When an object is rolling without slipping, the rolling friction force is much less than the friction force when the object is sliding; a silver dollar will roll on its edge much farther than it will slide on its flat side. When an object is rolling without slipping on a horizontal surface, we can approximate the friction force to be zero, so that and are approximately zero and and are approximately constant. Rolling without slipping means and . If an object is set in motion on a surface without these equalities, sliding (kinetic) friction will act on the object as it slips until rolling without slipping is established. A solid cylinder with mass and radius , rotating with angular speed about an axis through its center, is set on a horizontal surface for which the kinetic friction coefficient is??