Where Mindful Learning Begins
Where Mindful Learning Begins
Class 8 Science Curiosity NCERT Solutions Chapter 5 — every Probe and Ponder prompt, all 13 Activities, all 10 "Keep the Curiosity Alive" exercise questions, and all 5 "Discover, Design, and Debate" project prompts, solved and explained on one page.
This chapter builds up the idea of force from everyday pushes and pulls, through contact forces like friction, to non-contact forces like magnetism, static electricity, and gravity — and finally to weight, mass, spring balances, and why some objects float while others sink.
Chapter 5 follows two cyclists riding against the wind, up a rough hill, and back down a slope where "something seems to be pulling them downhill" — using this to build the concept of force from the ground up. It starts with the simple idea that a push or pull is a force, shows that force can start motion, change speed, change direction, or change shape, and then divides forces into two families: contact forces (muscular force, friction) that need physical touch, and non-contact forces (magnetic, electrostatic, gravitational) that act from a distance. It ends with weight, mass, spring balances, and why some objects float while others sink. Every Activity and exercise is solved here exactly as the textbook presents it.
A push or pull from one object's interaction with another — measured in newtons (N).
Muscular force and friction need touch; magnetic, electrostatic, and gravitational forces act from a distance.
Weight is gravity's pull, measured with a spring balance; buoyant force explains why objects float or sink.
What a force can do
A force is a push or pull on an object resulting from its interaction with another object. SI unit: newton (N). A force applied on an object may:
If an object is at rest, it doesn't necessarily mean no force is acting on it — it may mean the forces acting on it are balancing each other out (balanced forces, covered in higher grades). Also, whenever you push an object, you feel a reaction force back on your hand — the moment the interaction stops, that force disappears too.
Contact forces vs non-contact forces
| Type | Force | Description |
|---|---|---|
| Contact forces (need physical touch) | Muscular force | Caused by the action of our muscles — walking, lifting, pushing |
| Friction | Opposes motion between two surfaces in contact; depends on the nature of the surfaces | |
| Non-contact forces (act from a distance) | Magnetic force | Exerted by a magnet on another magnet or magnetic material; attractive or repulsive |
| Electrostatic force | Exerted by a charged body on another charged/uncharged body; attractive or repulsive | |
| Gravitational force | The Earth's pull on objects towards itself; always attractive |
Weight vs mass
| Mass | Weight | |
|---|---|---|
| What it measures | Amount of matter in an object | The gravitational force pulling the object down |
| SI unit | Kilogram (kg) / gram (g) | Newton (N) |
| Changes with location? | No — stays the same everywhere | Yes — varies with gravitational pull |
Weight of a 1 kg object on different worlds:
| Planet/Moon | Earth | Moon | Mars | Venus | Jupiter |
|---|---|---|---|---|---|
| Weight of 1 kg mass | 10 N | 1.6 N | 3.8 N | 9 N | 25.4 N |
Floating and sinking
A liquid pushes objects placed in it upward with a buoyant force (upthrust). If an object's weight is greater than the buoyant force acting on it, it sinks; if the two are equal, it floats.
Sample answer: pedalling uphill feels harder than on flat ground because, in addition to overcoming friction and air resistance, we must also work against a component of gravity pulling us back down the slope — the steeper the hill, the greater this effect. It's easier to slip on a wet surface because water reduces the friction between our shoes and the ground, since it fills in the tiny surface irregularities that would otherwise interlock and provide grip — with less friction, there's less force resisting sliding motion.
We feel "light" or like we're "floating" just after a swing reaches its highest point because, at that instant, our upward speed has slowed to zero and we're about to start accelerating downward under gravity — our body briefly experiences a sensation similar to what astronauts feel in free-fall, since for a moment we are momentarily unsupported and beginning to fall rather than being pushed up against the seat.
This is an open reflection prompt meant to set up the chapter's central ideas — friction, gravity, and how forces affect motion — all explained in full through the Activities below.
Answer: the box can be moved by pushing it, pulling it with a rope, lifting it up, or carrying it.
