Which forces produce a nonzero net force that changes an object's motion?

Unbalanced forces are the only forces that produce a nonzero net force, which is essential for changing an object's motion. When the forces acting on an object are unbalanced, it means that one force is greater than the opposing force, resulting in a net force that can cause the object to accelerate, decelerate, or change direction. This is in direct accordance with Newton's Second Law of Motion, which states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

In contrast, balanced forces occur when two or more forces acting on an object cancel each other out, resulting in a net force of zero. This means the object's motion remains unchanged, whether it is at rest or moving with a constant speed.

Resultant forces can refer to the net effect of all forces acting on an object, but if these forces are balanced, the resultant force will still be zero, which does not change the object's state of motion. Internal forces refer to interactions within the object itself and do not affect its motion when considering external forces.

Thus, unbalanced forces are essential for changing an object's motion due to the existence of a net force that can result in acceleration or a change in direction.