Work, Energy and Power is one of the most important chapters in Class 11 Physics. It connects force and motion with energy concepts and helps explain how work is done and how energy is transferred.
This chapter is crucial for:
- CBSE Board Exams
- JEE (Main + Advanced)
- NEET
👉 Core Idea: Energy is the ability to do work, and it is always conserved.
1. Work
Definition
Work is said to be done when a force is applied on a body and it causes displacement.
Formula
W = F · s cosθ
Where:
- F = force
- s = displacement
- θ = angle between force and displacement
Types of Work
Positive Work
Force and displacement in same direction
Negative Work
Force opposite to displacement
Zero Work
No displacement or perpendicular force
2. Energy
Definition
Energy is the capacity to do work.
SI Unit
Joule (J)
3. Kinetic Energy (KE)
Definition
Energy possessed by a body due to its motion.
Formula
KE = (1/2)mv²
Work-Energy Theorem (Very Important)
Statement
Work done on a body equals change in kinetic energy.
W = ΔKE
👉 Concept Clarity:
Work changes the motion (energy) of object.
4. Potential Energy (PE)
Definition
Energy possessed due to position or configuration.
Gravitational Potential Energy
PE = mgh
Elastic Potential Energy
Energy stored in spring:
PE = (1/2)kx²
5. Conservative and Non-Conservative Forces
Conservative Forces
Work done is independent of path
Examples: Gravity, spring force
Non-Conservative Forces
Work depends on path
Example: Friction
6. Mechanical Energy
Definition
Sum of kinetic and potential energy.
E = KE + PE
Law of Conservation of Energy
Statement
Energy can neither be created nor destroyed, only transformed.
👉 Example: Falling object (PE → KE)
7. Power
Definition
Power is the rate of doing work.
Formula
P = W/t
Instantaneous Power
P = F · v
8. Collisions (Important Concept)
Elastic Collision
- KE conserved
- Momentum conserved
Inelastic Collision
- KE not conserved
- Momentum conserved
9. Centre of Mass and Energy (Basic Idea)
Centre of mass is the point where entire mass is assumed concentrated.
Important Numericals (CBSE + JEE Level)
Numerical 1
Find work done when force 10 N moves object 5 m
W = Fs = 10 × 5 = 50 J
Numerical 2
Find KE of 2 kg object moving at 3 m/s
KE = (1/2)mv² = (1/2)(2)(9) = 9 J
Numerical 3
Find potential energy of 5 kg object at height 10 m
PE = mgh = 5×10×10 = 500 J
Numerical 4
Find power if 100 J work done in 5 s
P = W/t = 100/5 = 20 W
Numerical 5 (JEE Level)
A body falls from height. Calculate velocity using energy conservation.
Important Formula Sheet
- W = Fs cosθ
- KE = (1/2)mv²
- PE = mgh
- P = W/t
- E = KE + PE
JEE / NEET Focus
- Work-energy theorem
- Conservation of energy
- Collision problems
CBSE Board Strategy
- Write definitions clearly
- Show steps in numericals
- Use correct formulas
Common Mistakes
- Ignoring angle in work formula
- Sign errors in work
- Confusing KE and PE
SEO Keywords (Mobotes Optimized)
- Work Energy and Power Class 11 Notes
- Class 11 Physics Chapter 6 Notes
- Work Energy Theorem Notes
- Conservation of Energy Physics
Conclusion (Teaching Insight)
Work, Energy and Power is a scoring chapter if concepts are clear. It simplifies many physics problems using energy conservation.
👉 Focus on concept clarity + numericals + formula application.