Class 8 Science Chapter 5: Exploring Forces

Moving a Box

Put a large cardboard box on the floor and try to move it in as many ways as you can think of. You can shove it along from behind. You can take hold of an edge and drag it towards you. You can get both arms under it, lift it and carry it across the room.

Push, or Pull

Now look at what those have in common. Every one of them is either a push or a pull. Dragging is a pull. Lifting is a pull as well, upwards instead of sideways. However many ways you find, they sort into those two.

The Word Science Uses

A push or a pull applied to something is what science means by a force. That is the whole definition at this stage, and it is worth noticing how ordinary it is: you have been applying forces all day without using the word once.

It Takes Two Objects

Here is the part that does need care. Push a table and your hand is one object while the table is another, and the force exists because those two are interacting. A force is never something an object carries about inside it on its own.

And Both of Them Feel It

Push a table hard and look at your hand. You can feel the table pushing back. Whenever two objects interact, each of them feels a force from the other, which is why the hand that pushes is also the hand that aches.

Stop Pushing and It Goes

Take your hand away and the force on it disappears at once. Nothing is left over and nothing is stored. The force existed for exactly as long as the interaction did.

What a Force Can Do: Start It Moving

So what does applying a force actually achieve? Four things, and the first is the obvious one: something that was standing still can be made to move. The box on the floor does not set off by itself.

Change Its Speed

The second is that something already moving can have its speed changed. Hit a travelling ball with a bat and it leaves faster than it arrived. Squeeze the brakes on a bicycle and it slows. Both are forces.

Change Its Direction

The third is a change of direction. A fielder can deflect a ball without making it travel any faster, and turning the handle of an autorickshaw changes where it is heading while the speed stays put.

Change Its Shape

The fourth is the one most often left off a list. Press an inflated balloon between your hands and it changes shape. Roll a chapati and the dough flattens. Stretch a rubber band. Nothing in any of those has to go anywhere.

Or Several at Once

A single force often does more than one of the four. A friend grabbing your moving bicycle from behind slows it and may well turn it too. The four are a list of what can happen, not a set of alternatives.

The Other Way Round

And the list runs backwards just as well. If the speed of something changes, or its direction, or its shape, then a force acted on it. None of those happens on its own, which is a useful thing to know when you cannot see what is doing the pushing.

When Nothing Happens

That raises a fair question. If a box sits on the floor and nothing is changing, does that mean no force acts on it? It does not. The Earth is pulling it down and the floor is pushing it up, and the two are balancing one another. Balanced forces are a topic for later grades, but the trap is here already: stillness is not the absence of force.

The Unit of Force

Force is measured, like length and time, and its unit is the newton. A spring balance, which comes in a later topic of this chapter, is marked in newtons and reads a force off directly.

Writing It Properly

Two small conventions are worth getting right now. The unit written out in words takes a small n, as newton, because that is how units named after people are written. Its symbol is a capital N.

What To Carry Forward

A force is a push or a pull, it arises between two interacting objects, and both of them feel it. It can start a resting object moving, change the speed or direction of a moving one, or change a shape, singly or several at once. It is measured in newtons. And an object at rest may have forces on it that simply balance.

Scenario:

In a single over of a school cricket match, four things happen to the ball. The bowler releases it from a standstill in her hand. The batter hits it and it flies off in a new direction, faster than it arrived. A fielder running in stops it dead with both hands. Afterwards the umpire finds the ball has a flat patch where it struck the bat. A pupil writing this up says that a force acted only when the ball was hit, because that is the only moment anybody heard anything.

Analysis:

Each of the four moments is one of the effects a force can have. The pupil has mistaken the loudest for the only one. The flat patch is the clue worth dwelling on. Nothing moved anywhere as a result of it, and a force was plainly involved.

Activity:

List four things that happened to something in your own day and name the effect of the force in each.

Title:

Try it at home: four effects, one rubber band

Safety:

Flick the paper gently and away from faces. Nothing harder than a paper ball for this.

What You Notice:

The rubber band is the one worth noticing. Nothing travelled anywhere, yet a force was clearly applied, which is why a change of shape sits on the list beside the other three.

