Exploring Forces — Class 8 Science (Curiosity)
"It's thrilling! It seems something is pulling us downhill, what could it be?" — Sonali, Curiosity, Grade 8, page 63
1. About the Chapter
This is Chapter 5 of Curiosity (pages 62–79, Reprint 2026-27). It opens with Sonali and Ragini cycling out of their village on a windy day — pushing pedals, fighting the wind, finding some stretches of road rougher than others, and then racing downhill without pedalling at all. Almost every force in the chapter is already in that first page, unnamed.
| Section | Question it answers |
|---|---|
| 5.1 | What is a force? |
| 5.2 | What can a force do to the objects it acts on? |
| 5.3 | Are forces an interaction between two or more objects? |
| 5.4 | What are the different types of forces? |
| 5.5 | Weight and its measurement |
| 5.6 | Floating and sinking |
Two things to know before you start.
- There is not one formula in this chapter. No
F = ma, noW = mg, no value of g. Everything is qualitative, and that is deliberate. - Where the chapter reaches the edge of what Class 8 can handle, it says so and stops — balanced forces, why some metals charge positively, and the density that buoyant force depends on are all named and openly deferred. A good answer stops where the book stops.
2. What a Force Is
Activity 5.1 — the box
Take a large cardboard box and move it in as many ways as you can invent. Fig. 5.1 shows pushing (a), pulling (b), and lifting — which the book calls pulling up — and carrying (c). Slide it, tip it, roll it, drag it with a rope, get a friend to help.
In all the ways that you might have used to move the box, you had to apply a push or pull to the box. Generally, the push or pull applied on an object is called force in science.
The point is not the length of the list. It is that however inventive you are, you cannot find a way that is neither a push nor a pull.
What a force can do
The chapter's list, from Activity 5.2 and Table 5.1:
| Effect | Example |
|---|---|
| Make an object move from rest | Kicking a stationary football; opening a drawer |
| Change the speed of a moving object | Pedalling harder; applying brakes |
| Change the direction of motion | Hitting a moving ball with a bat; turning a steering handle |
| Change the shape of an object | Pressing a balloon; stretching a rubber band; rolling a chapati |
| Cause some or all of these | A fielder stops a ball and throws it back |
The fifth item is not a fifth effect — it is there to stop you thinking the first four are a menu from which exactly one is chosen.
The rule, read backwards
The chapter's characters ask whether every change in 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 more powerful direction. It turns an observed change into evidence for a force — even one you cannot see. The whole discovery of friction, a few pages later, rests on it.
3. A Force Needs Two Objects
When you push a table, your hand is one object and the table is the other. Run through every row of Table 5.1 and you never find a force with only one object in it.
A force is a push or pull on an object resulting from the object's interaction with another object. The SI unit of force is newton (written with a small 'n') and its symbol is N.
The habit worth forming now: before answering any force question, write down both objects. "The fruit has gravity" is not an answer; "the Earth pulls the fruit" is. Questions that look hard usually collapse the moment both names are on paper.
Two boxes that matter
An object at rest is not force-free. It means that the forces acting on the object are balancing one another. A book on a table is pulled down exactly as a falling fruit is — slide it off the edge and it drops at once. The chapter then stops: You will learn about balanced forces in higher grades.
Both objects feel it. Whenever two objects interact, each object experiences a force from the other. As soon as the interaction ceases, the two objects no longer experience the force. Push a table and you feel it in your hand too.
4. Contact Forces
Forces of this type which act only when there is physical contact between the objects are called contact forces. Contact may be direct (a hand) or indirect (a bat, a rope) — what matters is that the chain of touching is unbroken.
Muscular force
The force resulting due to the action of muscles is known as muscular force. It occurs when muscles contract and elongate while doing any activity.
Walking, running, lifting, jumping, stretching. Animals, birds, fish and insects use it for movement and survival. Humans have long used the muscular force of animals — bullocks at a plough, camels under a load.
And a great deal of it happens without your deciding anything: chewing, pushing food along the alimentary canal, and the heart expanding and contracting so that blood circulates.
Friction — a force discovered by argument
A ball rolling on flat ground stops by itself. A bicycle slows when you stop pedalling. No force appears to be acting on the objects, yet their speed gradually decreases.
But speed never changes without a force. So one must be acting — and the chapter goes looking for it.
Activity 5.3. Push a flat-based object across a table. It stops. Push it the opposite way. It stops again. A sloping table or a draught could not do that; a force that always opposes the motion could.
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. Friction always acts in a direction opposite to the direction in which the object is moving or trying to move.
