Law of conservation of energy — Science definition
Defined in 2 courses:
- Energy is conserved when stored changes and all transfers across the chosen boundary are counted. An isolated system exchanges neither matter nor energy; a system closed to matter can still exchange energy. (Physical Science: Matter, Forces & Energy)
- The total energy of an isolated system never changes; energy is only transformed or transferred, never created or destroyed. (Physics & Environmental Science)
Where it is taught
- Conservation of Energy — Energy & Heat Transfer, Physical Science: Matter, Forces & Energy, Grade 8
- Conservation of Energy — Work, Energy & Momentum, Physics & Environmental Science, Grade 12
Questions using law of conservation of energy
Problem 1
A Newton's cradle eventually stops swinging. Does this violate the law of conservation of energy? Explain.
Answer and explanation
No — the energy isn't destroyed. It transforms into small amounts of sound and thermal energy from friction and air resistance at each collision, until almost none of the original energy is left as visible swinging motion.
Problem 2
A real roller coaster loses mechanical energy on every hill. Explain where that energy goes, and whether this breaks the law of conservation of energy.
Answer and explanation
The 'lost' mechanical energy is not destroyed. Friction in the wheels and track and air resistance convert it into thermal energy (warming the parts and the air) and sound. Total energy is still conserved — it has just been dissipated into forms that are spread out and no longer drive the car's motion.
Taught alongside
- mechanical energy
- energy dissipation
- model
- evidence
- system boundary
- assumption
- conservation of mechanical energy
- dissipation
- efficiency