The hand that pushes a table is pushed back by the table, which is why pushing something heavy makes your hand ache as well as moving the furniture.

A change of shape counts as an effect of a force even though nothing travels anywhere, which is why pressing a balloon belongs on the same list as kicking a ball.

A box lying perfectly still may have two forces on it that balance exactly, so stillness is never proof that nothing is acting.

START, SPEED, STEER, SHAPE — The four effects of a force. Three are about motion and the fourth is not.

IT TAKES TWO — A force is never owned by one object. It arises between two, and both of them feel it.

Does It Have To Touch?

Every force in the last topic involved touching something. A hand on a box, a bat on a ball, fingers round a balloon. Does a force always need the two objects to be in contact? It does not, as the next topic shows, but a great many forces do, and those are the ones to sort out first.

Direct, and Indirect

Contact does not have to mean skin on the object. Pushing a box with your palm is direct contact. Shoving it with a stick, or hauling it in with a rope, is indirect. In both of those something solid runs unbroken from you to the box, so both count.

The Name for That Kind

A force that acts only while the two objects are in contact is called a contact force. That is the test, and it is worth stating as a test: take the contact away and ask whether anything is left.

Muscular Force

The first kind is the one you supply yourself. Walking, running, lifting, pushing, jumping, stretching: in each of them the force comes from your muscles working, as they contract and lengthen again. A force produced that way is called a muscular force.

Animals, and Work

Animals, birds, fish and insects all move and survive by the same means. And people have used the muscles of animals for a very long time to get work done: bullocks hauling a plough or a cart, horses carrying loads, camels crossing sand.

Muscles You Never See

Some of your muscular force never leaves your body. Muscles chew your food and then move it along the digestive canal. The muscle of your heart tightens and relaxes to drive blood round your body, which is not optional work.

A Ball That Stops Itself

Now a puzzle. Roll a ball across flat ground and it slows down and stops. Stop pedalling on a level road and the bicycle does the same. On a rough road it stops sooner than on a smooth one. Nobody touched any of them.

Something Must Be Acting

But you already know that a speed cannot change unless a force acts. So either that rule is wrong, or there is a force here that nobody has yet named. The second is correct, and finding it is what the next activity is for.

Push It, and Watch

Put something with a flat base on a table, give it a gentle shove and let go. It slides a short way and stops. Since it slowed down, a force acted on it, and since it slowed rather than sped up, that force pointed backwards against the sliding.

Push It the Other Way

Now shove it in the opposite direction. It slides and stops again, so the force must have turned round with it. Whatever this force is, it does not have a fixed direction of its own: it always opposes whichever way the object is going.

Friction, Named

The force that appears when one object slides, or tries to slide, over another surface is called friction. It acts against the motion, or against the attempt at motion, and it needs two surfaces touching, which makes it a contact force.

Why It Exists

Where does it come from? From the surfaces themselves. Even a surface that looks perfectly smooth is covered in minute bumps and hollows. Press two of them together and the bumps of one sit in the hollows of the other, and sliding means dragging all of them past each other.

Rough and Smooth

That explains the next result immediately. The rougher the surfaces, the larger the bumps, and the harder they are to drag past one another. So friction is greater on rough surfaces and smaller on smooth ones, though it is never quite absent.

Five Floors, Five Distances

The chapter tests this properly. Push the same object with the same shove across glass, cloth, wood, a ceramic tile and sand, and it travels a different distance on each. Since only the surface changed, the surfaces must be what friction depends on.

Air and Water Too

One more extension, and it matters. Friction is not confined to solid surfaces. Air, water and other liquids all resist anything moving through them. That is why an aeroplane, a ship, a boat and a fast train are all given shapes meant to cut down the resistance they meet.

What To Carry Forward

A contact force acts only while two objects touch, directly or through something solid. Muscular force is the kind you make yourself, inside your body as well as outside it. Friction appears between touching surfaces, always opposes the motion, grows with roughness and never quite vanishes, in air and water as much as on a floor.

