[填空题]
A block of mass m slides without frictisj23 dg:xs3 sxon zdzx596/dnbtr iz+o 1along the looped track shown in Fig. 6–39. If the block is to remain on the track, even at the top of the circle (whose radius is r), from what minimum heighzxzrd +6 tozbdni15/9t h must it be released? r