Friction is a force that resists motion when the surface of one object comes in contact with the surface of another. It is defined as the resistance offered by the surfaces that are in contact when they move past each other. Friction depends on the nature of the two surfaces that are in contact with each other, and the force that is acting on these surfaces. The roughness or smoothness of the surfaces in contact determines the amount of friction between them. Friction is proportional directly to the normal force. There are four main types of friction: static friction, sliding friction, rolling friction, and fluid friction.