Friction

Friction is the force that stops things from moving easily when they touch each other. It happens when solid things, like wood, or fluids, like oil, slide or press together. There are different kinds of friction, such as dry friction, fluid friction, and internal friction. People have studied friction for more than 2,000 years, and this study is called tribology.

Friction can have big effects. For example, rubbing wood together can make enough heat to start a fire. But friction can also cause wear, meaning things can get damaged or work worse over time. Many things affect friction, like tiny bumps on surfaces or changes in the materials. When two objects move against each other, some energy turns into heat or is lost. This loss of energy makes it hard for things to keep moving. Because friction is complex, it is often easier to study it by looking at real examples.

Types

Friction is the force that makes it hard to move things when they touch each other. There are several kinds of friction:

Dry friction happens when two solid things rub against each other. It can stop things from starting to move or slow them down when they are moving.
Fluid friction occurs when layers of a thick liquid, like oil or honey, move past each other.
Lubricated friction is when a special liquid, called a lubricant, is put between two solids to help them slide more easily.
Skin friction is the force that slows down a body moving through a fluid, like water or air.
Internal friction is the resistance inside a solid material when it is being bent or stretched.

History

Many ancient thinkers like Aristotle, Vitruvius, and Pliny the Elder studied friction. They noticed differences between moving things and things that are still.

Later, Leonardo da Vinci discovered important rules about friction, but they were not shared until much later. Guillaume Amontons rediscovered these rules in 1699. Other scientists like Charles-Augustin de Coulomb added more understanding. They looked at how materials, surface area, pressure, and time affect friction.

Scientists also learned that friction creates heat. For example, Julius Robert Mayer showed that rubbing things together makes them hot. By the 1900s, researchers could study friction on very tiny scales.

Dry friction

Dry friction stops two solid objects from moving against each other. There are two types: static friction between objects that aren’t moving, and kinetic friction between objects that are moving.

Friction always works against movement. For example, a stone sliding on ice feels a force slowing it down. When a car accelerates, the tires push forward against the road, and friction pushes the car forward.

Scientists discovered the basic rules of kinetic friction a long time ago. These rules are:

When the mass is not moving, the object experiences static friction. The friction increases as the applied force increases until the block moves. After the block moves, it experiences kinetic friction, which is less than the maximum static friction.

The force of friction depends on how hard the objects are pressed together.
The force of friction does not change based on how much area is touching.
Kinetic friction does not change based on how fast things are moving.

Coulomb friction, named after Charles-Augustin de Coulomb, helps us estimate the force of dry friction. It depends on two things: how hard the objects are pressed together, and a special number called the coefficient of friction that changes based on what the objects are made of.

Static friction

Static friction stops objects from starting to move. For example, it keeps a box from sliding down a slope. The force of static friction can change up to a certain maximum.

When you push on an object and it doesn’t move, static friction matches your push exactly. Only when your push gets too strong does the object start to move, and then static friction turns into kinetic friction.

An example of static friction is a car wheel rolling without slipping. The part of the tire touching the road isn’t moving relative to the road, so it’s static friction.

Kinetic friction

Kinetic friction happens when two objects rub against each other while moving, like a sled sliding on the ground. The force of kinetic friction depends on how hard the objects are pressed together.

Role of the normal force

The normal force is the force that pushes two surfaces apart. For an object resting on a flat surface, this is just the force of gravity pulling it down. The force of friction depends on this normal force.

Role of angle

Sometimes it’s helpful to think about friction in terms of angles. The angle of friction tells us how steep a slope can get before an object will start to slide.

Coefficient of friction

Materials	Static Friction, μ s {\displaystyle \mu _{\mathrm {s} }}	Kinetic/Sliding Friction, μ k {\displaystyle \mu _{\mathrm {k} }\,}
Dry and clean	Lubricated	Dry and clean	Lubricated
Aluminium	Steel	0.61		0.47
Aluminium	Aluminium	1.05–1.35	0.3	1.4–1.5
Gold	Gold			2.5
Platinum	Platinum	1.2	0.25	3.0
Silver	Silver	1.4	0.55	1.5
Alumina ceramic	Silicon nitride ceramic				0.004 (wet)
BAM (Ceramic alloy AlMgB₁₄)	Titanium boride (TiB₂)	0.04–0.05	0.02
Brass	Steel	0.35–0.51	0.19	0.44
Cast iron	Copper	1.05		0.29
Cast iron	Zinc	0.85		0.21
Concrete	Rubber	1.0	0.30 (wet)	0.6–0.85	0.45–0.75 (wet)
Concrete	Wood	0.62
Copper	Glass	0.68		0.53
Copper	Steel	0.53		0.36	0.18
Glass	Glass	0.9–1.0	0.005–0.01	0.4	0.09–0.116
Human synovial fluid	Human cartilage		0.01		0.003
Ice	Ice	0.02–0.09
Polyethene	Steel	0.2	0.2
PTFE (Teflon)	PTFE (Teflon)	0.04	0.04		0.04
Steel	Ice	0.03
Steel	PTFE (Teflon)	0.04−0.2	0.04		0.04
Steel	Steel	0.74−0.80	0.005–0.23	0.42–0.62	0.029–0.19
Wood	Metal	0.2–0.6	0.2 (wet)	0.49	0.075
Wood	Wood	0.25–0.62	0.2 (wet)	0.32–0.48	0.067–0.167

Sources of friction

Friction happens because of many processes that lose energy. A simple way to think about it is that tiny bumps on surfaces, called asperities, touch each other. When you push harder, more of these bumps touch, which makes friction stronger. When these bumps move past each other, they change shape in different ways, which creates the force we call friction.

