(Solved): Leonardo and his friend are riding a roller coaster that has a semicircular dip of radius 25.0 ...

Leonardo and his friend are riding a roller coaster that has a semicircular dip of radius $25.0\mathrm{m}$. As he goes through the bottom of the dip, he feels the seat of the car pushing up on him with a force equal to 2.5 the (magnitude) of his weight. (a) How fast is the car moving at the bottom of the dip? (b) How much force from the seat (in the perpendicular direction) does his $68.5-\mathrm{kg}$ friend feel when the car is $90^{\circ }$ above the dip and is traveling at three quarters of the speed at the bottom?