Final answer:
The term that refers to the sum of all forces acting on an object is 'net force', which is the vector sum of all individual forces acting on that object.
Step-by-step explanation:
The term that refers to the sum of all the forces acting upon an object is the net force. The net force is essential in understanding the motion of objects because it determines the object's acceleration according to Newton's Second Law of Motion. If multiple forces are applied to an object, some forces might cancel each other out, and the object will accelerate in the direction of the net force. This is explained by Essential Knowledge 3.B.1, which states that the net force is the vector sum of the individual forces.
In this context, internal force refers to forces acting between elements within the system and external force relates to the forces exerted upon a system from outside. For example, friction is considered an external force that can affect the net force acting on an object. Nevertheless, when addressing the initial question, the net force concept is critical to understanding how forces combine to influence the motion of an object.