Well, I was just browsing through this website:http://www.physicsclassroom.com/mmedia/circmot/rcd.html it explains stuff about the centripetal force, but how come for a rollercoaster, you add the normal force and gravity force to come up with the net force or the centripetal force? Could you also take the tangential velocities of the cart and subtract V_f by V_i to get the inward acceleration that the cart experiences? I know that they want to find the net force, but why don't they consider the tangential veolcities vectors also? Why do they only add the normal force and gravity? Doesn't the rollercoaster cart have a constantly changing velocity also? Wait, velocity is not a force right? Are the only forces the cart experiences the norm and gravity? I think I made a mistake in the last paragraph.. I remember from newton's laws that objects do not require a force to cause it to move, is this why there isn't a x component force acting on the cart? Thanks for your help!