Do not drop the words "or tries to move". Push a heavy almirah that does not budge and friction is acting all the same.
Where it comes from. 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). Those bumps lock into each other and resist sliding.
The surprising part: no surface is actually smooth. Smooth and rough are two ends of a scale, which is why friction is never zero — only smaller or larger.
Activity 5.4. Slide the same object over glass, cloth, wood, ceramic tile and sand. It travels a different distance on each. The force of friction depends upon the nature of the surfaces in contact. Friction is greater on rough surfaces.
Liquids and gases too. Air, water, and other liquids also exert force of friction on the objects moving through them, which is why aeroplanes, ships, boats and high-speed trains are designed with specific shapes to reduce it. Ragini fighting the wind on page 63 was meeting this force before it had a name.
5. Non-Contact Forces
There are forces whose effect can be experienced even if the objects are not in contact.
Magnetic force
Activity 5.5. Slide one ring magnet onto an upright wooden stick, then a second above it with like poles facing. The second one floats, with a visible gap. Push it down and you feel it push back. Reverse both magnets and it still floats — like poles are still facing.
The stick is not holding it up. Wood is not magnetic; it only stops the rings sliding off sideways.
The force exerted by a magnet on another magnet or a magnetic material is called magnetic force. It acts from a distance, so it is a non-contact force.
Electrostatic force
Activity 5.6. Rub a plastic scale vigorously with polythene — without touching the rubbed part, or the charge escapes and nothing happens. Bring it near small paper pieces and they jump up to meet it.
When two objects of certain materials are rubbed together, electrical charges build up on their surfaces. These charges are called static charges as they do not move by themselves.
Activity 5.7. Hang two balloons so they do not touch, rub both with a woollen cloth, release them — they move apart. Now bring the cloth near one — they attract.
The inference, step by step:
- Both balloons were charged the same way, so they carry similar charges.
- They repelled → like charges repel.
- Both the rubbing object and the rubbed object get charged but they acquire opposite kind of charges.
- The cloth attracted a balloon → unlike charges attract.
- The two kinds of static charges are said to be 'positive' and 'negative'.
That there are exactly two kinds of charge is not something anyone can see. It is deduced from a balloon being pushed away by one object and pulled towards another.
The force exerted by a charged body on another charged body or an uncharged body is called electrostatic force.
And the link back to Chapter 4: When the charges move, they constitute an electric current. Static charges give you this section; moving charges give you the last chapter. Same thing, two states.
Gravitational force
Activity 5.8. Throw a ball up. It comes down. Throw it harder — it goes higher and still comes down. That second throw is the point of the activity: this is not a pull you can beat by trying harder.
The force with which the Earth attracts objects towards itself is called the gravitational force, also called the force of gravity or simply gravity. It is a non-contact force.
The sharpest fact in the chapter, and easy to read past:
Gravitational force is always an attractive force, unlike magnetic force or electrostatic force, which can either be attractive or repulsive.
Magnets have north and south; charges have positive and negative; gravity has no opposite kind at all. There is nothing to turn over and no way to make it push.
Vertical motion (Fig. 5.11). Thrown straight up, an object moves up straight, slows down, stops momentarily at the top, and then takes a straight vertical path downwards. One steady downward force produces three of the four effects from section 5.2 — and it does not stop acting at the top. If it did, the ball would stay there.
6. Weight, and How to Measure It
Activity 5.9. Hang different objects from the same spring. The stretch differs each time, so the Earth pulls different objects with different forces.
That single idea — an invisible force turned into a visible length — is the whole basis of the spring balance.
The force with which the Earth pulls an object towards itself is called the weight of the object. And since the weight is a force, it is measured in the same unit as that of force. Therefore, SI unit of weight is also newton (N).
Reading the instrument
The chapter makes you examine the balance before using it, exactly as it made you examine a thermometer in Grade 6.
| Activity | Question | Answer for Fig. 5.13 |
|---|---|---|
| 5.10 | What is the maximum it can measure? | 10 N — range 0 to 10 N |
| 5.11 | Weight between two bigger marks? | 1 N |
| 5.11 | Divisions between them? | 5 |
| 5.11 | So one small division reads | 1 ÷ 5 = 0.2 N |
The method is what you are learning, not the number — your school laboratory may have spring balances for which the range and the value of the smallest division may be different.
Activity 5.12 adds the safety rule: never hang an object heavier than the maximum, otherwise it may get damaged. An over-stretched spring does not just fail on that object — every later reading it gives is wrong, and nothing about its appearance says so.