Scenario:

A class pushes one lunch box with the same gentle shove across five different surfaces: a glass sheet, a ceramic tile, a wooden desk, a stretch of cloth and a tray of sand. They mark where it comes to rest each time. On glass it slides furthest and on sand it barely moves. One pupil writes that the box has more force on glass. Another writes that there is no friction on glass at all, since the box kept going. A third asks what would happen if they pushed the box the other way round.

Analysis:

The box was given the same push every time. So what changed is the surface rather than the push. Both written conclusions over-read the result in the same direction. And the third pupil has asked the one question that tests which way friction points.

Activity:

Push one object across five surfaces with the same shove and mark where it stops each time.

Title:

Try it at home: find the grippiest floor

Safety:

Use a light box on a clear floor, and keep the pushes gentle so nothing slides into anybody.

What You Notice:

The shortest distance belongs to the roughest surface, and the box slows down whichever way you push it. That second result is the one that shows friction follows the motion rather than the push.

Some of your muscular force never leaves your body: muscles chew your food, move it along the digestive canal and drive the heart.

Friction is not confined to floors; air and water resist anything moving through them, which is why a fast train and a ship are shaped as they are.

Pushing an object the other way turns the friction round with it, which shows the force has no direction of its own.

TOUCH IT, AND TEST — A contact force exists only while the objects touch. Take the contact away and nothing is left.

FRICTION FOLLOWS THE MOTION — It always opposes the way the object is going. Push the other way and it turns round too.

A Force With a Gap In It

Everything in the last topic needed the two objects to touch. Is contact part of what a force is, or just a common way for one to arise? The answer is the second, and this topic is about the forces that reach across a gap with nothing solid in between.

Two Ring Magnets on a Stick

Stand a wooden stick upright on a table and slide a ring magnet down it so it rests at the bottom. Now slide a second ring magnet on above it, turned so that like poles face each other. The second magnet does not come down to meet the first. It stops short and hangs there.

Press It Down

Push the upper magnet down gently with a finger and you can feel it resist. Something is holding it up, and nothing solid is touching it from below. A push you can feel is a force by any definition.

Reverse the Poles

Turn both magnets over so unlike poles face each other instead, and the upper one drops straight down onto the lower. Like poles push apart and unlike poles pull together, which you met in Grade 6 and which works just as well with a gap.

Magnetic Force

Magnetic force is the name for what a magnet exerts on a second magnet, or on any magnetic material. Since it reaches across a gap without any contact, it is a non-contact force. The electromagnet from the earlier chapter of this book behaves the same way.

What a Magnet Acts On

It is worth being exact about what it reaches. A magnet attracts magnetic materials such as iron and steel. It does not pick up paper, plastic or copper, and that fact turns out to be the way of telling magnetism apart from the next force in the list.

Rub a Scale

Find a plastic straw, or a plastic scale, and rub it hard with a piece of polythene. Afterwards keep fingers and anything metal clear of the rubbed stretch. Now hold it just above some small scraps of paper on a table, close but not touching.

Why the Paper Jumps

The scraps leap up and cling to the scale. Something has changed about the scale, and it is not magnetism: a magnet would ignore paper entirely. Rubbing two particular materials together makes electrical charges build up on both of their surfaces.

Static, and Why

Those charges sit where they are rather than travelling anywhere, which is why they are called static charges. An object that has acquired them is called a charged object, and a charged object attracts uncharged things of certain materials, which is exactly what the paper scraps are.

Two Balloons

Now a second experiment, which answers a different question. Inflate two balloons and hang them side by side on threads so they do not quite touch. Rub both of them with the same woollen cloth and let them hang free.

They Push Apart

They swing away from each other. Both were rubbed the same way, so both have picked up the same sort of charge, and the same sort of charge pushes apart. That is a repulsion, and it needs no contact either.

Bring the Cloth Back

Now hold the woollen cloth near one of the rubbed balloons. The two move towards each other. The cloth and the balloon must therefore carry different sorts of charge, because different sorts attract.

Two Kinds of Charge

Put those two results together and you have something worth having. There are two kinds of static charge, called positive and negative. Like charges repel each other; unlike charges attract. And both objects in a rubbing get charged, with opposite kinds, which is why the cloth behaves as it does.

Electrostatic Force

The force a charged object exerts on another charged object, or on an uncharged one, is called electrostatic force. Like magnetic force it works across a gap, so it is a non-contact force too.

