Normal force

The train on a roller coaster ride experiences a 42.0 kN normal force at the bottom of a smooth, circular, vertical loop. Take the mass of  the train and passengers to be 600 kg. a) What normal force acts on the train at the top of the loop? b) Now assume that the radius of the loop is 10.0 m with the same normal force acting on the bottom of the loop as before. What is the radius of the largest loop the train will just be able to completely pass through? Note: Use g=10 m/s2