= Nonpenetration constraint
{title2=$w-f\geq0$}
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.
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