Loop-the-Loop (a) A block of mass $m$ slides from rest on a frictionless loop-the-loop track, as shown in Figure $8-32$. What is the minimum release height, $h,$ required for the block to maintain contact with the track at all times? Give your answer in terms of the radius of the loop, $r$ (b) Explain why the release height obtained in part (a) is independent of the block's mass.