Exploring Forces
Why This Matters
Picture two friends, Sonali and Ragini, going for a cycle ride on a windy day. As they ride, the wind pushes hard against them. They have to press the pedals harder just to keep moving. Going uphill is even tougher. But on the way down the hill, something amazing happens. They stop pedalling — and the cycle still rushes down faster and faster, as if something is pulling them.
What is going on here? Why does the wind push them back? Why is going uphill so hard? Why do they slip more easily on a wet road? And what unseen thing pulls them down the hill?
The answer to all of these is one simple idea: force. A push here, a pull there. Forces are acting on you every second of the day, even when you are sitting still. You just do not notice most of them.
In this chapter, we are going to make the invisible visible. By the end, you will be able to point at almost anything happening around you — a falling leaf, a sliding box, a magnet on the fridge — and say exactly which force is at work and why. Let us explore.
The Big Idea
A force is simply a push or a pull. Nothing fancy. When you open a door, kick a ball, or stretch a rubber band, you are applying a force. A force is never alone — it always needs two objects: one to give the push or pull, and one to receive it. A force can do four things: start a still object moving, change the speed of a moving object, change its direction, or change its shape. Some forces need the two objects to touch (like friction). Others act across empty space without touching (like the Earth pulling everything down). We measure force in a unit called the newton (N).
Let’s Break It Down
What a force is — push or pull
Imagine a big cardboard box on the floor. How could you move it? You could push it forward. You could pull it towards you with a rope. You could lift it up (which is really a pull, upwards). In every single case, you did one of two things: a push or a pull.
That is the whole secret. In science, a push or a pull applied on an object is called a force.
But here is the part NCERT mentions and then rushes past — and it really matters. A force is never a one-sided thing. It always involves at least two objects interacting. When you push a table, your hand is one object and the table is the other. Without a second object, there is nothing to push or pull. A force is what happens between two objects when they interact.
Here is a small test of this idea. Push your hand flat against a wall. Do you feel the wall pressing back on your hand? You do. The moment you stop pushing, that feeling vanishes. This shows the force only exists while the two objects (your hand and the wall) are interacting.
A force is not just how strong the push is. It also has a direction. Pushing a box to the right is different from pushing it to the left. So we describe a force with two things: its size (how strong) and its direction (which way). We draw a force as an arrow — a longer arrow means a stronger force, and the arrowhead shows the direction. Figure 5.1 below shows this for a push and a pull.
So whenever you see something being pushed or pulled, picture the arrow: how big, and which way. That little arrow is how scientists think about every force.
A boy lifts his school bag straight up off the floor. Is this a push or a pull, and what two objects are interacting?
Lifting something up is a pull — the boy pulls the bag upwards, against the way the Earth tries to pull it down. The two objects interacting are the boy (through his hand) and the bag. Remember, a force always needs two objects: one to apply it and one to receive it.
What a force can do
So you apply a force. What can it actually do to the object? It turns out a force can have four kinds of effects. Let us go through them with everyday examples.
- Start a still object moving. A football lying still on the ground does nothing on its own. Kick it (a push) and it shoots forward. The force made it move from rest.
- Change the speed of a moving object. A cyclist who is already moving presses the brakes. The brakes apply a force, and the cycle slows down. A force can speed things up too — pressing the pedals harder makes the cycle go faster.
- Change the direction of a moving object. A batsman hits a ball that was coming straight at him, and it flies off sideways. The bat’s force changed the ball’s direction of motion.
- Change the shape of an object. Press an inflated balloon between your hands and it goes flat on the sides. Roll a ball of dough with a belan (rolling pin) and it spreads into a flat chapati. The force changed the object’s shape.
A single force can even cause more than one of these at once. Figure 5.2 below shows all four effects together.
Here is a useful flip-side of this rule. If you ever see an object change its speed, direction, or shape, you can be sure a force is acting on it. None of these changes happen by themselves. This is a powerful detective tool: spot a change in motion, and you know a force is hiding somewhere.
Balanced and unbalanced forces — why motion changes
Now think about a tricky question. A book lies on your table, perfectly still. Is any force acting on it? You might say no, because it is not moving. But that is wrong! Gravity is pulling the book down all the time, and the table is pushing the book up by the same amount. The book stays still because these two forces cancel each other out.
This brings us to a really important idea: balanced and unbalanced forces.
