Final answer:
An object increases in speed due to unbalanced forces, as per Newton's second law of motion.
Step-by-step explanation:
If an object increases in speed, it is due to unbalanced forces acting upon it. According to Newton's second law of motion, an acceleration (which includes an increase in speed) occurs when a net force is applied to an object. Friction, gravitational attraction, and air resistance can influence the speed of an object, but they do not directly account for an increase in speed unless they result in an unbalanced force. For instance, air resistance typically acts in the opposite direction to the motion, slowing objects down rather than speeding them up. Increased friction also generally acts to reduce speed by opposing motion. An object falling under the influence of gravity can only accelerate if the gravitational force is not balanced by other forces such as air resistance.