It decreases.
Explanation
The relationship between the work done on an object and its motion is defined by the Work-Energy Theorem. This theorem states that the net work done on an object by all forces acting on it is equal to the change in its kinetic energy.
Analysis
Mathematically, the theorem is expressed as:
Wnet = ΔK = Kfinal - Kinitial
Where Wnet is the net work done, and K represents kinetic energy.
When the net work done is negative (Wnet < 0), the change in kinetic energy is negative. This implies that the final kinetic energy is less than the initial kinetic energy (Kfinal < Kinitial). Physically, this occurs when the net force acts in a direction opposite to the object's displacement (such as friction or a braking force), causing the object to decelerate. Consequently, the speed decreases, leading to a decrease in kinetic energy.
Key Takeaway:
Positive net work increases an object's kinetic energy, while negative net work decreases its kinetic energy by opposing its motion.