Friction

A force called friction works when things rub together.

This force tries to stop things from sliding. It can happen between solid objects. It can even happen in liquids.

Tiny bumps on surfaces make friction happen. These bumps catch on each other. This can make things feel warm. Rubbing wood together can start a fire.

Friction can also cause wear. This means things might get damaged. It can make parts break over time. Friction is a very strong force. It is all around us.

Friction is a force that resists motion. It happens when surfaces rub or grind together. This can occur between solid objects or layers of liquid.

Most surfaces are not perfectly smooth. They have tiny bumps called asperities. These bumps catch on each other. This makes it hard for objects to slide.

There are different kinds of friction. Dry friction happens between two solid surfaces. It has two parts. Static friction keeps things from moving. Kinetic friction happens when things are already sliding.

Friction can also happen in fluids. A fluid is a liquid or a gas. This is called fluid friction. You might also see skin friction. This is a part of drag. Drag is the force that resists a fluid moving across a surface.

Friction can change energy. When things rub, they make heat.

Rubbing wood together can even start a fire. Friction also causes wear. This can damage parts over time. In fact, friction uses about 20% of the world's total energy.

Friction is a force that resists motion. It happens when solid surfaces, liquid layers, or other materials rub or grind against each other.

This force is very important in our daily lives. It can help us, like when we rub wood together to start a fire. However, it can also cause wear and damage to machine parts. In fact, friction uses about 20% of all the energy used in the world. Scientists who study these processes call the field tribology.

How does friction actually work? Most surfaces are not perfectly smooth when you look closely. They have tiny bumps called asperities. When two objects touch, these bumps catch on each other.

This interaction can change mechanical energy into heat. This is why your hands feel warm when you rub them together.

There are different ways this happens. Dry friction occurs between solid surfaces. Static friction, or "stiction," keeps objects from moving at first. Kinetic friction is the force you feel when surfaces are already sliding.

People have wondered about friction for a very long time. Ancient authors like Aristotle and Pliny the Elder studied its causes. In 1493, Leonardo da Vinci wrote about the laws of sliding friction in his notebooks. These ideas were later rediscovered by Guillaume Amontons in 1699. He created three laws to describe how dry friction works. Later, Charles-Augustin de Coulomb studied how materials and pressure affect friction in 1785. Scientists like Leonhard Euler and John Leslie also helped us understand these complex forces.

There are many specific facts about how friction behaves. Amontons' first law says friction is proportional to the weight pressing surfaces together. His second law says friction does not depend on the size of the contact area. Coulomb's law shows that kinetic friction stays the same even if sliding speed changes.

In 1845, James Joule studied how mechanical work turns into heat. In 1950, researchers Bowden and Tabor found that the actual contact area is very small. This is because the tiny bumps only touch at a few points.

You can see friction in many places around you. A curling stone slows down on ice because of kinetic friction. If a car tire did not have friction, the wheels would just spin on the road. You can also feel fluid friction when moving through water or air. This is often called drag. Even the way a lubricant works is based on these rules. A lubricant is a fluid that helps separate two solid surfaces to reduce friction. Understanding these rules helps engineers build better machines and tools.

Friction is a retarding force that resists relative motion between surfaces. This resistance occurs between solid surfaces, fluid layers, or material elements. It can happen when objects slide or grind against one another. Friction is a massive factor in global energy use. It accounts for about 20% of the total energy expenditure of the world. Scientists who study these processes call the field tribology.

To understand friction, we must look at the microscopic level. Most surfaces are not perfectly smooth. They are covered in tiny surface features called asperities.

When two bodies touch, these asperities interact. This interaction causes mechanical energy to transform into heat. It can also create changes in local structure or electrical charges. The total energy lost per unit of distance is the frictional force. This complex process makes calculating friction from first principles very difficult. Scientists often use empirical methods to study it instead.

There are several distinct types of friction. Dry friction occurs between two solid surfaces in contact. This is subdivided into static friction and kinetic friction. Static friction, or "stiction," acts between surfaces that are not moving. Kinetic friction acts between surfaces that are already in motion.

Fluid friction describes the resistance between layers of a viscous fluid. Lubricated friction is a specific type of fluid friction. In this case, a lubricant fluid separates two solid surfaces. Skin friction is a component of drag, which resists fluid moving across a body. Finally, internal friction resists motion between elements within a solid during deformation.

Humans have studied friction for over 2,000 years. Ancient authors like Aristotle and Pliny the Elder investigated its causes. In 1493, Leonardo da Vinci documented laws of sliding friction in his notebooks. However, these notes remained unknown for a long time. Guillaume Amontons rediscovered these laws in 1699. He described friction using surface irregularities and the force pressing surfaces together. Later, Leonhard Euler helped distinguish between static and kinetic friction. In 1785, Charles-Augustin de Coulomb expanded this by studying material types and pressure.

Modern science provides specific mathematical models for these forces. Amontons' first law states friction is proportional to the applied load. His second law says friction is independent of the apparent contact area. Coulomb's law states that kinetic friction is independent of sliding velocity.

We use the coefficient of friction to describe these interactions. This is an empirical property of the specific materials in contact. In the static case, friction acts as a threshold called traction. Once the force exceeds this threshold, motion begins.

History shows how our understanding of energy changed with friction. In 1798, Benjamin Thompson reported on heat created by friction. In 1845, James Joule published work on the mechanical equivalent of heat. He used a rotating paddle wheel in water to measure this. This helped prove that friction converts mechanical work into heat. In 1950, Bowden and Tabor showed that actual contact area is tiny. They proved that true solid-to-solid contact only happens at a few asperity points. This explains why friction increases when you increase the pressure.

Friction connects to many different fields of science. It is essential to the study of thermodynamics. For example, Max Planck described heat from rubbing as an irreversible process. It also relates to fluid dynamics through the study of viscous flow. Engineers must account for friction to prevent wear and damage. Wear can lead to the degradation of machine components. Understanding these forces allows us to control everything from car tires to microscopic machines.