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NCERT Solutions Curiosity Chapter 5 Chapter-end exercise, Questions 1–10 — Keep the curiosity alive

Book page 77 Updated on2026-09-05

Q1.
Match items in Column A with the items 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.
Answer
Column AColumn BWhy this pair
(i) Muscular force(b) A child lifting a school bagThe upward pull comes from the child's arm muscles contracting — a contact force
(ii) Magnetic force(e) A compass needle pointing NorthThe magnetised needle is turned by a magnetic force acting on it without contact
(iii) Frictional force(a) A cricket ball stopping on its own just before touching the boundary lineFriction between the ball and the ground acts opposite to the ball's motion and stops it
(iv) Gravitational force(c) A fruit falling from a treeThe Earth attracts the fruit towards itself — a non-contact, always attractive force
(v) Electrostatic force(d) Balloon rubbed on woollen cloth attracting hair strandsRubbing charges the balloon; a charged body attracts uncharged hair across a gap
Tip: sort the five by contact first. Only (i) and (iii) need touching, and only one of the examples involves a person's muscles — that settles two rows immediately.
Q2.
State whether the following statements are 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.
Answer

(i) True. Speed cannot change on its own. Whether an object starts moving, speeds up, slows down or stops, a force must be acting on it — that is the central conclusion of Table 5.1.

(ii) False. Friction acts opposite to the direction in which the ball is rolling, so it decreases the speed and finally brings the ball to rest. It can never increase the speed. (A rolling ball on a rough ground stops sooner than on a smooth one — the very opposite of what this statement claims.)

(iii) False. Electrostatic force is a non-contact force, so it acts across the gap: like charges repel and unlike charges attract. Two rubbed balloons hanging apart push each other away (Activity 5.7) without ever touching.

Q3.
Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why?
Answer

They will move away from each other — they repel.

Both balloons were rubbed with the same woollen cloth, so both have been charged in the same way and have therefore acquired similar (like) charges. Like charges repel each other, so each balloon pushes the other away and the threads holding them slant outwards (Fig. 5.9b).

Why it happens: rubbing does not create charge on one object alone. Charges build up on both the rubbing object and the rubbed object, and they are of opposite kinds. So the two balloons end up alike (and repel), while either balloon and the cloth are unlike (and attract). Bring the woollen cloth near one of the balloons and you will see it swing towards the cloth.
Check it yourself: the effect is a force, so it obeys the rules of a force — it acts on both balloons, and it acts without contact, which is why we call it an electrostatic force, a non-contact force.
Q4.
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.
Answer

Both objects have two forces on them in water — the gravitational force pulling them down and the buoyant force (upthrust) of the water pushing them up. Which one wins decides whether the object sinks or floats.

  • The coin: a coin is small, so it pushes very little water aside, and the upthrust it receives is small. But it is made of metal, so even that small coin is heavy. Here the gravitational force is more than the buoyant force, and the coin sinks to the bottom.
  • The wooden block: the block is much bigger, so as it settles into the water it pushes a large amount of water aside and receives a large upthrust. Wood is light for its size, so the block does not have to sink far before the upthrust has grown equal to its weight. At that point the two forces are equal, and the block floats with part of it above the surface.
Coin gravity > upthrust → sinks Wooden block gravity = upthrust → floats gravitational force buoyant force (upthrust)
The arrows are drawn to scale: on the coin the downward pull is the longer arrow, on the block the two arrows are equal.
Why the size is misleading: it is tempting to say the block floats because it is wood and the coin sinks because it is metal — but the real comparison is between the weight of the object and the upthrust it can gather, and the upthrust depends on how much water the object pushes aside. The bigger block gets a bigger upthrust precisely because it is bigger. This is Archimedes' Principle: the upward force on an immersed object equals the weight of the liquid it displaces.
Q5.
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. (i) During its upward motion (ii) During its downward motion (iii) At its topmost position
Answer

Two forces act on the ball while it is in the air: the gravitational force of the Earth, which is always vertically downwards, and the force of friction due to the air, which always acts opposite to the ball's motion.

Stage of the flightForces acting on the ballDirection of eachWhat it does to the speed
(i) During its upward motionGravitational force; force of friction due to airGravity — downwards; air friction — downwards (opposite to the upward motion)Both oppose the motion, so the speed keeps decreasing
(ii) During its downward motionGravitational force; force of friction due to airGravity — downwards; air friction — upwards (opposite to the downward motion)Gravity is the larger, so the speed keeps increasing
(iii) At its topmost positionGravitational force onlyDownwardsThe ball is momentarily at rest, so there is no motion for air friction to oppose; gravity at once starts it moving down
(i) During upward motion G F both forces downwards (ii) During downward motion G F air friction now acts upwards (iii) At the topmost position G at rest for an instant — only G G = gravitational force (always downwards) F = friction due to air direction of motion
The Earth's pull never changes direction; the friction of the air flips over, because it always opposes whichever way the ball is going.
Why the ball stops at the top: going up, both forces act downwards, so the speed falls steadily until it becomes zero. Gravity does not stop acting at that instant — it is still pulling downwards — so the ball cannot stay there. Its direction of motion changes and it begins to fall, gaining speed all the way down.
Tip: if the question asks only for the force named in this chapter's section on gravity, the gravitational force alone is enough. Mentioning air friction is the fuller answer, since the book has told us that air also exerts a force of friction on objects moving through it.
Q6.
A ball is released from the point P and moves along an inclined plane and then along a horizontal surface as shown in the Fig. 5.16. It comes to stop at the point A on the horizontal surface. Think of a way so that when the ball is released from the same point P, it stops (i) before the point A (ii) after crossing the point A.
Answer

The ball is released from the same point P every time, so it always arrives at the bottom of the incline in the same way. The only thing left that can decide where it stops is the friction between the ball and the horizontal surface — so change that.

