Newton's First Law:
The Law of Inertia
Friction forces are common enough that it is often difficult to observe an object in motion without forces on it.
An air track supports a glider on a thin layer of air so that the friction forces on an air track glider are very, very close to zero.
It's fairly easy to understand how and why an air track glider, initially at rest, remains at rest.
We must supply a force to it to cause it to move from rest. (this involves Newton's second law).
What keeps an object moving once it is in motion?
That is the very nature of motion that is described by Newton's First Law. An object continues in motion simply because it was in motion if there are no forces acting on it to change its motion.
Force Mass Return to ToC, Ch5, Newton's Laws of Motion (c) Doug Davis, 2001; all rights reserved