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.

${\displaystyle F_{k}=\mu _{k}\cdot N}$ 

Where F_k, is the force of kinetic friction. μ_k is the coefficient of kinetic friction, and N is the normal force.

• 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

The motion of sliding friction can be modelled (in simple systems of motion) by Newton's Second Law

${\displaystyle \sum F=ma}$ 

${\displaystyle F_{E}-F_{k}=ma}$ 

Where ${\displaystyle F_{E}}$  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.
• 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 planed plane edit

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:^[4]

${\displaystyle F_{g}=mg\sin {\theta }}$ 

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

${\displaystyle N=mg\cos {\theta }}$ 

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

${\displaystyle F_{k}=\mu _{k}\cdot mg\cos {\theta }}$ 

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 ${\displaystyle \theta }$ 

${\displaystyle \sum F=ma=0}$ 

${\displaystyle F_{k}=F_{g}}$  or ${\displaystyle \mu _{k}mg\cos {\theta }=mg\sin {\theta }}$ 

Here it is found that:

${\displaystyle \mu _{k}={\frac {mg\sin {\theta }}{mg\cos {\theta }}}=\tan {\theta }}$  where ${\displaystyle \theta }$  is the angle at which the block begins moving at a constant velocity^[5]