Take a look back at Newton's first law of motion.
It says something like
"Inertia is that property of matter by which it tends to
remain in constant, uniform motion until acted on by
an external force."
There you are. No external force ===> constant, uniform motion.
The net force required to maintain constant velocity is zero.
(Completely opposite to everything we have ever seen in
everyday life ... where a moving object with no force on it
always stops moving.)
