When an object slides along a surface, the surface exerts a retarding force called the kinetic friction force.  The magnitude of this force is given by:

• Ffk = mk*Fn

where Ffk is the magnitude of the kinetic friction force, Fn is the magnitude of the normal force on the object due to the surface, and mk is the  coefficient of kinetic friction.  The direction of the frictional force is opposite to the direction of the motion of the object with respect to the surface.