Kinetic Friction

Once the applied force exceeds the maximum static friction and the object begins to slide, the surface continues to exert an opposing frictional force — now called kinetic friction ($f_k$, also called sliding or dynamic friction). To keep the object moving at constant velocity once sliding, an applied force exactly equal (and opposite) to kinetic friction must be maintained.

The empirical relation for kinetic friction is:

where $\mu_k$ is the coefficient of kinetic friction. Crucially, experiments consistently show

meaning it is always somewhat easier to keep an object sliding than it was to start it sliding. …