Friction

When one object moves, or tries to move, over the surface of another, there is always a force that opposes the motion. This force that opposes the relative motion between two surfaces in contact is called friction. Friction always acts along the surfaces in contact and in the direction opposite to the motion (or attempted motion). It is the reason a rolling ball eventually stops, a sliding box slows down, and we are able to walk without slipping. Friction is caused by the roughness of surfaces: even surfaces that look smooth have tiny bumps and hollows that interlock and resist sliding.

There are three main types of friction, depending on how the surfaces move over each other. Static friction is the friction that acts when an object is at rest and a force is trying to move it but it has not yet moved. It is what we must overcome to start an object moving. Sliding friction (also called kinetic friction) is the friction that acts when an object is actually sliding over a surface. Rolling friction is the friction that acts when an object rolls over a surface, as a wheel or ball does.

These three types differ in strength (magnitude). Static friction is the largest — it is generally a little harder to start an object moving than to keep it moving. Sliding friction is smaller than static friction, which is why, once a heavy box starts sliding, it becomes a bit easier to keep pushing it. Rolling friction is the smallest of the three — much smaller than sliding friction. This is why it is far easier to roll a heavy drum than to slide it, and why wheels and ball bearings are used to reduce friction in machines and vehicles.

So the order of these frictional forces, from greatest to least, is: static friction > sliding friction > rolling friction. Knowing the types of friction and their relative sizes helps us understand why it is hard to start moving a heavy object, why sliding is easier than starting, and why rolling is the easiest of all. This understanding is the basis for studying what affects friction, its advantages and disadvantages, and how it can be increased or reduced.

The amount of friction between two surfaces is not fixed — it depends on certain factors. By understanding what affects friction, we can predict when friction will be large or small, and we can control it (increase it when we need grip, or reduce it when it gets in the way). The two main factors that affect friction are the nature of the surfaces in contact and the force pressing the surfaces together (the normal force).

The first factor is the nature (roughness or smoothness) of the surfaces in contact. Rougher surfaces have more and larger bumps that interlock, so they produce more friction. Smoother surfaces have fewer, smaller irregularities, so they produce less friction. For example, there is much more friction between a tyre and a rough, gravelly road than between the tyre and a smooth, wet road. This is why polishing or oiling a surface, which makes it smoother, reduces friction, while a rough surface like sandpaper has very high friction.

The second factor is the force pressing the two surfaces together, often called the normal force. The harder the surfaces are pressed together, the more their bumps interlock, and so the greater the friction. A heavier object presses down harder on the surface beneath it, so it experiences more friction than a lighter object on the same surface. This is why it is harder to push a heavily loaded box than an empty one across the same floor, and why pressing a brake pad harder against a wheel produces more friction and stops it faster.

It is also useful to know what does not affect friction much: for ordinary surfaces, the area of contact has little effect on the friction — what matters is the roughness and the force pressing the surfaces together, not how large the touching area is. By controlling the two main factors — choosing rough or smooth surfaces, and changing how hard the surfaces are pressed together — we can increase or decrease friction as needed. This understanding leads us to study the advantages and disadvantages of friction and the ways to change it.

Friction is often thought of as a nuisance because it slows things down, but in fact friction is essential for everyday life. Without friction, the world would be impossible to live in — we could not walk, hold things, write, or stop a moving vehicle. Friction has many important advantages, and in countless situations we actually need and increase friction on purpose.

One of the most basic advantages is that friction allows us to walk. When we walk, our feet push backward against the ground, and the friction between our shoes and the ground pushes us forward and stops our feet from slipping. On a surface with very little friction, such as smooth ice or a freshly polished floor, walking becomes difficult and we tend to slip. Friction also lets vehicles move and stop: friction between the tyres and the road gives the grip needed to drive, turn, and especially to brake safely.

Friction is also what lets us hold and grip objects. The friction between our fingers and an object stops it from slipping out of our hands; this is why we can hold a pencil, grip a doorknob, or carry a glass. Friction makes writing possible — the friction between a pen or pencil and the paper lets the ink or graphite mark the page; on a very smooth, oily surface a pen would simply slide without writing. Striking a matchstick to light it, tying knots and shoelaces, and the grip of a nail in wood all depend on friction.