When two forces of equal size act in opposite directions on an object, they cancel out. We say the forces are balanced. The leftover force — what we call the net force — is zero. With balanced forces, the object’s motion does not change. A still object stays still; a moving object keeps moving the same way.
When the forces do not cancel — maybe one is bigger, or they point in directions that do not fully oppose — there is a leftover push or pull. We say the forces are unbalanced. There is a net force in one direction. And here is the key: only an unbalanced force can change how an object moves. It can start motion, speed it up, slow it down, or change direction.
The clearest way to see this is a tug-of-war. Figure 5.3 below shows both cases.
Now we can answer Sonali and Ragini’s mystery from the start. Going uphill, gravity pulls the cycle back down the slope, and that pull is not balanced — so it slows them down, and they must push the pedals hard to overcome it. Coming downhill, that same unbalanced pull of gravity now points the way they are going, so it speeds them up even without pedalling. It is all about which way the leftover force points.
A heavy almirah does not move even when one person pushes it hard. Are the forces on it balanced or unbalanced? Why does it not move?
The forces are balanced. The person pushes the almirah, but friction between the almirah and the floor pushes back by the same amount in the opposite direction. The two cancel out, the net force is zero, so the motion does not change and the almirah stays still. To move it, you would need a bigger push so the forces become unbalanced.
Contact forces — muscular force and friction
Now let us sort forces into two big families. The first question to ask about any force is simple: do the two objects have to touch?
If a force acts only when the two objects are touching, it is a contact force. The touch can be direct (your hand on a box) or through something in between (a rope, a stick). Let us meet two contact forces.
Muscular force. When you walk, lift, push, jump or stretch, the force comes from the muscles in your body. The force made when our muscles work is called muscular force. Muscles make this force by squeezing in (contracting) and stretching out. It is a contact force because your muscles must be in contact with whatever they move — you cannot lift a bag without holding it.
Muscular force is not only for humans. Animals use it too. A bullock pulls a cart, a horse pulls a tonga, a dog runs — all using muscular force. And here is a surprise: muscular force works inside your body as well. Your heart muscles squeeze and relax to pump blood. Muscles in your food pipe push food down when you swallow. So muscular force keeps you alive, not just moving.
Friction. Here is a puzzle. Push a lunch box across a table and let go. It slides a little, slows down, and stops. But nobody pushed it backwards. So what stopped it? A force must have, because we just learned that only a force changes speed.
The hidden force is friction. Friction is the force that comes into play when one surface moves, or tries to move, over another surface. It is a contact force, because it only happens where the two surfaces touch. And friction has a special rule: it always acts opposite to the direction the object is moving (or trying to move). That is why it slows the sliding box and finally stops it.
But why does friction exist at all? NCERT states that friction comes from “irregularities” and moves on — but let us actually see why. No surface is perfectly smooth, even ones that look and feel smooth. Zoom in close enough and every surface is covered in tiny bumps and dips. When two surfaces sit together, the bumps of one fit into the dips of the other and lock together, like two combs pressed teeth-to-teeth. To slide one surface over the other, you have to drag all those locked bumps past each other, and they resist. That resistance is friction. Figure 5.4 below shows this zoomed-in picture.
This explains a lot. On a rough surface (sand, a kachcha road) the bumps are big, so friction is large and the box stops quickly. On a smooth surface (glass, a polished floor) the bumps are small, so friction is small and the box slides far. So friction is greater on rougher surfaces.
Friction can be a friend or a trouble-maker, depending on what you want.
- Where friction helps us: Friction lets you walk without slipping — your shoe grips the ground. It lets brakes stop a cycle or car. It lets you hold a pen, write with chalk, and strike a matchstick. Without friction, the world would be impossibly slippery.
- Where friction is a problem: Friction wears out the soles of shoes and the moving parts of machines. It makes machines heat up and waste energy. It slows down vehicles, so they need more fuel.
Because friction is sometimes unwanted, we have ways to reduce it: oil and grease in machines (a smooth layer that keeps the bumpy surfaces apart), ball bearings, and smooth, streamlined shapes. In fact, air and water cause friction too — they push back on anything moving through them. That is why aeroplanes, ships and fast trains are built with smooth, pointed shapes, so they cut through the air or water with less friction.
You give a notebook the same gentle push twice. The first time you slide it on a glass table; the second time on a rough cloth. On which surface does it travel farther, and why? Name the force that finally stops it.