Studies show that friction and wear depend on more than just how things are pressed together. The tiny changes near the surface, the bits of material that break off, and chemical changes all play a part. These factors can change how materials wear down. The pieces of material that form between surfaces can either make things wear faster or protect them by stopping the surfaces from touching directly. Different conditions, like how hard you push or how fast you move, can change how friction behaves. Environmental factors can also affect how materials crack and how friction changes over time.

Breakdown of the Coulomb model

The Coulomb model is a simple way to understand friction. It works well in many computer programs that study how objects move or how grains of sand behave.

This model says that only a tiny part of two touching surfaces actually touches each other. It also says that the force of friction depends on how hard the surfaces are pushed together, not on how big the touching area is. Even though this model isn’t perfect, it is useful for many real-world problems.

However, the Coulomb model doesn’t always work. For example, it doesn’t work well when surfaces really stick together, like with adhesive tape or special tires used in drag racing. Even so, it remains a helpful tool for many engineering and science tasks.

Dry friction and instabilities

Dry friction can change how machines work when they need to stay steady. This happens because friction changes with speed, heat from rubbing can alter materials, or stretchy materials moving together can create new actions.

These changes can make car brakes squeak or create sounds in musical instruments like violins. Scientists have found that friction can create new patterns where surfaces rub together, helping some materials last longer by reducing wear.

Friction at the nanoscale

Main article: Nanotribology

In 2008, scientists moved a single atom across a surface for the first time and measured the forces needed. They used special tools and very cold temperatures to move a cobalt atom and a carbon monoxide molecule over copper and platinum.

By 2012, researchers found something surprising: under certain conditions, friction can increase when the force pushing the objects together decreases. This was seen when moving a tiny tool over a special material called graphene with oxygen present. Normally, more force means more friction, but this study showed the opposite can happen.

Friction at this tiny level happens because creating new surfaces takes energy, which is released as heat. Some materials, like a special form of graphite, can have almost no friction at all, a state called superlubricity.

Fluid friction

Main articles: Viscosity and Drag (physics)

Fluid friction happens when layers of fluid move past each other. This resistance to movement is called viscosity. In simple terms, viscosity is how "thick" a fluid feels. Water is thin and has low viscosity, while honey is thick and has high viscosity. The less thick a fluid is, the easier it moves.

All real fluids, except superfluids, have some resistance when they move, which means they are viscous. For learning, people sometimes talk about an ideal fluid that has no resistance and is not viscous.

Lubricated friction

Main article: Lubrication

Lubricated friction happens when a liquid stops two solid things from touching. We use something called a lubricant to help reduce damage to parts that move. This liquid makes pressure that helps the objects move smoothly without rubbing hard against each other. Good lubrication helps machines work well with little wear and tear. If lubrication stops working, the parts can rub together hard, making heat and possibly breaking the machine.

Skin friction

Main article: Parasitic drag

Skin friction happens when a fluid, like air or water, touches the surface of an object. This friction depends on how much of the object’s surface is in contact with the fluid. It gets stronger as the object moves faster.

We can reduce skin friction in two main ways. First, we can shape the object so that the fluid flows smoothly around it, like the curved shape of an airplane wing. Second, we can make the object shorter and thinner to help lower friction.

Internal friction

Main article: Plastic deformation of solids

Radiation friction

In 1909, Einstein said that light could push on objects, creating something called "radiation friction." When an object moves, more light pushes back on the front of it than on the back. This makes a force that slows the object down. The faster the object moves, the stronger this slowing force becomes.

Other types of friction

Rolling resistance

Main article: Rolling resistance

Rolling resistance is the force that makes it hard for a wheel or round object to roll on a surface. This happens because the object or the surface gets a little squished. Usually, rolling resistance is weaker than the force that stops things from sliding.

Braking friction

Any wheel with brakes can create a strong force to slow down or stop a vehicle or machine. Braking friction is different from rolling friction because braking friction is made strong on purpose by choosing special materials for brake pads.

Triboelectric effect

Main article: Triboelectric effect

When you rub two things together, they can share tiny particles called electrons or ions. This sharing uses up some of the energy that would otherwise be used for moving. It can also build up electrostatic charge.

Belt friction

Main article: Belt friction

Belt friction is what happens when a belt wraps around a pulley and one end is pulled. This creates tension on both ends of the belt. Engineers use this to figure out how many times the belt needs to wrap around the pulley so it doesn’t slip. People who climb mountains or work on sailing boats use this idea all the time.

Friction reduction

Devices like wheels, ball bearings, roller bearings, and air cushion or other types of fluid bearings can change sliding friction into a much smaller type of rolling friction.

A common way to reduce friction is by using a lubricant, such as oil, water, or grease, which is placed between two surfaces. This often makes friction much smaller. The science of friction and lubrication is called tribology.

Applications

Friction is very important in many areas of engineering.

Transportation

Automobile brakes use friction to slow down cars by turning motion into heat. Disk brakes work by squeezing brake pads against a spinning disc, while drum brakes press brake shoes against a spinning cylinder. Disc brakes work better because they cool more easily.
The grip between train wheels and rails, called rail adhesion, depends on friction.
How slippery roads are is a key safety factor for cars. Split friction happens when one side of a car has very different grip than the other, which can be dangerous. The texture of the road also changes how tires grip the surface.

Measurement

A tribometer is a tool that measures how much friction a surface has.
A profilograph measures how rough a road surface is.

Household usage

Rubbing a matchstick against a special surface creates enough friction to light it.
Sticky pads help keep things from sliding off smooth surfaces by increasing the grip between the object and the surface.