Contact slip
= Contact slip
{title2=$\mathbf w$}
<Contact slip> is the tangential <relative velocity> between the two material points instantaneously in contact. For a nonrotating box on a belt it is $w=v-U$; for a <rigid body>, use the <velocity> of its contact point, including the contribution from <angular velocity>. Zero <contact slip> defines <rolling without slipping> at a rolling contact, even when the <center of mass> moves.