Conservation of Energy
Roller-coaster track — E = PE + KE stays constant; bars show the exchange.
Key Notes
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.
Formulas
Mechanical energy conservation
Valid when only conservative forces act.
Including non-conservative work
W_nc = work done by non-conservative forces (usually negative for friction).
Pendulum (mass-energy)
Height swung ↔ speed at lowest point.
Spring-mass
A = amplitude; energy is fixed if no damping.
Important Points
Mechanical energy is conserved only when ALL forces are conservative.
Friction always reduces mechanical energy (converts to heat).
Energy is FRAME-INDEPENDENT for total energy — but individual contributions (KE, U) are frame-dependent.
If you observe a 'violation', look for missing energy form: heat, light, sound, chemical, etc.
Real-world devices (engines, motors) are < 100% efficient — energy goes to friction, heat, sound.
Power plants follow energy conservation: chemical (coal) → thermal → mechanical → electrical.
Conservation of Energy notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 11 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.