A skier at an exhibition wants to loop the loop of radius 
on a
special (frictionless) ice track set up as shown. How high (in terms
of ) must the starting slope be for the skier to get around the
track if she needs to feel one ``gravity'' of normal-force
acceleration at the top of the track to stay in control? (Note well:
This means that you should have 
(down) at the top of the track
instead of 
for the case where the skier barely loops the loop).