Because friction is so useful, we often deliberately increase it where we need more grip. The soles of shoes and tyres are made with grooves and treads to increase friction; sportspersons wear spiked shoes for better grip; sand is spread on slippery or icy roads; and rough handles are used on tools. Far from being only harmful, friction is a friend that makes movement, grip, writing, and safety possible. The challenge is to have the right amount of friction — enough where we need grip, and as little as possible where it wastes energy.

Although friction is very useful, it also has important disadvantages, especially in machines and moving parts, where friction is often unwanted. In these situations, friction wastes energy, damages parts, and reduces efficiency. Understanding these drawbacks helps us see why engineers work hard to reduce friction wherever it is not needed.

One major disadvantage of friction is wear and tear. When surfaces rub against each other, friction gradually wears them away — this is why the soles of shoes wear thin, tyres become bald, and the moving parts of machines wear out and need replacing. A second disadvantage is the production of heat. Friction between rubbing surfaces produces heat, which can be wasteful and even harmful; for example, the parts of a fast-running machine can become very hot, and rubbing our hands together makes them warm. A third disadvantage is the loss (waste) of energy and reduced efficiency: because friction opposes motion, extra energy (and fuel) must be used to overcome it, so part of the energy supplied to a machine is wasted in working against friction instead of doing useful work.

Because of these drawbacks, we use several methods to reduce friction in machines and moving parts. Lubrication — applying oil, grease, or other lubricants between surfaces — fills the tiny gaps and forms a slippery layer so the surfaces slide easily; this is the commonest method, used in engines, hinges, and machinery. Ball bearings and roller bearings are used to change sliding friction into the much smaller rolling friction in wheels, fans, and machine shafts. Polishing surfaces makes them smoother, reducing friction.

Two further methods are also important. Streamlining is giving objects a smooth, tapered shape so that they move through air or water with less fluid friction (drag) — this is why cars, aeroplanes, ships, and even fast trains have smooth, pointed shapes. In some cases, using wheels to replace sliding with rolling reduces friction greatly. By choosing the right method — lubricating, using bearings, polishing, or streamlining — we can cut down the wear, heat, and energy loss caused by friction, making machines and vehicles work better and last longer. The aim is always to keep friction where it helps and reduce it where it harms.

Friction does not occur only between solid surfaces; it also occurs when an object moves through a fluid (a liquid or a gas). The friction (resistance) that a fluid exerts on an object moving through it is called fluid friction or drag. When you swim, you feel the water resisting your movement; when you run or cycle fast, you feel the air pushing back against you; and a falling raindrop is slowed by air resistance. In each case, the fluid is exerting fluid friction (drag) that opposes the object's motion.

Fluid friction depends on a few things: the speed of the object (the faster it moves, the greater the drag), the shape of the object, the area it presents to the fluid, and the nature of the fluid (a thick fluid like honey resists more than a thin one like air). Because drag increases with speed and with the area facing the fluid, fast-moving vehicles and animals experience large drag forces that they must overcome — wasting energy — unless they are designed to reduce it.

The main way to reduce fluid friction is by streamlining — giving an object a smooth, tapered shape (rounded at the front and narrowing toward the back) so that the fluid flows smoothly around it instead of being pushed aside. A streamlined shape lets the object cut through the fluid with much less drag. This is why cars, buses, aeroplanes, ships, fast trains, and rockets are built with smooth, streamlined shapes, allowing them to move faster while using less fuel.

Nature shows streamlining too. Fish have smooth, tapered bodies that let them glide through water with little resistance; birds have streamlined bodies and wings for efficient flight; and fast-swimming animals like dolphins are beautifully streamlined. So fluid friction (drag) is the resistance offered by liquids and gases to objects moving through them, and streamlining is the clever shaping — used by engineers and found in nature — that reduces this drag. With this, we complete our study of friction: a force that is sometimes our helper and sometimes our hindrance, always present whenever surfaces or fluids meet moving objects.