal distances in equal intervals of time, so it is speeding up or slowing down. (You met uniform and non-uniform motion in Measurement of Time and Motion, Curiosity Grade 7.)
   2. The chapter's rule: a force is essential to change the speed of an object, and none of these take place without the action of force.
   3. A changing speed therefore means a force is acting.

   ✦ You are not being asked to identify the force — only to conclude that there is one. That is what makes this a good question. A bicycle freewheeling to a stop, a ball rolling to rest, a fruit falling faster and faster — in each case the force is different (friction, friction, gravity), but the conclusion that some force is acting follows from the changing speed alone.

   The chapter used this exact reasoning to discover friction on page 67: a rolling ball slows down, no force appears to be acting, and yet one must be, because otherwise the speed could not have changed.

   A change of direction at constant speed also counts as a force acting, since change the direction of motion of an object is on the same list.

9. 94 marksCuriosity Grade 8, Chapter 5, page 78, exercise 9

   The weight of an object on the Moon becomes one-sixth of its weight on the Earth. What causes this change? Does the mass of the object also become one-sixth of its mass on the Earth?

   Hint. Two questions with opposite answers — and the chapter's table lets you check the first.

   What causes the change in weight. Weight is the gravitational force with which the Earth (or another planet) pulls an object. The Moon pulls a given object much less strongly than the Earth does, so the same object weighs less there. Nothing about the object has changed — the body pulling it has.

   Does the mass change? No. Mass is the amount of matter in an object and is measured in grams (g) or kilograms (kg). Its value remains the same at every place. The object contains exactly as much matter on the Moon as on the Earth. The chapter's summary line: weight can change, but mass does not.

   Check it against the chapter's own table (page 75):

   	Earth	Moon
   Mass of the object	1 kg	1 kg — unchanged
   Weight of the object	10 N	1.6 N

   And 10 ÷ 6 ≈ 1.7 N, which agrees with the table's 1.6 N. So the one-sixth figure in the question is consistent with the table rather than a separate claim.

   ✦ The heart of the answer is that weight is a force and mass is not. Weight depends on two things — the object and whatever is pulling it. Mass depends on the object alone. Change the planet and only the first can change.

   A practical consequence. A spring balance measures a force, so it would read differently on the Moon. A beam balance compares the object against a known mass, and both would be affected equally, so it would give the same answer — but that goes beyond what the chapter states.

10. 104 marksCuriosity Grade 8, Chapter 5, page 79, exercise 10

    Three objects 1, 2 and 3 of the same size and shape but made of different materials are placed in water. They dip to different depths as shown in Fig. 5.17 — object 1 sinks in deepest, object 3 least. If their weights are w₁, w₂ and w₃, then:

    (i) w₁ = w₂ = w₃ (ii) w₁ > w₂ > w₃ (iii) w₂ > w₃ > w₁ (iv) w₃ > w₁ > w₂

    Hint. All three are floating. What must be true of the forces on a floating object?

    Answer: (ii) w₁ > w₂ > w₃.

    The reasoning, in three steps.

    1. All three are floating, each resting at its own depth. For a floating object the two forces balance: if the two forces are equal, the object floats. So for each one, buoyant force = weight.
    2. The deeper an object sits, the more water it pushes aside. By Archimedes' Principle the upward force is equal to the weight of the liquid it displaces, so more water displaced means a larger buoyant force.
    3. Putting the two together: a deeper-floating object has a larger buoyant force, and therefore a larger weight.

    Reading the figure, object 1 is submerged the most, object 2 less, and object 3 least of all. So w₁ > w₂ > w₃.

    Object	How deep it dips	Water displaced	Buoyant force	Weight
    1	Most	Most	Largest	Largest
    2	Middle	Middle	Middle	Middle
    3	Least	Least	Smallest	Smallest

    ✦ Why the question says 'the same size and shape'. So that depth is a fair measure of how much water is displaced. Objects of different shapes could sit at the same depth and displace quite different amounts, and the comparison would fall apart. With shape and size fixed, the only thing that differs is the material, and so the weight.

    Option (i) is the tempting wrong answer. Same size and shape sounds as though it should mean the same weight — but they are made of different materials, which is exactly why their weights differ, and why they float at different depths.

    Consistent with exercise 4: it is not size that decides how an object floats.

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.