Work–Energy Theorem
Definition
The net work done on an object equals its change in kinetic energy — push something and the work you do shows up as speed.
Unit: joule (J)
Technical definition
W_net = ΔKE = ½mv² − ½mu². The theorem follows from integrating Newton's second law over displacement and holds for any net force, constant or varying.
Example
A 2 kg block pushed with a net force of 10 N over 5 m gains 50 J of work, so its kinetic energy rises by 50 J — from rest it reaches √(2·50/2) ≈ 7.1 m/s.
Common confusion
Students apply the theorem with one force's work instead of NET work. Friction's negative work must be counted — the books only balance when every force doing work is included.