Final answer:
If a rigid body has a net force of zero acting on it, it must either be at rest or moving with a constant velocity, as per Newton's First Law of Motion.
Step-by-step explanation:
When multiple forces act on a rigid body, and the resultant or net force is zero, it indicates that the body is either at rest or moving with a constant velocity. This outcome is a direct consequence of Newton's First Law of Motion, often referred to as the Law of Inertia, which states that a body at rest will remain at rest, and a body in motion will continue in motion with a constant velocity unless acted upon by a net external force. This means that the body is not experiencing any acceleration.
As an example, if a car is cruising down the street at a steady pace, the forces, such as the frictional force between the tires and the road and the drag force from air resistance, are balanced, resulting in no net force acting on the car. Thus, the car maintains a constant velocity. However, if the car were to accelerate, a net force would need to be applied, altering the state of motion for the car.