# Work-energy theorem The net work done on an object equals its change in kinetic energy. Physics (university) · Part 03: Work, energy and momentum · Card 3 of 9 · Last updated 2026-10-19 ## Analogy Work is the money paid in, kinetic energy is the balance. ## Example **From rest** W = 1/2 m v^2 5000 = 500 v^2 v = 3.16 m/s ## In practice 5000 J of net work on a 1000 kg car from rest gives it about 3.2 m/s. ## Practice ### Flashcards - **What does the work-energy theorem say?** Net work equals the change in kinetic energy. ## Questions people ask ### When should I use the work-energy theorem instead of F = ma? Use it when you know distances and speeds but not time, or when the force changes along the path. It needs only scalars, so it avoids splitting vectors. For example, finding a car's braking distance is quick with work equals change in kinetic energy. ### Does the theorem work with friction? Yes. Count friction as one of the forces and include its negative work in the net work. The result equals the change in kinetic energy. Alternatively, treat the lost mechanical energy as heat. Both methods give the same answer if you account for everything. ## Related concepts - [Work](https://studyvisually.com/learn/science/physics-university/physics-university-part-3/physics-university-part-3-concept-1) - [Kinetic energy](https://studyvisually.com/learn/science/physics-university/physics-university-part-3/physics-university-part-3-concept-2) - [Gravitational potential energy](https://studyvisually.com/learn/science/physics-university/physics-university-part-3/physics-university-part-3-concept-4) - [Conservation of energy](https://studyvisually.com/learn/science/physics-university/physics-university-part-3/physics-university-part-3-concept-5) Web page: https://studyvisually.com/learn/science/physics-university/physics-university-part-3/physics-university-part-3-concept-3 Study this card in the app: https://studyvisually.com/app/learn?category=physics-university&card=physics-university-part-3-concept-3