Exploring Forces

Think about the first ten minutes of an ordinary school day. You push a blanket aside, pull open a cupboard, press a switch, lift a bag on to your shoulder, and kick a stone out of the way on the road. Every one of those actions is either a push or a pull. Science gives all of them a single name: force.

The first thing to notice is that a force is never a lonely thing. Something applies it and something receives it. Your hand and the cupboard door. The bat and the ball. The Earth and a falling mango. If you cannot point to both objects, you have not yet found a real force. So a good habit while answering questions is to finish the sentence: this object pushes (or pulls) that object.

The second thing to notice is that you never actually see a force. You see the result of it: a trolley that starts rolling, a rubber band that stretches, a shuttlecock that turns back in the air. A force is known only by what it does.

To describe a force completely you need two pieces of information.

• How strong it is. A gentle nudge and a hard shove are both pushes, but they are not the same force.
• Which way it acts. Pushing a cart forward and pulling it backward are opposite in effect, even if the two are equally strong.

Force is measured in a unit called the newton, written with the symbol N. A newton is a small amount of force: the Earth pulls a one-kilogram packet of sugar with a force of a little under 10 N. In the laboratory a spring balance measures force directly. The harder you pull its hook, the more the spring inside stretches, and the further the pointer moves along the scale.

When more than one force acts on the same object along the same line, the forces combine into a single net force.

• Forces acting in the same direction add up. Two friends pushing a stalled scooter, one with 60 N and the other with 40 N, together give a push of 100 N.
• Forces acting in opposite directions subtract. If you pull a box towards you with 30 N while a friend pulls it the other way with 20 N, the two pulls leave 10 N acting towards you. Friction from the floor pushes back as well, so the box actually moves your way only if that leftover 10 N is bigger than the friction.

One last caution about the word itself. In everyday speech we say a leader has a forceful personality, or that someone was forced to do something. That is not the meaning here. In science, force has one narrow and very useful meaning: a push or a pull that one object applies to another.

Forces are sorted into two big families, and the test is simple. Does the object applying the force have to touch the object receiving it? If the answer is yes, it is a contact force.

Muscular force is the force we apply using our muscles. Lifting a bucket of water from a well, pedalling a bicycle, kneading dough, pushing a door shut and carrying a younger cousin are all muscular force at work. Animals use it too: bullocks pulling a plough, a horse pulling a cart, a camel carrying a load across sand.

Muscular force is not only for the work you can see. Inside your body, muscles are pushing all day long. The muscular wall of the food pipe squeezes food downward towards the stomach, the muscles of the heart push blood out into the blood vessels, and the muscles of the ribs and the diaphragm move air in and out of the lungs. All of that is muscular force, and all of it is a contact force, because each muscle presses on something it is in contact with: the food pipe squeezes the food itself, the heart squeezes the blood inside it, and the rib muscles and the diaphragm pull on the walls of the chest, widening and narrowing the space that the lungs sit in.

Friction is the second contact force you meet everywhere. Whenever one surface slides, or even tries to slide, over another, friction acts along the surfaces and opposes that motion. Push a book across a table and let go: it slows down and stops, because friction is pushing back on it the whole time. Friction gets two sections of its own later, because it is both a helper and a nuisance.

Three other contact forces are worth naming, because questions often describe them without using the word force at all.

• The push of a surface. A book lying on a table is not sinking through the table, so the table must be pushing up on the book. Any firm surface pushes outward on whatever rests against it.
• The pull of a stretched spring, rope or rubber band. Stretch a rubber band and you can feel it pulling back on your fingers. The rope that lifts a bucket out of a well applies this kind of force to the bucket.
• The resistance of air and water. Air pushes back on a cyclist, and water pushes back on a swimmer. These are contact forces too, because the moving object is touching the air or the water. Together they are called fluid friction, or drag.

Here is a quick way to check yourself. Imagine a thin gap suddenly opening between the two objects. If the force would stop at that instant, it was a contact force. If it would keep acting across the gap, it belongs to the family in the next section.

Some forces do their work across a gap. Nothing touches, yet the pull or the push is completely real. These are non-contact forces, and there are three you should know well.

Magnetic force. Bring a bar magnet slowly towards an iron nail lying on a table and the nail jumps across to the magnet before the two ever meet. A magnet attracts objects made of iron, and also of nickel and cobalt. Magnets act on each other as well: two like poles, north with north or south with south, push each other apart, while two unlike poles, north with south, pull each other together. The freely turning needle of a compass swings around and settles pointing north because of this force, with nothing touching it at all.

Electrostatic force. Rub a plastic comb or a plastic scale briskly on dry hair and hold it just above tiny bits of torn paper. The paper bits leap upward and stick to the comb. The rubbing has given the comb an electric charge, and a charged object attracts light uncharged objects lying near it. Charged objects also act on one another: two objects carrying the same kind of charge push each other apart, and two carrying opposite kinds of charge pull each other together. This is why a balloon rubbed on your hair will cling to a wall, and why your hair itself sometimes rises and follows the comb. On a dry winter day the effect is strong; on a humid day it is weak, because the charge leaks away into the moist air.

