Final answer:
In physics, a nonzero net external force acts on an object in scenarios where there is a change in the object's motion. Examples include being pushed or pulled with a greater force in one direction and having forces that cancel each other out, but do not sum to zero.
Step-by-step explanation:
In physics, a nonzero net external force acts on an object in scenarios where there is a change in the object's motion. This is described by Newton's second law, which states that the acceleration of an object is directly proportional to the net external force acting on it and inversely proportional to its mass.
In Case A, if an object is pushed or pulled with a greater force in one direction, there will be a nonzero net external force acting on it, causing it to accelerate in that direction.
In Case B, if an object is subject to forces that cancel each other out but do not sum to zero, there is a nonzero net external force acting on the object. For example, if a person pushes an object with a force of 10 N to the right and another person pushes it with a force of 5 N to the left, the net external force is 5 N to the right.