Conservation of Energy, Explained
Updated 2026-07-12
The law of conservation of energy states that energy cannot be created or destroyed, only changed from one form to another. In a closed system with no friction, the total energy stays constant, so kinetic and potential energy trade off but their sum does not change.
Conservation of energy is a bookkeeping law: whatever energy you start with, you still have at the end, just in different forms. That lets you skip the messy details of the motion and compare only the start and the finish.
Energy changes form
A ball at the top of a drop has gravitational potential energy and no motion. As it falls, that potential energy converts into kinetic energy of motion. The total, potential plus kinetic, stays the same the whole way down.
Start equals finish
Because the total is constant, you can set the total energy at the start equal to the total at the end and solve for what you want, without tracking every instant in between.
Worked example
An object is dropped from a height of 5 m. How fast is it moving just before it hits the ground? (Use g = 9.8 m/s^2.)
- At the top, all energy is potential: m g h. At the bottom, all of it is kinetic: (1/2) m v^2.
- Set them equal: m g h = (1/2) m v^2, and the mass cancels.
- Solve for v: v = square root of (2 g h) = square root of (2 times 9.8 times 5).
Answer: v is about 9.9 m/s.
The common mistake
Wrong: Forgetting that friction or air resistance removes mechanical energy.
Fix: Conservation of the mechanical total only holds when there is no friction. If there is, some energy becomes heat, and you must account for that loss.
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Frequently asked questions
What is the law of conservation of energy?
It says energy cannot be created or destroyed, only transformed from one form to another. In a closed system with no friction, the total amount of energy stays constant.
How do you use conservation of energy to find speed?
Set the potential energy at the start equal to the kinetic energy at the end. The mass cancels, giving v equal to the square root of 2 g h.