Here's why:

* Newton's First Law of Motion: This law states that an object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

* Constant velocity: Constant velocity means the object is moving at a steady speed in a straight line.

* Unbalanced force: For an object to accelerate (change velocity), there must be a net force acting on it. Since the velocity is constant, there is no acceleration.

Therefore, if the velocity is constant, the net force acting on the object must be zero.