Sliding is a type of frictional motion between two surfaces in contact. This can be contrasted to rolling motion. Both types of motion may occur in bearings. The relative motion or tendency toward such motion between two surfaces is resisted by friction. Friction may damage or 'wear' the surfaces in contact. However, wear can be reduced by lubrication. The science and technology of friction, lubrication, and wear is known as tribology. Sliding may occur between two objects of arbitrary shape, whereas rolling friction is the frictional force associated with the rotational movement of a somewhat disclike or other circular object along a surface. Generally, the frictional force of rolling friction is less than that associated with sliding kinetic friction. Typical values for the coefficient of rolling friction are less than that of sliding friction. Correspondingly sliding friction typically produces greater sound and thermal bi-products. One of the most common examples of sliding friction is the movement of braking motor vehicle tires on a roadway, a process which generates considerable heat and sound, and is typically taken into account in assessing the magnitude of roadway noise pollution. Sliding friction Sliding friction (also called kinetic friction) is a contact force that resists the sliding motion of two objects or an object and a surface. Sliding friction is almost always less than that of static friction; this is why it is easier to move an object once it starts moving rather than to get the object to begin moving from a rest position. Where , is the force of kinetic friction. is the coefficient of kinetic friction, and N is the normal force. Examples of sliding friction Sledding Pushing an object across a surface Rubbing one's hands together (The friction force generates heat.) A car sliding on ice A car skidding as it turns a corner Opening a window Almost any motion where there is contact between an object and a surface Falling down a bowling lane Motion of sliding friction The motion of sliding friction can be modelled (in simple systems of motion) by Newton's Second Law Where is the external force. Acceleration occurs when the external force is greater than the force of kinetic friction. Slowing Down (or Stopping) occurs when the force of kinetic friction is greater than that of the external force. This also follows Newton's first law of motion as there exists a net force on the object.