Once static friction between the body and the ground has been overcome—i.e., once the critical force is reached—the body begins to move. During this motion, a frictional force continues to act on the body, known as kinetic friction. It depends on the normal force, is generally smaller than static friction, and is typically independent of the velocity of the moving body. Analogous to static friction, the relationship between the normal force and kinetic friction is given by:
$$
\vec{F}_\mathrm{GR} = \mu_\mathrm{GR} \vec{F}_N
$$
The proportionality constant $\mu_\mathrm{GR}$ is called the coefficient of kinetic friction.