does inertia apply when a body moves on a curved surface?

Inertia applies to bodies which are in a state of rest or of uniform motion along a straight line. It is the inability of such bodies to change their state by themselves.

 

However, when a body moves on a curved surface, it is not moving uniformly along a straight line as its direction of motion changes at each point which gives it a non-uniform motion. An external force is required to move a body along a curved surface.