Conservation of Energy

Objective: Apply K + U conservation when nonconservative transfer is negligible.

Mechanical energy is the sum of selected kinetic and potential energies, commonly E_mech = K + U_g + U_s. If a system experiences negligible friction, drag, and external work, its mechanical energy can be approximately constant. For a falling object, a decrease in gravitational potential energy accompanies an increase in kinetic energy. Starting at rest 5 m above a chosen zero level, the ideal model gives K = mgh immediately before reaching that level, provided other stores are negligible. The total energy of an appropriately enlarged system remains conserved even when mechanical energy changes. The assumptions and reference level must be stated.

Objective: Apply conservation of energy to calculations, evidence, and specified situations.

Energy conservation requires a defined system and accounting for transfers. In ideal motion without dissipative transfer, K+U is constant. With friction or drag, mechanical energy can decrease while thermal energy increases. Total energy is not the same as mechanical energy. Efficiency is useful output divided by input, and depends on what is counted as useful. A device can transfer energy to several destinations; include those destinations rather than describing energy as destroyed.