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PhysicsMediumClass 11
Conservation of Energy — KE + PE = constant
Work, Energy & Power
7
JEE Qs
8%
Hard
75
min
Always begin by identifying all forces and classifying them as conservative or non-conservative to determine if mechanical energy is conserved, and diligently choose a consistent potential energy reference level.
🧮 Key Formulas
E_mech = KE + PE
KE = (1/2)mv^2
PE_g = mgh (for gravitational potential energy near Earth's surface)
PE_s = (1/2)kx^2 (for elastic potential energy of a spring)
If only conservative forces do work: KE_initial + PE_initial = KE_final + PE_final
If non-conservative forces (W_nc) also do work: W_nc = Delta_E_mech = (KE_final + PE_final) - (KE_initial + PE_initial)
✅ Key Points for JEE
- 1Conservation of Mechanical Energy (KE + PE = constant) is applicable ONLY when all forces doing work on the system are conservative (e.g., gravity, spring force, electrostatic force).
- 2Non-conservative forces (e.g., friction, air resistance, applied external force, tension, normal force) cause a change in the total mechanical energy of the system. In such cases, apply W_nc = Delta_E_mech.
- 3Carefully choose a zero reference level for potential energy (e.g., ground, lowest point of motion) at the beginning of the problem; consistency is key.
- 4System definition is crucial: ensure all energies and works considered correspond to the chosen system. Internal conservative forces can be accounted for as potential energy, while external or internal non-conservative forces do work on the system.
- 5This principle often simplifies problems involving variable forces or complex paths, allowing direct calculation of speeds and positions without detailed force analysis.
⚠️ Common Mistakes
- ✕Applying conservation of mechanical energy when non-conservative forces are doing work, without including the work done by them.
- ✕Incorrectly setting the reference level for potential energy or changing it midway through a problem.
- ✕Mixing up signs for work done by non-conservative forces (e.g., work done by friction is usually negative as it opposes motion).
- ✕Forgetting to account for both gravitational and elastic potential energies when both types of conservative forces are present in the system.
NCERT Chapters
- Class 11 Physics Ch 6: Work, Energy and Power