An initially motionless car of mass 1000 kg slides down from a height 40 m on a frictionless roller coaster track. It enters a circular vertical loop-the-loop of radius 10 m. What acceleration does a passenger feel at the bottom of the loop just as the curvature starts?
a) 5g
b) 7g
c) 9g
d) 11g
e) 13g