Gravitational force. Let go of a pen and it falls. Throw a ball straight up and it slows, stops and comes back down. Water in a river runs downhill. In every one of these cases the Earth is pulling the object towards itself, and it does so without touching it. This pull is the force of gravity. It acts on every object, everywhere, at every moment, including an object sitting perfectly still on the ground, and it works even where there is no air.

Gravity is not only an Earth thing. Every object attracts every other object. Two school bags sitting on a bench attract each other as well, but that pull is far too tiny to notice. You feel gravity only when one of the two objects is enormous, like the Earth, the Moon or the Sun.

The force with which the Earth pulls an object is called the weight of that object, and being a force, it is measured in newton. Weight is not the same thing as mass. Mass is the quantity of matter in an object, it is measured in kilogram, and it stays the same wherever you carry the object. Weight is a pull, so it becomes smaller in a place where the pull of gravity is weaker, such as on the Moon.

All three non-contact forces share one habit worth remembering: they grow weaker as the two objects move further apart. Hold the magnet far from the nail and nothing happens at all.

Once a force acts, what actually changes? At this level, everything a force can do fits neatly into three effects, and very often two of them happen together.

1. A force can change the speed of an object. This one effect covers four everyday situations.

• Starting motion. A football lying still on the ground stays still until somebody kicks it.
• Speeding up a moving object. A cyclist who pedals harder goes faster, and wind pushing a sailing boat from behind makes it move quicker.
• Slowing down a moving object. Squeeze the brake lever and the bicycle loses speed.
• Stopping motion. A goalkeeper brings a fast-moving ball to rest with her hands.

2. A force can change the direction of motion. Here the object may keep moving just as fast, but along a new path. A batter meets a ball coming towards her and sends it away towards the boundary. A carrom striker bounces off the side of the board and travels off in a new direction. A fielder deflects a ball with an outstretched hand. A cycle turns a corner because the road pushes sideways on its tyres.

3. A force can change the shape or the size of an object. Press a lump of dough and it flattens. Stretch a rubber band and it becomes longer and thinner. Squeeze an empty plastic bottle and it caves in. Sit on a cushion and it sinks. A potter shaping clay on a wheel is using force mainly for this third effect, since the clay stays on the wheel the whole time.

Two ideas are worth adding to this list.

First, one force can produce more than one effect at the same time. When a bat strikes a ball, the ball changes direction, changes speed, and is squashed flat for a fraction of a second before springing back into its round shape. All three effects, from a single hit.

Second, a force does not always produce a change you can see. Push hard against a classroom wall and nothing appears to happen. The wall does not move and it does not look any different. The force is certainly real, as your tired arms will tell you, but the change it produces is far too small to notice. Whether a force actually produces motion depends on what else is acting on the object at the same moment, and that is exactly the idea in the next section.

Keep this short checklist ready: speed, direction, shape. Whenever a question asks what a force has done, the answer is one or more of these three.

Objects usually have several forces acting on them at once. What happens next depends entirely on whether those forces cancel each other out.

Balanced forces are forces that cancel, so the net force on the object is zero. When the forces on an object are balanced, its state of motion does not change. An object at rest stays at rest, and an object already moving keeps moving in the same direction at the same speed.

Three everyday pictures of balanced forces:

• A book lying on a table. The Earth pulls it down; the table pushes it up by exactly as much. The book stays exactly where it is.
• A tug of war in which neither team is winning. Each side pulls the rope equally hard in opposite directions, and the knot in the middle stays over the same spot on the ground.
• A car driven at a steady speed along a straight, level road. The forward push that keeps it going is exactly cancelled by friction and air resistance, so the speed does not change.

Look at that tug-of-war rope carefully, because it teaches something students often miss. The rope is not moving, so the forces on it are balanced, and yet the rope is stretched tight and is slightly longer than it was before. Balanced forces cannot change the state of motion, but they can still change the shape of an object. Squeeze a rubber ball equally hard from both sides and you will see the same thing happen.

Unbalanced forces are forces that do not cancel. Something is left over, and the object responds to whatever is left. An unbalanced force can start a still object moving, speed up or slow down a moving object, stop it altogether, or turn it on to a new path.

Working this out is simple arithmetic as long as the forces act along one line. Suppose a crate is pushed to the right with 20 N while friction pushes it to the left with 12 N. The net force is 20 N minus 12 N, that is 8 N to the right, so the crate speeds up towards the right. If friction were also 20 N, the net force would be zero, the forces would be balanced, and a crate at rest would simply refuse to budge no matter how red your face got.

This idea clears up something that puzzles a lot of students. A cycle rolls along a flat road and slowly comes to a stop even though nobody touched it. Nothing pushed it backwards on purpose. But once the rider stops pedalling, friction from the road and resistance from the air are no longer balanced by anything, and that leftover force quietly eats away the speed until the cycle halts.

