Answer:
Newton's first law states that; A body will remain in its state of rest or of motion until an external force acts on the body. The body will have inertia of motion or inertia of rest unless an external force is applied on it to change its state of inertia(state or rest).
As per the Law, a body will remain in its state of rest or of motion unless an external force acts on it, in the similar manner roller coaster will not run unless a force is applied to run it and again force of brakes is applied to stop the roller coaster from moving. At rest or at motion for both the cases an external force is applied on the roller coaster.