Mass is not weight
| Mass | Weight | |
|---|---|---|
| What it is | The amount of matter in an object | The gravitational force pulling it |
| Unit | Gram (g), kilogram (kg) | Newton (N) |
| Changes with place? | No — the same everywhere | Yes |
| Measured with | Beam balance (by comparison) | Spring balance |
The chapter's table for a 1 kg object:
| Earth | Moon | Mars | Venus | Jupiter | |
|---|---|---|---|---|---|
| Mass | 1 kg | 1 kg | 1 kg | 1 kg | 1 kg |
| Weight | 10 N | 1.6 N | 3.8 N | 9 N | 25.4 N |
The top row never moves; the bottom row varies by a factor of more than fifteen.
Why we confuse them and get away with it. Since the weight of an object remains almost the same everywhere on the Earth, so for all practical purposes it is acceptable to weigh an object to find its mass. The shopkeeper's balance works because both of you are standing on the same planet.
But the weight of the wheat bag is 10 kg borrows the unit of mass and the word for weight. The chapter is blunt: in scientific use, this is not correct.
7. Floating and Sinking
Activity 5.13. Push a tightly closed empty bottle down into a bucket of water. You feel an upward push. Let go and it springs back to the surface.
The force applied by a liquid on an object in the upward direction is known as upthrust or buoyant force.
The rule. 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.
| Comparison | Result |
|---|---|
| Gravitational force more than buoyant force | Sinks |
| The two forces equal | Floats |
Archimedes' Principle makes it sharper: when an object is fully or partially immersed in a liquid, it experiences an upward force which is equal to the weight of the liquid it displaces. So a floating object has pushed aside water weighing exactly as much as itself.
What decides it is not size. A coin sinks and a much bigger wooden block floats. A large stone sinks and a small matchstick floats. Compare each object with the water it displaces — never with the other object.
Pumice makes the point unforgettably: a rock that floats. Lava full of gas cools quickly, traps the bubbles, and the result is a light, porous rock — filled with air pockets which is less dense than water. A steel ship floats for the same reason: it is mostly air.
The chapter notes that buoyant force depends on the density of the liquid — and immediately adds You will learn about density in a later chapter of this book. Everything in this chapter can be answered by comparing two forces, without density at all.
8. Summary
Snapshots, as the chapter gives them:
- A force is a push or pull resulting from an object's interaction with another object. SI unit: newton (N).
- Forces act with or without contact. Muscular force and friction are contact forces; magnetic, electrostatic and gravitational forces are non-contact.
- Force can change an object's speed, its direction, or both — and can change its shape.
- Friction acts when an object moves or tries to move over another surface, always opposite to that motion.
- Magnetic force is exerted by a magnet on another magnet or a magnetic material.
- Electrostatic force is exerted by a charged body on another charged or uncharged body.
- Gravitational force is the Earth's pull, and it is always an attractive force.
- Weight is the force with which the Earth pulls an object; SI unit newton (N).
- The mass of an object remains unchanged whereas its weight may vary from place to place.
- Upthrust (buoyant force) is the upward force a liquid applies on an object placed in it.
Appendix — What Belongs to Other Years, Not This Chapter
An earlier version of this page taught a great deal of material that is not in Curiosity Grade 8 Chapter 5. It is listed here so you know where it belongs, and so you do not use it in a Class 8 answer.
| Topic | Where it actually belongs |
|---|---|
| Newton's three laws of motion; inertia; action–reaction | Class 9 Science — Force and Laws of Motion |
F = m × a | Class 9 Science |
Law of universal gravitation, F = Gm₁m₂/r², the value of G | Class 9 Science — Gravitation |
W = m × g and the value of g | Class 9 Science |
| Balanced and unbalanced forces; equilibrium | Class 9 — and explicitly deferred by this chapter: "You will learn about balanced forces in higher grades" |
| Friction classified as static, sliding and rolling | Not in this book at all |
| Tension, normal force, applied force, nuclear forces | Not in this book — it names exactly five forces |
| Lubrication and methods of reducing friction | Beyond this chapter, which only mentions shaping vehicles to reduce fluid friction |
Also removed: unsourced claims. The earlier page carried specific historical assertions about who discovered gravity and when, and a list of modern achievements, none of it sourced. The chapter itself names only Archimedes, for the principle about displaced liquid. If you want to write about the history of these ideas, take it from a cited source rather than from memory.
What this chapter genuinely does say about going further is worth repeating, because it is a model of honest limits: balanced forces come later; why copper is a positive electrode and zinc a negative one comes later; density comes later in this same book. Naming the boundary is part of the science.