Charges That Move

One connection forward, which ties this chapter to an earlier one. When charges stop sitting still and move instead, they are an electric current. That is the same current that lights a lamp, warms a wire and makes a coil into a magnet.

What To Carry Forward

A force does not need contact. A magnet acts on magnets and on magnetic materials across a gap. A charged object acts on charged and uncharged things across a gap. Like repels and unlike attracts in both cases, and the test that tells them apart is simple: offer the object a steel clip and a scrap of paper and see which it lifts.

Scenario:

Two groups set up demonstrations side by side. The first slides two ring magnets onto a vertical wooden stick and the upper one hangs in the air above the lower one. The second rubs a plastic scale with polythene and holds it over scraps of paper, which leap up and cling to it. A visitor says both are magnetism, since both work across a gap. A pupil in the second group then holds the rubbed scale over a steel paper clip, and nothing happens at all.

Analysis:

Acting across a gap is what the two demonstrations have in common. It is not enough to make them the same force. The paper clip is the decisive test. A magnet would have picked it up at once, and the charged scale leaves it where it is.

Activity:

Rub a scale and test it on paper, on a steel clip and on a plastic button, and write down which it lifts.

Title:

Try it at home: charge a balloon

Safety:

Do not touch the rubbed part of a balloon with your fingers, and keep balloons away from faces when they burst.

What You Notice:

The two balloons push apart and the cloth pulls one towards it. Both balloons were charged the same way and the cloth the opposite way, which is how you know there are two kinds.

Both objects in a rubbing end up charged, and with opposite kinds, which is why the woollen cloth attracts the very balloon it just charged.

A charged object attracts uncharged things as well as charged ones, which is what the scraps of paper in the activity are.

Static charges that start moving are an electric current, the same current that lights a lamp, warms a wire or makes a coil into a magnet.

LIKE PUSHES, UNLIKE PULLS — True of magnetic poles and true of charges. One rule covers both halves of this topic.

CLIP OR PAPER? — Offer the object a steel clip and a paper scrap. A magnet takes the clip; a charged object takes the paper.

Throw It Up

Take a ball outside and throw it straight up. It comes back down. There is nothing surprising in that until you try to say what brought it back, because nothing visible touched it at any point in the flight.

Throw It Harder

So throw it harder. It goes higher and takes longer, and it still comes down. Think of anything you have ever thrown in any direction: it finished on the ground. Whatever is doing this does not get tired and cannot be escaped by throwing more firmly.

Something Pulls It Down

Since the ball changed speed and then changed direction, a force acted on it. Since everything falls towards the Earth, it is the Earth doing the pulling. That is the only candidate that explains why the rule has no exceptions.

Gravitational Force

What the Earth exerts on everything near it, drawing it inwards, is the gravitational force. It is also called the force of gravity, or just gravity, and all three names mean the same thing.

Only Ever a Pull

Here is where gravity differs from the two forces in the last topic. Magnets can attract or repel, depending on the poles. Charges can attract or repel, depending on the kinds. Gravity only ever attracts. Nothing is pushed away from the Earth.

No Contact Needed

And it reaches without touching. The Earth does not have to be in contact with a falling leaf to pull it, so gravitational force joins magnetic and electrostatic force as a non-contact force. That makes three.

Going Up, and Coming Down

Watch the whole flight carefully. Drop something from a height and it takes a straight path vertically downwards. Throw it straight up and it rises while slowing, stops for an instant at the top, then comes straight back down, speeding up as it falls. Motion in a vertical line under gravity like that is called vertical motion. Notice that gravity pulled downwards the whole time, including while the ball was rising, which is exactly why it slowed.

Weight

Now a second question: does the Earth pull every object just as hard? How hard it draws one particular object inwards is what we call the weight of that object. Weight is a measure of how strongly that object is being pulled.

The Unit of Weight

Weight is a force, so it is measured in the unit of force. The SI unit of weight is therefore the newton, the same as for any other force, and a weight is properly written with the symbol N after the figure.

