please need help with this hard physics problem?

Question

chapter 7 #29
a roller coaster car has a mass of 500 kg when fully loaded with passengers. if the vehicle has a speed of 20 m/s at point a (point a radius 10 meter) what is the force of the track on the vehicle at this point. what is the maximum speed the vehicle can have at point b (point b radius 15 meter) in order for gravity to hold it on the track.

Answer 1

I will assume these are inside loops where the car is going upside down.. The points are at the top of the loop. B may be an outside loop, doesn't matter for the answer.

the centripetal acceleration is v^2/r
therefore
a)
F-500*20^2/10+500*9.81=0
F=15095 N

b)
500*9.81-500*v^2/15=0
12.13 m/s

j