Common feature: each of these ways involves applying a push or a pull to the box. A push or pull applied on an object is called a force.
| Action | Push/Pull | Effect |
|---|---|---|
| Kicking a football at rest | Push | Makes the stationary ball start moving |
| Stretching a rubber band | Pull | Change in shape (length) of the rubber band |
| Applying brakes on a moving bicycle | Push (on the brake lever) | Decreases the bicycle's speed |
Conclusion: a force can make an object start moving from rest, change its speed, change its direction of motion, change its shape, or cause a combination of these effects.
Note: an object at rest doesn't necessarily mean no force is acting on it — the forces acting on it may simply be balancing each other out (balanced forces, studied in later grades).
Observation: the object slides for some distance and then stops on its own. It stops after travelling some distance in the opposite direction too.
Answer: yes. A force acts on the object between its surface and the table's surface, in the direction opposite to its motion, bringing it to rest. This is the force of friction — a contact force, since it arises only when the two surfaces are touching.
Observation: no. The object travels the farthest on glass and stops soonest on sand — the stopping distance is different for every surface.
Conclusion: the force of friction depends on the nature of the surfaces in contact. Friction is greater on rough surfaces and smaller on smooth surfaces.
Reason: all surfaces, even ones that look smooth, have microscopic irregularities. When two surfaces touch, these irregularities lock into each other and oppose motion — rougher surfaces have larger irregularities, so they produce more friction.
Going further: friction also acts on objects moving through air and water. This is why aeroplanes, ships, and high-speed trains are given streamlined shapes — to reduce this friction (drag).
Observation: with like poles facing each other, the second magnet floats above the first without touching it, and pushing it down produces a force that pushes it back up. With the poles reversed, the second magnet no longer floats — it is pulled down onto the first.
Conclusion: a magnet can exert a force on another magnet without contact. Like poles repel; unlike poles attract. This is called magnetic force, and since it acts without contact, it is a non-contact force.
Observation: the paper pieces get pulled towards the rubbed scale or straw and stick to it, without it touching them.
Reason: rubbing two certain materials together builds up electrical charges on their surfaces, called static charges. An object carrying static charge is a charged object, and it attracts nearby uncharged objects like paper — a force that acts without contact.
Observation 1: the two rubbed balloons move away from each other — they repel.
Observation 2: the rubbed balloon and the woollen cloth move towards each other — they attract.
Inference: both balloons carry the same (like) type of charge, since both were rubbed the same way — like charges repel. The balloon and the cloth carry opposite (unlike) charges, since they attract — unlike charges attract. The two kinds of static charge are called positive and negative. The force between charged bodies is called electrostatic force, a non-contact force.
Going further: moving charges form an electric current in a circuit — the same current responsible for the heating and magnetic effects covered in the previous chapter.
Observation: yes. Whether thrown gently or hard, the ball always falls back to the ground. Any object thrown up in any direction eventually comes back down.
Reason: the Earth attracts objects towards itself. This is called gravitational force, or gravity — a non-contact force, since it acts without touching the object. Unlike magnetic or electrostatic force, gravity is always attractive, never repulsive.
Observation: no. The spring stretches by a different amount for each object — more for heavier objects, less for lighter ones.
Reason: the spring stretches due to the gravitational force pulling the hanging object down. Since the Earth pulls different objects with different amounts of force, different objects have different weights — which is exactly the principle a spring balance uses to measure weight.
Answer: the maximum weight this spring balance can measure is 10 N. This means the scale has a range of 0 to 10 N — trying to measure anything heavier than this could damage the spring balance.
Weight difference between two bigger marks (e.g., between 0 and 1 N, or between 1 N and 2 N): 1 N.
Number of smaller divisions between these two bigger marks: 5 divisions.
Smallest value one small division can read: 1 N ÷ 5 = 0.2 N. So the smallest value this particular spring balance can measure is 0.2 N.
Important takeaway: different spring balances may have different ranges and different smallest-division values, so it's always necessary to examine any spring balance (or measuring instrument) carefully before using it, following this same method to work out its precision.
