According to the work-energy theorem, the work done on an object equals what?

• A. The force applied to the object
• B. The velocity of the object
• C. The time over which the force acts
• D. The change in the object's kinetic energy

Answer: (D)

The work-energy idea is that work transferred to an object changes its motion energy. The work-energy theorem states that the work done on the object equals the change in its kinetic energy, W = ΔK, where K = (1/2) m v^2. So the work you do increases or decreases the object's kinetic energy depending on whether it speeds up or slows down. If you push to speed up, work is positive and kinetic energy increases; if you slow it down, work is negative and kinetic energy decreases. Time or the force magnitude alone don’t determine the energy change—the force must act through a distance and align with the displacement to do work, and the resulting change in kinetic energy depends on the initial and final speeds.