Motion with Constant Velocity and Zero Net Force

Summary: We introduce a simply model based on Newton's revolutionary insight - that the natural state of motion of an isolated body is motion with constant velocity.
We now explore our first model - motion with constant velocity.  According to Newton, this is the natural state of motion.  If the net external force acting on a body is zero, the body will move with constant velocity (with respect to an inertial coordinate system).

Mathematically, motion with constant velocity is quite straightforward: the velocity is constant and the position increases linearly with time.  The hard part is sometimes to determine why the net force is zero when some forces are more obvious than others.