* Friction: The biggest factor is friction. If there's no friction, no force is needed to keep an object moving at a constant velocity. This is due to Newton's First Law of Motion (inertia).

* Air resistance: Air resistance also acts as a form of friction, and the force needed to overcome it depends on the object's speed and shape.

* Other forces: There might be other forces acting on the object, like gravity or magnetic forces, which might require a force to counteract.

So, here's the breakdown:

* No friction: No force is needed to keep an object moving at a constant velocity.

* Friction present: You need a force equal to the force of friction to keep the object moving at a constant velocity. This force is often called the "force of friction."

* Other forces: You need a force that balances out all the other forces acting on the object.

Important Note: The force required to *start* an object moving (overcoming its inertia) is always greater than the force needed to keep it moving at a constant velocity. This is because you need to overcome static friction initially, which is usually greater than kinetic friction.