Friction is the force that opposes the sliding of one surface over another surface it is touching. It always acts along the surfaces, and it always acts in the direction that opposes that sliding, whether the sliding is already happening or is only being attempted. Push a box to the right and friction on that box acts towards the left.

Where does friction come from? No surface is perfectly smooth. A sheet of glass, a marble floor, even a polished metal plate is covered with tiny bumps and hollows, far too small for your eye to catch. When two surfaces are placed together, the bumps of one settle into the hollows of the other. To make the surfaces slide, all these interlocked bumps have to be dragged past one another, and that resistance is what we feel as friction. This single picture explains two useful rules at once.

• Rougher surfaces give more friction, because the bumps are larger and lock together more deeply. Dragging a box across a rough cement floor is far harder than dragging it across smooth marble.
• Pressing the surfaces together harder gives more friction, because the bumps are forced into each other more tightly. An almirah packed with books is much harder to push than the same almirah when it is empty.

Friction turns up in three forms, depending on what the object is doing.

• Static friction acts while the object is still not moving. Push a heavy table gently and it does not shift at all, because static friction pushes back with exactly as much force as you are applying. Push harder and harder, and at some point the table finally breaks free.
• Sliding friction acts once the object is actually sliding. You may have noticed that after a heavy table finally starts moving, keeping it moving feels a little easier than starting it did. That is because sliding friction is usually somewhat smaller than the largest static friction.
• Rolling friction acts when an object rolls over a surface instead of sliding along it. For the same pair of surfaces, rolling friction is much smaller than sliding friction. This one fact is the reason behind wheels, rollers, trolleys, suitcases with wheels and ball bearings.

There is a fourth member of the family. When an object moves through air or water, the fluid pushes back against it. This is fluid friction, usually called drag. It is why a cyclist feels the air shoving against her face, why running inside a swimming pool is exhausting, and why drag builds up quickly as speed increases.

Finally, a warning about a common confusion. Friction is not a substance stored inside an object, and no object simply has friction on its own. Friction appears only where two surfaces meet, and only while one of them is sliding or trying to slide over the other.

Friction is sometimes described as a necessary evil, and the description is fair. We could not manage a single day without it, and it also costs us a great deal.

Where friction helps us

• Walking. Your foot pushes backwards against the ground and friction gives it grip. Without friction your foot would simply slip, which is exactly what happens on a wet marble floor or on a banana peel.
• Stopping. The brakes of a cycle, a scooter or a car work by pressing a pad hard against a moving part, and friction between them removes the speed. The grip of the tyres on the road does the rest of the job.
• Holding and joining. Friction lets you hold a glass of water without it sliding out of your fingers, lets a knot stay tied, keeps a nail sitting in a wall and stops a screw from working itself loose. Wet the same glass and it turns slippery, because the film of water lowers the friction.
• Writing. A pencil leaves a mark only because graphite rubs off on the paper as the tip drags across it. Writing with a pencil on a very smooth glazed surface hardly works.
• Lighting a matchstick. Rubbing the match head along the rough strip produces enough heat to start the flame.

Where friction costs us

• Wear and tear. Shoe soles get worn smooth, tyres wear out, the teeth of gears slowly wear down, and the pages of a much-used book fray at the corners. Friction rubs material away.
• Heat and wasted energy. Rub your palms together quickly and they get warm. The same thing happens in every joint and bearing of a machine, so part of the fuel or electricity supplied is lost as heat instead of doing useful work.
• Extra effort. Moving heavy loads takes far more force than it otherwise would, because friction has to be overcome first.

How we increase friction when we want a better grip

• Grooves called treads are cut into the soles of shoes and into the tyres of vehicles.
• Sprinters and footballers wear shoes with spikes or studs that dig into the ground.
• Sand or ash is spread over a slippery road or a wet floor.
• Handlebars, bat handles and tool grips are covered with rubber.
• Gymnasts rub a coarse chalk powder on their hands, and some players rub soil on their palms, for a firmer hold. The powder has to be a rough one; a fine, smooth powder like talcum does the opposite and makes the hands slip.

How we reduce friction when it only wastes effort

• Lubricants such as oil, grease and graphite powder spread a thin layer between the surfaces and hold them slightly apart, so the bumps can no longer lock together. In some machines a cushion of air does the same job.
• Wheels, rollers and ball bearings replace sliding with rolling, and rolling friction is much smaller. Ball bearings are exactly this trick, built into the moving parts of cycles, fans and machines.
• Polishing a surface flattens the bumps and so lowers the friction.
• Streamlining cuts down drag. Boats, aeroplanes, fast trains and racing cars are given smooth, tapering shapes for this very reason, and fish and birds have such shapes naturally.

Now try imagining a world with no friction at all. The slightest slope would send everything sliding off a shelf, no knot would hold, no vehicle could start or stop, and you could not take a single step. We do not really want friction removed. We want the right amount of it, in the right place.