[问答题]
A wheel of mass M has radius4r 6v+-umdop n R. It is standinq 7ipmofvd/e .qt t-6,g vertically on the floor, and we want to exert a horizontal force F at its axle so that it will clqt/ev 7-td.ifop 6m, qimb a step against which it rests (Fig. 8–55). The step has height h, where h