Friction

Introduction

Friction is a force between two surfaces that are sliding, or trying to slide, across each other.

Friction always slows a moving object down. Air resistance is a type of friction.

This section includes:

• What is friction?
• Useful friction and reducing friction
• Air resistance
  
  

  What is friction?

  Friction is a force between two surfaces that are sliding, or trying to slide, across each other. For example when you try to push a book along the floor friction makes this difficult.
  Friction always works in the direction opposite from the direction the object is moving, or trying to move. Friction always slows a moving object down.

  More or less friction

  The amount of friction depends on the materials from which the two surfaces are made. The rougher the surface, the more friction is produced.
  For example, you would have to push a book harder to get it moving on a carpet than you would on a wooden floor. This is because there is more friction between the carpet and the book than there is between the wood and the book.

  Heat

  Friction also produces heat. For example, if you rub your hands together quickly, they get warmer.
  
  

  Useful friction and reducing friction

  Friction can be a useful force because it prevents our shoes slipping on the pavement when we walk and stops car tyres skidding on the road.
  Ice causes very little friction, which is why it is easy to slip over on an icy day. But this is a good thing for ice skating and sledging.

  Reducing friction

  Sometimes we want to reduce friction. For example we use oil to reduce the friction between the moving parts inside a car engine. The oil holds the surfaces apart, and can flow between them. The reduced friction means there is less wear on the metal, and less heat produced.
  
  

  Air resistance

  Air resistance is a type of friction between air and another material. When an aeroplane flies through the air, for example, air particles hit the aeroplane, making it more difficult for the aeroplane to move through the air.
  Some shapes, known as streamlined shapes, cause less air resistance than others. Aeroplanes and cars are streamlined, so that they move through the air as easily as possible.
  Source : BBC UK