Friction

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.