Each force element in the Forces palette has a mode parameter that determines how the normal contact force is modeled.

There are three modes:

Linear spring and damper,

Linear spring and limited damper, and

Hunt and Crossley.

For each mode, the compression force, $f_c$, and damping force, $f_d$, are computed and then combined to arrive at the normal force, $f_n$. The compression force depends on the displacement, $u$, and the uncompressed length, $L_0$. The damping force depends on the velocity, $v$. The uncompressed length depends on the geometries of the contacting elements; for example, for two spheres it is the sum of their radii, while for a sphere and a disk it is the sum the radius of the sphere and the thickness of the disk.

If a contacting body penetrates more than half the uncompressed length, it penetrates the surface. To prevent penetration, the compression and damping factors should be increased.