Guidance: the exact weight recorded for each object (like a pencil box or a partially filled water bottle) will depend on the actual objects used and the spring balance's own readings, since these are measured directly rather than calculated. A model way to record this:
| Object | Weight (N) |
|---|---|
| Pencil Box | e.g., 1.4 N (record your own reading) |
| Partially filled water bottle | e.g., 3.0 N (record your own reading) |
Important safety note: always check that an object isn't heavier than the spring balance's maximum measurable weight before hanging it, since exceeding this range can permanently damage the spring.
Observation: yes. Pushing the bottle in requires overcoming a noticeable upward push, and releasing it makes it bounce back to the surface.
Reason: water (and every liquid) exerts an upward force on objects placed in it, called upthrust or buoyant force. Gravity pulls the object down while buoyant force pushes it up; if gravity is stronger, the object sinks, and if the two forces are equal, it floats.
Going further — Archimedes' Principle: an object immersed in a liquid experiences an upward force equal to the weight of the liquid it displaces. If this is less than the object's own weight, it sinks; if equal, it floats.
Muscular force isn't just for walking or lifting — it plays a role inside our bodies too. It helps us chew food and push it through the alimentary canal during digestion, and the continuous expansion and contraction of our heart muscles is what circulates blood throughout our body, a process essential for survival.
Most rocks sink, but pumice, formed during volcanic eruptions, can float on water. When lava rich in gas and water vapour cools very quickly, it traps tiny bubbles of gas inside, creating a light, porous rock full of air pockets — making it less dense than water, so it floats.
Archimedes, a famous Greek scientist, discovered that an object immersed in a liquid experiences an upward force equal to the weight of liquid it displaces — known today as Archimedes' Principle. It explains not just floating and sinking, but also why a mug feels lighter while still under water when drawing water from a bucket.
| Column A (Type of force) | Matches with (Column B) |
|---|---|
| (i) Muscular force | (b) A child lifting a school bag |
| (ii) Magnetic force | (e) A compass needle pointing North |
| (iii) Frictional force | (a) A cricket ball stopping on its own just before touching the boundary line |
| (iv) Gravitational force | (c) A fruit falling from a tree |
| (v) Electrostatic force | (d) Balloon rubbed on woollen cloth attracting hair strands |
(i) True. Changing the speed of a moving object always requires a force acting on it — this is one of the defining effects a force can have.
(ii) False. Friction opposes motion, so it causes a rolling ball's speed to decrease gradually, eventually bringing it to a stop — not increase.
(iii) False. Electrostatic force is a non-contact force, meaning charged objects can exert a force (attraction or repulsion) on each other even without touching — so there is indeed a force between two charged objects placed a small distance apart.
Answer: the two balloons would move away from each other, repelling one another.
Reason: since both balloons were rubbed with the same material (wool) in the same way, they acquire the same (like) type of static charge. Like charges repel each other, which is why the two similarly charged balloons push apart rather than attract.
Answer: whether an object sinks or floats depends on its weight compared to the buoyant force water exerts on it. The coin has a small volume, so it displaces very little water — the buoyant force generated is far less than its weight, so it sinks.
The wooden block, despite being bigger, is made of a much less dense material. Its larger size lets it displace enough water to generate a buoyant force equal to its own weight, so it floats.
(i) Upward motion: gravitational force, directed downward — slowing the ball down.
(ii) Downward motion: gravitational force, directed downward — speeding the ball up.
(iii) At the topmost position: gravitational force, directed downward — this is what makes the ball start falling again, even though its speed is momentarily zero.
(i) To stop before point A: increase the friction on the horizontal surface — for example, cover it with sandpaper or cloth instead of a smooth surface. Higher friction slows the ball down faster. Releasing the ball from a lower point on the incline would also work, giving it less initial speed.
(ii) To stop after crossing point A: decrease the friction on the horizontal surface — for example, use glass or polished wood instead. Lower friction lets the ball travel further. Releasing the ball from a higher point on the incline would also work, giving it more initial speed.
Answer: friction depends on the irregularities of the two surfaces in contact. Ice and polished floors have very few, very small irregularities, so there's very little friction between them and our feet.
With so little friction, there isn't enough grip to stop our feet from sliding when we walk or shift our weight — which is why we slip on such surfaces.
