A block of mass m slides without friction along the looped track shown in the figure. If the block is to remain on the track, even at the top of the circle (whose radius is r), from what minimum height h should it be released? (Hint: What forces or force would be centripetal when the block is at the top of the loop? Now, thinking about the minimum centripetal force on the block when it is at the top of the loop, what would be the centripetal acceleration corresponding to this?)