Sliding (motion)

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.
formula_1
Where , is the force of kinetic friction. is the coefficient of kinetic friction, and N is the normal force.
Motion of sliding friction.
The motion of sliding friction can be modelled (in simple systems of motion) by Newton's Second Law
formula_2
formula_3
Where formula_4 is the external force. 
Motion on an inclined plane.
A common problem presented in introductory physics classes is a block subject to friction as it slides up or down an inclined plane. This is shown in the free body diagram to the right.
The component of the force of gravity in the direction of the incline is given by:
formula_5
The normal force (perpendicular to the surface) is given by:
formula_6
Therefore, since the force of friction opposes the motion of the block,
formula_7
To find the coefficient of kinetic friction on an inclined plane, one must find the moment where the force parallel to the plane is equal to the force perpendicular; this occurs when the block is moving at a constant velocity at some angle formula_8
formula_9
formula_10 or formula_11
Here it is found that:
formula_12 where formula_8 is the angle at which the block begins moving at a constant velocity