Answer: yes. Non-uniform motion means an object's speed or direction is changing, and changing speed or direction always requires a force. So some force — friction, gravity, an applied push/pull, or a combination — must be acting on it.
Cause: weight depends on the strength of gravitational pull, and the Moon's gravity is about one-sixth as strong as Earth's. A weaker pull means a smaller weight, so the object weighs one-sixth as much on the Moon.
Does mass change too? No. Mass is the amount of matter in an object and does not depend on gravity — it stays the same on Earth, the Moon, or anywhere else. Only weight changes with gravity; mass stays constant.
Principle: for objects of the same size and shape, the one that dips deepest is the heaviest, since it needs to displace the most water to generate enough buoyant force to support it.
Answer: (iv) w3 > w1 > w2. Object 3 is submerged deepest, object 1 to an intermediate depth, and object 2 the least — as shown in Fig. 5.17.
These five prompts are hands-on investigations, creative projects, and class discussions rather than fixed-answer questions. Here's guidance on how to approach each one.
Guidance: systematically rub each material against several others and record whether the rubbed material attracts small paper pieces afterward. Combinations like plastic or polythene rubbed with wool or silk tend to charge readily and attract paper strongly. Metals typically don't hold a static charge well, since they are conductors — any charge that builds up tends to flow away quickly rather than staying put on the surface. A good research paper would tabulate every pair tested, note which combinations charged and which didn't, and try to explain the pattern based on which materials are good insulators (which hold static charge) versus conductors (which don't).
Guidance: think through what this chapter has taught about gravity's role — it's what makes things fall, keeps our feet on the ground, gives objects weight, and holds water in a glass or a bucket. A strong story might explore consequences like: people and objects floating away with no "up" or "down," water floating out of containers in blobs, difficulty in eating, walking, or even breathing normally (since our body relies on gravity for many everyday functions), and everyday objects like furniture drifting freely. A cartoon strip could show a "before and after" contrast — a normal morning routine suddenly disrupted the moment gravity vanishes.
Where friction is a necessity: friction lets us walk without slipping, lets vehicle tyres grip the road (especially important for braking), allows us to hold and grip objects securely, lets a matchstick ignite when struck, and lets a nail or screw stay firmly in place in wood.
Where friction is a problem: friction causes wear and tear on machine parts over time, wastes energy as heat in engines and moving machinery (reducing efficiency), makes it harder to push or slide heavy objects, and increases fuel consumption in vehicles by resisting motion.
A balanced conclusion for the discussion: friction is neither purely good nor purely bad — it's necessary in the right amount and place (like tyres and brakes), but a problem where it causes unwanted energy loss or wear (like in engines), which is exactly why lubricants are used to reduce friction in machine parts while rough tyre treads are designed to increase it on the road.
Guidance on building and calibrating: with a teacher's help, use a spring and a pointer against a marked scale, and calibrate it by hanging objects of known standard weights and marking where the pointer settles for each.
Expected pattern: when you divide each object's measured weight (in newtons) by its known mass (in kilograms), the ratio should come out to be roughly the same value for every object — approximately 10 N per kg (matching the value used earlier in the chapter for Earth). This constant ratio is the strength of Earth's gravitational pull per unit mass, showing that weight is directly proportional to mass for any object on Earth's surface.
Guidance on testing: after assembling the electroscope as shown (a copper wire through a straw fixed in the jar's lid, with aluminium foil strips hanging from the wire inside the jar), bring a charged object — like a plastic scale or balloon rubbed to generate static charge — close to the exposed copper wire at the top, without touching it. If the wire and foil are charged by induction, the two foil strips should visibly move apart (diverge), since they now carry the same charge and repel each other.
Other possible uses to explore: testing whether various everyday objects (like different fabrics, plastics, or combs) carry a static charge after being rubbed; comparing how strongly different objects are charged, based on how far apart the foil strips move; and testing whether a charge slowly "leaks away" over time by observing whether the foil strips gradually come back together.
Now that forces are covered, move on to how pressure and force combine to explain wind and weather, revisit Chapter 4, or book a free demo class for personalised coaching.
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