A mass is accelerated when what type of force acts upon it?

A mass is accelerated when an unbalanced force acts upon it because, according to Newton's second law of motion, an object will only change its state of motion (accelerate) when there is a net force acting on it. An unbalanced force means that there is a greater force acting in one direction than the opposing forces, resulting in a change in velocity of the object. This fundamental principle explains how forces influence motion; if all forces acting are balanced, the object remains at rest or moves at a constant speed.

In practical terms, examples of unbalanced forces leading to acceleration include pushing a stationary object, where the push creates a net force, or an object accelerating due to gravity when falling freely. In contrast, balanced forces do not cause any change in motion; they keep an object at rest or moving consistently in a straight line at a constant speed. Frictional force and gravitational force can act as unbalanced forces when they lead to acceleration, but they are not by themselves the broad answer to the question about what type of force causes acceleration. The key concept is that it is the unbalanced nature of the force that ultimately leads to acceleration.