[问答题]
A wheel of mass M has radqj /dqa ;h/9)j9yynvx y 7(ydmius R. It is stan(wcm85:sf mdvlr.9anding vertically on the floor, and we want to exert a horizontal force F at its axle so that it will climb a step against which it rests (Fig. 8–55). Themvw8m (r9n sc:d f5.al step has height h, where h