Physics – Work, Energy and Power (JEE)
1. Concept of Work
Work is said to be done when a force applied on a body produces displacement
in the direction of the force.
Work = Force × Displacement × cosθ
If displacement is perpendicular to force, work done is zero.
2. Special Cases of Work
- θ = 0° → Maximum work
- θ = 90° → Zero work
- θ = 180° → Negative work
3. Variable Force and Work
When force varies with displacement, work is calculated as the area
under the force–displacement graph.
4. Kinetic Energy
Energy possessed by a body due to its motion is called kinetic energy.
Kinetic Energy = $\frac{1}{2}mv^2$
5. Work–Energy Theorem
Work done by net force = Change in kinetic energy
This theorem is valid for all types of forces.
6. Potential Energy
Energy possessed by a body due to its position or configuration.
Gravitational Potential Energy = $mgh$
7. Potential Energy in a Spring
Elastic Potential Energy = $\frac{1}{2}kx^2$
8. Conservative and Non-Conservative Forces
| Conservative | Non-Conservative |
|---|---|
| Path independent | Path dependent |
| Potential energy defined | No potential energy |
| Example: Gravity | Example: Friction |
9. Mechanical Energy
Mechanical Energy = Kinetic Energy + Potential Energy
10. Law of Conservation of Mechanical Energy
If only conservative forces act on a system,
total mechanical energy remains constant.
11. Power
Power is the rate of doing work.
Power = $\frac{Work}{Time}$
12. Instantaneous Power
Instantaneous Power = $\vec{F} \cdot \vec{v}$
13. Units and Dimensions
| Quantity | SI Unit | Dimension |
|---|---|---|
| Work | Joule | $ML^2T^{-2}$ |
| Energy | Joule | $ML^2T^{-2}$ |
| Power | Watt | $ML^2T^{-3}$ |
14. Collision and Energy
In elastic collisions, kinetic energy is conserved.
In inelastic collisions, kinetic energy is not conserved.
15. Important JEE Traps
- Confusing work done by force and net work
- Forgetting direction in power calculation
- Applying conservation of energy when friction is present
- Ignoring sign of work
16. Typical JEE Question Patterns
| Type | Concept Used |
|---|---|
| Block on incline | Work–energy theorem |
| Spring problems | Elastic potential energy |
| Collision | Energy conservation |
| Power problems | $P = Fv$ |
17. Final Revision Checklist
You have mastered this chapter if you can:
- Calculate work in all force cases
- Apply work–energy theorem correctly
- Use conservation of energy confidently
- Solve spring and collision problems
- Handle power problems with vectors