When an object experiences no external horizontal frictional force and its weight acts downward, it can move with a constant velocity due to the absence of any net external force acting on it. This situation is known as "uniform motion" or "constant velocity motion."
The key concept at play here is Newton's first law of motion, also known as the law of inertia. According to this law, an object at rest tends to stay at rest, and an object in motion tends to stay in motion with the same velocity unless acted upon by an external force. In other words, if there is no net force acting on an object, its velocity will remain constant.
In the scenario you described, the object's weight (the force of gravity acting on it) is directed downward. However, since there is no horizontal frictional force opposing its motion, the object does not experience any net force in the horizontal direction. Without a net force, the object will continue to move with a constant velocity in the absence of other influences like air resistance.
It's worth noting that if there were a horizontal force acting on the object, it would accelerate in that direction according to Newton's second law of motion (F = ma), where F represents the net force, m is the mass of the object, and a is the resulting acceleration. However, in the absence of such a force, the object will continue to move with a constant velocity in a straight line.