Problem # p025 
The car of a roller coaster ride is hitting the bottom of a hill
with v=20 m/s. To prevent the passengers from blacking out the
absolute value of the acceleration at the bottom of the hill is
not to exceed 3*g ( g=9.81 m/s^{2} ). As shown in the figure, around the bottom of the hill the track is described by a parabola given by : y = k * x^{2} k is a constant
