Conservation of Energy

Conservation of energy: total energy of an isolated system is constant — only converts between forms.

Mechanical energy = KE + PE. Conserved when only CONSERVATIVE forces act (gravity, spring).

Non-conservative forces (friction, air drag): mechanical energy is NOT conserved; some converts to heat or sound.

Total energy (mechanical + thermal + chemical + EM + nuclear + …): ALWAYS conserved (1st law of thermodynamics).

Examples: pendulum (KE ↔ PE), free fall (PE → KE), spring-mass (KE ↔ U_spring), hydroelectric (PE → KE → electrical), photosynthesis (light → chemical), nuclear (mass → energy).

Friction converts mechanical energy to HEAT — random molecular motion in the contact surfaces.

Energy can change FORM but never be created or destroyed.

Closed-system energy conservation is THE most powerful physical principle.