Work–Energy Theorem
Net work done equals the change in kinetic energy.
About this topic
The work-energy theorem states that the net work done on an object equals its change in kinetic energy: W_net = ½mv² - ½mu². It follows from Newton's second law and the equations of motion.
It often provides a faster route than analysing forces and time, especially with varying forces or curved paths. Add the work done by every force, including friction, which is negative, and set the sum equal to the change in KE. Positive net work speeds the object up and negative net work slows it.
Example: a 1000 kg car moving at 20 m/s brakes to rest with a constant force. The change in KE is -200,000 J, so if the stopping distance is 40 m, the force is 5000 N. If the speed is doubled, the same force needs four times the distance. Safety engineers use this to set barrier requirements and braking designs.
Misconceptions: it is the net work, not the work of one force alone, that matches the change in KE. If the net work is zero, the speed is constant, even though individual forces may do work. Also, the theorem relates to speed, not velocity direction.
Key ideas
- W_net = ΔKE
- Positive work speeds things up
- Negative work slows them down
Quick check
Use cases: where Work–Energy Theorem helps
- Homework: review the key idea behind Work–Energy Theorem before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Work–Energy Theorem without notes.
- Visual learning: play with the interactive lab to see how Work–Energy Theorem changes when you adjust its inputs.
- Teaching a friend: explain Work–Energy Theorem in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Work–Energy Theorem?
Net work done equals the change in kinetic energy.
What will I learn in Work–Energy Theorem?
W_net = ΔKE. Positive work speeds things up. Negative work slows them down.
Is Work–Energy Theorem part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Work–Energy Theorem lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Work–Energy Theorem?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.