- Identify the force at work. After you let go, no one is pushing the notebook, yet it slows and stops. So a backward force is acting on it. That force is friction, acting opposite to its motion.
- Recall what friction depends on. Friction comes from the tiny bumps on the two surfaces locking together. Rougher surfaces have bigger bumps, so they grip harder — meaning more friction.
- Compare the two surfaces. Glass is smooth (small bumps, small friction). Cloth is rough (big bumps, large friction). So friction is larger on the cloth.
- Reach the answer. Less friction means the notebook is slowed less, so it travels farther on the smooth glass. On the rough cloth, the larger friction stops it sooner. In both cases, the force that finally brings it to rest is friction.
Non-contact forces — magnetic, electrostatic and gravity
Now the second family. What if a force can act without the two objects touching at all, reaching across an empty gap? Such a force is a non-contact force. There are three you should know.
Magnetic force. Bring a magnet near an iron pin and the pin jumps to it — even before they touch. Bring two magnets close and they either pull together or push apart, again across a gap. The force a magnet exerts on another magnet or on a magnetic material (like iron) is the magnetic force. Since it works without contact, it is a non-contact force. Remember from earlier classes: like poles (North–North or South–South) repel, and unlike poles (North–South) attract. So magnetic force can be a pull or a push.
Electrostatic force. Rub a plastic comb or scale on your dry hair or a polythene bag, then hold it near tiny bits of paper. The paper jumps up and sticks to the comb — without the comb touching it first. Why? When certain materials are rubbed together, tiny electric charges build up on their surfaces. These are called static charges because they sit still and do not flow. An object that gathers these charges is charged. A charged object can pull (or push) other objects across a gap. This force is the electrostatic force, and it is a non-contact force.
There are two kinds of charge, called positive and negative. The rule is just like magnets: like charges repel, unlike charges attract. That is why two balloons rubbed the same way push apart (same charge), but a rubbed balloon is pulled towards the cloth that rubbed it (opposite charges).
Gravitational force. Throw a ball up, in any direction, and it always comes back down to the ground. Drop anything and it falls. Why does everything fall towards the Earth? Because the Earth pulls every object towards itself. This pull is the gravitational force, also called the force of gravity, or simply gravity. The Earth pulls the ball even though it is not touching it, so gravity is a non-contact force.
Gravity has one special feature. Magnetic and electrostatic forces can either pull or push (attract or repel). But gravity is always a pull — it only ever attracts, never pushes. The Earth can pull a ball down, but it can never push it away.
These three non-contact forces, together with the two contact forces, are easy to keep straight if you remember the one key question: do they need to touch? Figure 5.5 below sorts all five forces into this family tree.
Let us see gravity in more detail, because it does something interesting when you throw a ball up. Figure 5.6 below traces the full journey, and also shows what weight means.
This is also how we get weight. The weight of an object is the force with which the Earth pulls it towards itself. Since weight is a force, it is measured in newton (N), just like any force. A heavier object is pulled harder, so it weighs more. We can measure weight with a spring balance: hang an object from its hook, and the more the spring stretches, the greater the weight it shows on the scale.
Be careful not to mix up weight and mass, because students often do. Mass is the amount of matter in an object, measured in grams (g) or kilograms (kg). Mass never changes — it is the same on Earth, on the Moon, or anywhere. Weight is the pull of gravity on that object, measured in newton (N), and it can change from place to place because gravity changes. For example, a 1 kg object weighs about 10 N on Earth but only about 1.6 N on the Moon, because the Moon’s pull is weaker. The object’s mass stays 1 kg in both places — only its weight changed.
| What you compare | Mass | Weight |
|---|---|---|
| What it is | The amount of matter in the object | The Earth's gravitational pull on the object |
| Unit | Gram (g) or kilogram (kg) | Newton (N) |
| Does it change place to place? | No, it stays the same everywhere | Yes, it changes where gravity is different |
| On the Moon (vs Earth) | Same as on Earth | About one-sixth of its Earth value |
One last small idea the chapter touches on. Push an empty closed bottle down into a bucket of water and you feel water pushing it back up. When the bottle is released, it pops to the surface. This upward push from a liquid is called upthrust or the buoyant force. If the Earth’s downward pull (gravity) on an object is more than the upward buoyant force, the object sinks; if the two are equal, it floats. This is why a light wooden block floats while a small coin sinks. You will study this in more detail later.
Common Mistakes
These are the slip-ups students make most often with forces. Read them once and you will sidestep them.
