Push a book across a table. The kinetic force of friction, f_k is the force that brings the book to rest, opposing the motion.

f_k is proportional to the normal force:

f_k = m_k N

m_k is known as the coefficient of kinetic friction. It is a dimensionless number that depends only on the two surfaces in contact. Typical values are 0.1 - 1.0, but could be higher or lower.