A roller coaster starts from rest at a height h = 21 m at th…

A roller coaster starts from rest at a height h = 21 m at the left side of a loop-the-loop, as shown in the figure. It is not attached to the track in anyway, and there is no friction from the track or from air resistance. The radius of the loop is R = 6.0 m. What is the speed of the roller coaster at the top of the loop?