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.

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

Motion of sliding friction 
The motion of sliding friction can be modeled (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 is also follows Newton's first law of motion as there exists a net force on the object.
 Constant Velocity occurs when there is no net force on the object, that is the external force is equal to force of kinetic friction.

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:

The normal force (perpendicular to the surface) is given by:

Therefore, since the force of friction opposes the motion of the block,

To find the coefficient of kinetic friction on an incline 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 

 or 

Here it is found that:

 where  is the angle at which the block begins moving at a constant velocity

References 

Category:Classical mechanics