Hang It From a Spring

To find out whether the pull is equal on everything, hang a spring from a nail and hook things onto it one at a time, say a tiffin box, a stone and a pencil box. The spring stretches each time, because each object is being pulled down by the Earth.

Different Objects, Different Stretch

And the stretch is not the same for all of them. Different objects stretch it by different amounts, which answers the question directly. The Earth does not pull everything equally, so different objects have different weights.

A Spring Balance

That observation is the whole idea behind a spring balance: a spring anchored at the top, a hook hanging below it and a scale alongside. Hang an object from the hook, read where the pointer comes to rest, and the scale gives you the weight in newtons. Many balances carry a second scale in grams as well, marked on the assumption that the balance is being used here on Earth.

Its Range

Before trusting any instrument, look at it. The balance in the chapter reads up to 10 N, so its range runs from zero to 10 N. Hang something heavier than that on it and you may damage it rather than measure it.

Its Smallest Division

Then find the smallest weight it can show. Between two big marks the difference is 1 N, and there are five small divisions in that gap. One small division is therefore a fifth of a newton. Another balance may have a different range and different divisions, which is exactly why looking first is the habit to build.

Weight Is Not Mass

Now the distinction this topic exists for. Mass says how much matter an object contains, measured in grams or kilograms, and it is the same everywhere. Weight is the gravitational pull on that object, measured in newtons, and it changes with where the object is.

The Same Kilogram, Four Places

One kilogram of matter stays one kilogram wherever you take it. Its weight does not. On Earth it is pulled with about 10 N; on the Moon with far less than that; on Mars with under half the Earth figure; on Jupiter with well over twice. Same matter, four different pulls.

What To Carry Forward

Gravity is the Earth pulling everything towards itself, always attracting, always without contact. Weight is that pull on one object, measured in newtons with a spring balance whose range and smallest division you check first. And weight is not mass: matter travels unchanged, and the pull on it does not.

Scenario:

A class hangs a spring from a nail and suspends three things from it in turn: a tiffin box, a small stone and a pencil box. The spring stretches by a different amount each time, and the stone stretches it most. A pupil records the stretches and writes that the stone has the most mass and so the Earth pulls everything equally. Another pupil says the spring is simply getting tired. A third asks what the readings would look like if the whole experiment were done on the Moon.

Analysis:

The different stretches are the result worth reading carefully. They are evidence about the pull rather than about the spring. The first pupil has stated the right observation and drawn the opposite conclusion from it. And the third has asked the question that separates weight from mass.

Activity:

Hang three objects from one rubber band in turn and mark how far each one stretches it.

Title:

Try it at home: a rubber band balance

Safety:

Keep the objects light, and stand clear in case the band snaps. Nothing heavy or sharp on the hook.

What You Notice:

The heaviest object stretches the band furthest, and the stretches are not in equal steps. Your band is doing the job of a spring balance, which is why a real one has a scale marked in newtons.

Gravity pulls downwards on a ball for the whole of its upward flight, which is the reason the ball slows down rather than carrying on.

A spring balance usually carries two scales, and the one in grams is marked on the assumption that you are using the balance here on Earth.

The same kilogram of matter is pulled with about ten newtons on Earth, far less on the Moon and well over twice as much on Jupiter.

GRAVITY ONLY PULLS — Magnets and charges can push or pull. Gravity has one direction and never pushes anything away.

MATTER TRAVELS, PULL DOES NOT — Mass goes to the Moon unchanged. The weight of that same mass does not.

Why Does Not Everything Sink?

Drop a handful of objects into water and some settle on the bottom while others sit on the surface. The Earth is pulling downwards on every one of them, as the last topic established. So why do some of them stay up?

The Mug That Feels Lighter

Here is a clue you have met without noticing. Dip a mug into a bucket to take water out and the mug feels lighter while it is under the surface than it does when you lift it clear. Nothing about the mug has changed in that second.

Push a Bottle Under

Try it properly. Take an empty bottle, screw the lid on tightly, and press it down into a bucket full of water. You can feel the water resisting you. The harder you press, the more it pushes back.

Let It Go

Now take your hand away. The bottle does not stay where you left it: it bounces back up to the surface. Something sent it there, and the only candidate in the bucket is the water.

