For a sliding object, the kinetic friction force f_k opposes the motion. In the absence of external pushing, kinetic friction stops a moving object.

Basic feature:   f_k is proportional to the normal force:  f_k = μ_k N.

Here μ_k is the kinetic friction coefficient, a dimensionless number that depends only on the two surfaces in contact. Typical values are the range 0.1 - 1.0.