Consider a small frictionless puck perched at the top of a fixed sphere of radius r. if the puck is given a tiny nudge so that it begins to slide down, through what vertical height will it descend before it leaves the surface of the sphere? [hint: use conservation of energy to find the puck's speed as a function of its height, then use newton's second law to find the normal force of the sphere on the puck. at what value of this normal force does the puck leave the sphere? ]