If an object is not moving, then no force is acting on it.
In everyday life we only notice a force when something visibly moves, so a still object looks like a place where nothing is happening.
A still object can have many forces acting on it that simply balance out. A book on a table is pulled down by gravity and pushed up by the table at the same time, so the net force is zero and it stays put. 'No motion' means the forces are balanced, not that there are no forces.
Heavier objects must always have more mass and more weight, and the two words mean the same thing.
On Earth a heavier object really does have both more mass and more weight, so in daily talk we use the words as if they are identical.
Mass is the amount of matter (in kg) and never changes; weight is the gravitational pull on that matter (in N) and changes where gravity changes. The same object keeps its mass on the Moon but weighs only about one-sixth as much. They measure different things in different units.
Friction is a useless, annoying force that we should always try to get rid of.
We mostly notice friction when it works against us, wearing out shoes or slowing a cycle, so it feels like a pure nuisance.
Friction is often essential. Without it you could not walk, hold a pencil, or stop a vehicle, because everything would slip. Friction is helpful in many cases and unwanted in others, so we increase it where we need grip and reduce it where it just wastes energy.
Quick Check
Try these quick questions. Each one checks a single idea from the chapter.
A bowler throws a ball and a fielder catches it, bringing it to a stop. Which effect of force is the fielder's hand producing on the ball?
Two friends pull a rope in opposite directions, each with a force of 200 N, and the knot in the middle does not move. What can we say about the forces?
You rub a plastic comb on dry hair and it lifts small bits of paper without touching them first. Which force is this, and what kind is it?
A box slides across the floor and slowly stops on its own. Which way does the force of friction act on it?
Practice Problems
Try each one yourself first. Only then tap to reveal the full answer.
Easy
List the four effects a force can have on an object, with one everyday example of each.
A force can:
- Start a still object moving — kicking a still football makes it move.
- Change the speed of a moving object — pressing the brakes slows a moving cycle.
- Change the direction of a moving object — a bat sends an incoming ball off sideways.
- Change the shape of an object — pressing a balloon flattens it.
A single force can even cause more than one of these at the same time.
Sort these into contact forces and non-contact forces: muscular force, gravity, friction, magnetic force, electrostatic force.
The simple test is: do the two objects have to touch?
- Contact forces (objects must touch): muscular force and friction.
- Non-contact forces (act across a gap): gravity, magnetic force, and electrostatic force.
So muscular force and friction belong in one family, and the other three in the other.
Medium
Two balloons are rubbed with the same woollen cloth and brought near each other. What happens, and why?
The two balloons move apart — they repel each other.
Here is why. Rubbing each balloon with the wool gives it static charge. Because both balloons were rubbed in the same way with the same cloth, they pick up the same kind of charge. And the rule for charges is: like charges repel. So the two similarly charged balloons push each other away, through the electrostatic force, even without touching.
(If you instead brought the woollen cloth near one balloon, they would attract, because the cloth and balloon carry opposite charges, and unlike charges attract.)
Why do we sometimes slip on smooth surfaces like wet tiles or polished floors? Explain using friction.
Walking depends on friction. When you walk, your foot pushes back on the ground, and friction grips your shoe so it does not slide — that grip is what lets you move forward safely.
On a smooth or wet surface, the tiny bumps that cause friction are very small, or the water fills the gaps between them, so the surfaces barely lock together. This means there is very little friction. With little friction, your shoe cannot grip the floor. Your foot slides instead of pushing you forward, and you slip.
So slipping happens because smooth or wet surfaces give too little friction for your feet to hold on.
Challenge
A ball is thrown straight up. It rises, slows down, stops for a moment at the top, and then falls back. Name the force acting on the ball and give its direction during (i) the upward motion, (ii) the topmost point, and (iii) the downward motion. Explain why the ball slows down going up but speeds up coming down.
In all three parts, the force acting on the ball is the gravitational force (gravity), and it always points downwards, towards the Earth. Gravity never switches off and never pushes up — it only pulls down.
- (i) Going up: Gravity acts downwards, opposite to the ball’s upward motion. A force opposite to the motion slows the object, so the ball steadily loses speed as it rises.
- (ii) At the top: Gravity still acts downwards. For one instant the ball’s speed is zero, but the downward pull is still there — which is exactly why the ball does not stay up but begins to fall.
