When two objects rub against each other they cause ‘friction’. Friction is vitally important in our lives. Friction between our shoes and the ground stops us from slipping over when we walk. Friction between tyres and the road allows cars and Lorries to move forwards and prevents them from skidding. Friction also causes heat — you can start a fire by rubbing two sticks together.
Friction also slows things down. A ball rolling along the ground will gradually get slower until it stops, because of friction between the ball and the ground. Car and bicycle brakes also use friction to slow a moving vehicle down.
Friction is the resistance to motion of one object moving relative to another. It is not a fundamental force, like gravity or electromagnetism. Instead, scientists believe it is the result of the electromagnetic attraction between charged particles in two touching surfaces.
Scientists began piecing together the laws governing friction in the 1400s, but because the interactions are so complex, characterizing the force of friction in different situations typically requires experiments and can’t be derived from equations or laws alone.
For every general rule about friction, there are just as many exceptions. For instance, while two rough surfaces (such as sandpaper) rubbing against each other sometimes have more friction, very smoothly polished materials (such as plates of glass) that have been carefully cleaned of all surface particles may actually stick to each other very strongly.
There are two main types of friction, static friction and kinetic friction. Static friction operates between two surfaces that aren’t moving relative to each other, while kinetic friction acts between objects in motion. In liquids, friction is the resistance between moving layers of a fluid, which is also known as viscosity. In general, more viscous fluids are thicker, so honey has more fluid friction than water.
The atoms inside a solid material can experience friction as well. For instance, if a solid block of metal gets compressed, all the atoms inside the material move, creating internal friction. In nature, there are no completely frictionless environments: even in deep space, tiny particles of matter may interact, causing friction.