(i) To make it stop before the point A — increase the friction. Spread a rough material along the horizontal surface: sand, a piece of cloth, a jute mat or coarse paper. The rougher surface has bigger irregularities, so the force of friction on the ball is larger, its speed falls faster, and it comes to rest short of A.

(ii) To make it stop after crossing the point A — reduce the friction. Make the horizontal surface smoother: polish it, lay a sheet of glass or a smooth ceramic tile along it, or sprinkle a little talcum powder. With smaller irregularities the force of friction is less, the ball loses speed slowly, and it travels past A before stopping.

P (i) A (ii) (i) Sand or cloth on the surface → more friction → the ball stops before A (ii) Polish it or lay a glass sheet → less friction → the ball stops after A
Same starting point P, same ball — only the horizontal surface is changed, and with it the friction.
Careful: do not answer by releasing the ball from a higher point, or by giving it a push. The question fixes the starting point at P, so the answer must lie in the surface, not in the release.
Q7.
Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.
Answer

Because on such surfaces the force of friction between our feet and the ground is very small, and it is friction that normally gives our feet their grip.

Think about what happens in an ordinary step. Your foot presses backwards on the ground; friction, acting opposite to that attempted backward slide, holds the foot in place and lets you push yourself forward. Friction arises from the irregularities of the two surfaces locking into one another — and ice and polished floors have very few and very shallow irregularities. There is almost nothing for the sole of your shoe to lock into. So instead of gripping, your foot slides out from under you and you slip.

Why it happens: friction depends on the nature of the surfaces in contact, and is greater on rough surfaces (Activity 5.4). A smooth surface is simply the extreme case of a low-friction surface. A wet polished floor is worse still, because the film of water keeps the two surfaces apart and reduces the friction further.
Did you know? Friction is often called a nuisance, but this is the situation that shows how badly we need it. We spread sand or ash on an icy path, and shoes are given deep tread patterns, precisely to increase friction where a smooth surface has taken it away.
Q8.
Is any force being applied to an object in a non-uniform motion?
Answer

Yes. An object in non-uniform motion is one whose speed keeps changing — and a force is always required to change the speed of motion of an object.

So if a bus is speeding up, slowing down, or moving faster on one stretch than another, some force is acting on it. Friction is the commonest one: it is the reason a rolling ball or a coasting bicycle keeps losing speed until it stops, without anyone touching it.

Why it must be so: the chapter's central conclusion works both ways. A force can change the speed of an object — and, conversely, nothing changes an object's speed except a force. If the motion is not uniform, some force must be at work, whether or not you can see what is applying it.
Careful: the question is about non-uniform motion. If the object is moving in a curve at a steady speed, its direction is changing instead — and that too needs a force, since a force can change the direction of motion as well.
Q9.
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?
Answer

The cause: weight is the gravitational force with which a body pulls an object towards itself. The Moon's gravitational force is much weaker than the Earth's — about one-sixth as strong — so the Moon pulls the same object with about one-sixth of the force. That smaller pull is the object's smaller weight on the Moon.

No — the mass does not change at all. Mass is the amount of matter in an object. Carrying the object to the Moon does not remove any matter from it, so its mass on the Moon is exactly what it was on the Earth.

On the Earth: mass = 1 kg, weight = 10 N
On the Moon: mass = 1 kg (unchanged), weight = 10 N ÷ 6 ≈ 1.6 N

The book's own table shows exactly this — the mass stays 1 kg on the Earth, the Moon, Mars, Venus and Jupiter, while the weight reads 10 N, 1.6 N, 3.8 N, 9 N and 25.4 N.

Why the two behave differently: mass belongs to the object alone. Weight does not — it is the strength of the pull between the object and the world it is standing on, so it depends on that world as well. Change the world and the weight changes; the object itself is untouched.
Check it yourself: a spring balance taken to the Moon would show one-sixth of its Earth reading in newton, because it measures a force. A beam balance would still show the same result, because it compares the object with known masses and the Moon's weaker pull acts equally on both pans.
Q10.
Three objects 1, 2, and 3 of the same size and shape but made of different materials are placed in the water. They dip to different depths as shown in Fig. 5.17. If the weights of the three objects 1, 2, and 3 are w1, w2, and w3, respectively, then (i) w1 = w2 = w3 (ii) w1 > w2 > w3 (iii) w2 > w3 > w1 (iv) w3 > w1 > w2
Answer

Option (ii): w1 > w2 > w3.

In Fig. 5.17 all three objects are floating, so for each of them the buoyant force of the water is exactly equal to its weight. Reading the figure: object 1 dips the deepest, object 2 dips less, and object 3 dips the least.

Deeper the object dips → more water it pushes aside → greater the upthrust on it
Object floats → upthrust = weight of the object
Therefore: deepest dip → greatest weight
Depth of dip: 1 > 2 > 3, so w1 > w2 > w3
1 2 3 dips deepest w1 largest dips less w2 in between dips least w3 smallest
Same size and shape, different materials: the one that settles deepest displaces the most water, so it must be the heaviest.
Why the other options fail: (i) would need all three to dip to the same depth, which the figure contradicts. (iii) and (iv) both make object 1 — the one sunk deepest — lighter than another, which cannot be, since a floating object sinks in only far enough to gather an upthrust equal to its own weight.
Did you know? This is Archimedes' Principle in use: the upward force on an immersed object equals the weight of the liquid it displaces. The objects are of the same size and shape, so the only thing that can differ is the material — and the heaviest material makes the heaviest object, which must sink in furthest.
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