- (iii) Coming down: Gravity acts downwards, now in the same direction as the ball’s motion. A force in the direction of motion speeds the object up, so the ball falls faster and faster.
So the single, unchanging downward pull of gravity explains both effects: it opposes the upward motion (slowing the ball) and aids the downward motion (speeding it up). The direction of the force never changes — only its relationship to the motion does.
A 1 kg packet of sugar is taken from the Earth to the Moon. On Earth its weight is about 10 N. On the Moon, the gravity is about one-sixth of the Earth's. What is the packet's mass and weight on the Moon? Explain why one of them changes and the other does not.
On the Moon:
- Mass = 1 kg (unchanged).
- Weight = about 10 N divided by 6, which is roughly 1.6 N.
Why the difference? Mass is the amount of matter in the packet. Moving it to the Moon does not add or remove any sugar, so the mass stays exactly 1 kg — mass is the same everywhere.
Weight, on the other hand, is the gravitational pull on the packet, measured in newton. The Moon pulls things with only about one-sixth of the Earth’s strength. Since the pull is weaker, the weight becomes about one-sixth, that is roughly 1.6 N.
So the matter inside the packet (its mass) does not change, but the strength of the pull on it (its weight) does. That is the whole reason astronauts feel so light on the Moon — same body, but a much weaker pull.
Summary
Here is everything you can now explain to a friend:
- A force is a push or a pull, and it always needs two objects interacting. We describe it by its size and its direction, and we measure it in newton (N).
- A force can start a still object moving, change its speed, change its direction, or change its shape. Whenever you see one of these changes, a force is acting.
- Balanced forces are equal and opposite, so they cancel out — the net force is zero and motion does not change. Unbalanced forces leave a net force, and only an unbalanced force changes how an object moves.
- Contact forces act only when objects touch: muscular force (from our muscles) and friction.
- Friction opposes motion. It happens because tiny bumps on the two surfaces lock together. It is greater on rougher surfaces. It is helpful (walking, gripping, braking) and sometimes a problem (wear, wasted energy), so we reduce it with oil, smooth shapes, and ball bearings.
- Non-contact forces act across a gap: magnetic force and electrostatic force (each can attract or repel, since like poles/charges repel and unlike attract) and gravitational force, which is always a pull.
- Gravity is the Earth’s pull on objects. The weight of an object is this pull, measured in newton (N). Mass (in kg) is the matter in an object and never changes; weight can change where gravity changes.
What’s Next
You have just learned that a force is a push or a pull, and that even invisible forces like gravity act all around you. But there is one more invisible push we have not met yet — the push that air itself makes.
Have you ever felt the wind shove against you on a stormy day, or seen a roof fly off in a cyclone? That is air pushing with real force. In the next chapter, Chapter 6 — Pressure, Winds, Storms and Cyclones, you will find out how a force spread over an area becomes pressure, why air presses on everything, and how moving air builds into winds, storms and powerful cyclones. The exploring continues!
Frequently Asked Questions
What is a force in simple words?
A force is a push or a pull on an object. It always comes from one object acting on another object. The SI unit of force is the newton, written with a small n and shown by the symbol N. A force can make a still object move, change the speed of a moving object, change its direction, or change its shape.
What is the difference between balanced and unbalanced forces?
Balanced forces are equal forces acting in opposite directions on an object, so they cancel out and the leftover force is zero. They do not change how the object moves. Unbalanced forces do not cancel out, leaving a net force in one direction. Only an unbalanced force can start motion, change speed, or change direction.
What is the difference between contact and non-contact forces?
Contact forces act only when two objects are touching, either directly or through something like a rope or stick. Muscular force and friction are contact forces. Non-contact forces act even when the objects are not touching, across a gap. Magnetic force, electrostatic force, and gravitational force are non-contact forces.
Why does friction happen and which way does it act?
Friction happens because no surface is perfectly smooth. Even surfaces that look smooth have tiny bumps and dips. When two surfaces touch, these bumps lock into each other and resist sliding. Friction always acts opposite to the direction in which the object moves or tries to move. Rougher surfaces have bigger bumps, so friction is greater.
What is weight and how is it different from mass?
Weight is the gravitational force with which the Earth pulls an object towards itself, so it is measured in newton. Mass is the amount of matter in an object, measured in grams or kilograms. Mass stays the same everywhere, but weight can change from place to place because gravity can change, for example weight on the Moon is about one-sixth of that on Earth.