Upthrust

So water applies a force to an object placed in it, and that force points upwards. It has two names, which are used interchangeably: upthrust, and buoyant force. Both mean the same upward push.

It Is Not Only Water

And it is not a peculiarity of water. Every liquid does this. That matters for the next part, because the amount of the push turns out to depend on which liquid you are using.

Two Forces, One Object

Now put the two topics together. An object sitting in a liquid has the Earth pulling it downwards and the liquid pushing it upwards at the same time. Whether it floats or sinks is settled by comparing those two.

When It Sinks

If the downward pull of the Earth is the larger of the two, the object goes down. The upward push is still there and still acting; it is simply not enough. A stone dropped in a bucket is pushed up the whole way to the bottom.

When It Floats

Where the two match exactly, the object floats. Nothing is left over to move it either way, so it stays where it is on the surface. That is the whole of the rule, and it explains the mug too: the upthrust takes part of the load off your hand, so the mug feels lighter without becoming lighter.

What Upthrust Depends On

One of the things the push depends on is the density of the liquid, which is a word for a later chapter of this book. For now it is enough to know that different liquids push with different strength, so an object can float in one and sink in another.

Archimedes

A Greek scientist, Archimedes, worked out the rule that makes this exact. When an object is put into a liquid, wholly or partly, the upward force on it equals the weight of the liquid that the object pushes out of the way. That statement is known as the principle named after him.

Displacement, Weighed

Read it carefully, because it turns a vague idea into something you could measure. If the liquid shoved aside weighs less than the object, the object sinks. If the liquid shoved aside weighs the same as the object, the object floats. Nothing else has to be known.

A Rock That Floats

A rock that floats sounds impossible, and there is one. Pumice forms when lava carrying a great deal of gas and water vapour cools very quickly, trapping tiny bubbles inside as it hardens. The result is a light porous rock, and it floats.

The Same Clay, Two Shapes

The experiment worth doing is with modelling clay. Rolled into a tight ball it sinks. Pressed into a wide shallow bowl and set gently on the water, the very same clay floats. Its matter never changed, so what changed was how much water it pushed aside, and therefore the upthrust on it.

What To Carry Forward

A liquid pushes upwards on anything placed in it, and that push is called upthrust or buoyant force. It acts on sinking objects as much as on floating ones. Compare it with the pull of the Earth: larger pull means sinking, equal forces mean floating. And the push equals the weight of the liquid pushed out of the way, which is why shape matters as much as matter.

That also closes the chapter tidily. It began with a force as a push or a pull between two interacting objects, and sorted the forces by whether contact is needed. Upthrust is the last of them, and the first that has to be weighed against gravity rather than simply listed beside it.

Scenario:

A group fills a bucket with water and tries three things. They push a tightly closed empty bottle under the surface and feel it resist, then let go and watch it leap back up. They drop a small stone in and it goes straight to the bottom. Finally they lower a mug of water with their hand still on the handle, and the mug feels lighter than it did in the air. One pupil writes that the water pushes up on the bottle and the mug but not on the stone, since the stone sank.

Analysis:

The three results look like three different rules and are one rule applied three times. The upthrust acts in every case; what differs is how it compares with the pull of the Earth on that object. The mug is the useful one, because the load changed while the mug did not.

Activity:

Hold a closed bottle under water, let it go, then try the same with a stone and write down what differs.

Title:

Try it at home: make the heavy thing float

Safety:

Water in a bowl on a steady surface, and mop up spills so nobody slips.

What You Notice:

The same clay sinks as a ball and floats as a bowl. Its mass never changed, so what changed is the upthrust, which is why a steel ship floats when a steel bolt does not.

A liquid pushes upwards on a sinking stone the whole way to the bottom; the push is simply smaller than the pull of the Earth on it.

A mug feels lighter under water without becoming lighter, because the upthrust takes part of the load off your hand.

Pumice is a rock that floats, because lava full of gas and water vapour cooled quickly and set with the bubbles still inside it.

COMPARE THE TWO ARROWS — Pull down against push up. Larger pull means sinking and equal forces mean floating.

WEIGH WHAT MOVED — The upward push equals the weight of the liquid the object pushed out of the way.