Nonpenetration constraint ($w-f\ge 0$)

For a body lying above an obstacle with height $f$, its normal displacement graph $w$ must obey the displayed inequality. Equality allows contact and strict inequality means separation. This is a unilateral mechanical constraint: motion away from the obstacle is allowed, but motion through it is forbidden. In nonadhesive frictionless contact the associated normal contact force can push the body outwards but cannot pull it into the obstacle.