Work, Energy and Power for NEET Physics
Master the concepts of work, kinetic & potential energy, work-energy theorem, power, and collisions — essential for solving mechanics problems, explained for NEET aspirants.
🔍 Detailed concept explanation
💪 1. Work
Work done by a constant force is W = F·d·cosθ, where θ is the angle between force and displacement. Work is a scalar, unit: joule (J). Positive work: force helps motion (θ<90°); negative work: force opposes motion (θ>90°); zero work: force perpendicular (θ=90°).
⚡ 2. Energy
Kinetic Energy (KE) = ½mv², energy due to motion. Potential Energy (PE) is stored energy. Gravitational PE near Earth = mgh. Spring PE = ½kx² (where x is compression/extension).
🔄 3. Work-Energy Theorem
Net work done by all forces on a body equals change in its kinetic energy: Wnet = ΔKE = ½mvf² – ½mvi². This is a powerful alternative to equations of motion.
📊 4. Conservation of Mechanical Energy
If only conservative forces (gravity, spring force) act, total mechanical energy (KE + PE) remains constant. For non-conservative forces (friction), work done equals change in mechanical energy.
🏎️ 5. Power
Rate of doing work. Average power Pavg = W/t. Instantaneous power P = F·v (dot product). Unit: watt (W).
💥 6. Collisions
Elastic collision: KE and momentum conserved. Inelastic collision: only momentum conserved, KE not conserved (some lost as heat/sound). Perfectly inelastic: bodies stick together. Coefficient of restitution e = relative speed after / relative speed before (e=1 elastic, e=0 perfectly inelastic).
📋 Complete formula sheet
(variable force: W = ∫F·dx)
PEspring = ½kx²
| Quantity | Formula |
|---|---|
| Conservation of ME | KEᵢ + PEᵢ = KEf + PEf |
| Perfectly inelastic collision | vcommon = (m₁v₁ + m₂v₂)/(m₁+m₂) |
| Loss in KE (inelastic) | ΔKE = ½ μ vrel² (μ = reduced mass) |
| Spring potential energy | U = ½kx² |
✏️ Solved NEET-level examples
Example 1 (Work & KE)
A 10 kg block is pulled 5 m on a horizontal surface by a force of 50 N at 37° to horizontal. If μk = 0.2, find work done by each force and final speed if starts from rest. (g=10, sin37=0.6, cos37=0.8)
Example 2 (Conservation of energy)
A block of mass 2 kg slides down a frictionless incline from height 5 m. Find speed at bottom and if it compresses a spring (k=1000 N/m) at bottom, find maximum compression.
Example 3 (Collision)
A 2 kg ball moving at 3 m/s collides elastically with a stationary 1 kg ball. Find velocities after collision.
📈 Important graphs & key points
- Force–displacement : area under F–x graph = work
- PE–distance for spring : parabola U = ½kx²
- KE & PE in SHM : interchange, total constant
- Power–time : area under P–t = work
⭐ Work done by conservative forces is path-independent and equals negative change in potential energy.
⚡ Quick revision box
⚠️ Common mistakes to avoid
- Using work formula W = F·s without considering angle θ.
- Forgetting that work against friction is negative and dissipates energy.
- Applying conservation of mechanical energy when non-conservative forces are present.
- In collisions, mixing up velocity signs – use consistent sign convention.
- Assuming power P = Fv always; only valid when force and velocity are in same direction.
🧠 Exam strategy tips
- Identify conservative vs non-conservative forces first.
- Use work-energy theorem to avoid kinematics when forces are variable.
- In collision problems, draw before/after diagrams with velocity directions.
- For spring problems, energy conservation is often easier than force methods.
- Memorize standard results for elastic collisions (they save time).
❓ Frequently asked questions
📥 Download toppers notes (free)
| 🔬 Physics NEET Toppers Notes | Download → |
| 🧬 Biology NEET Toppers Notes | Download → |
| ⚗️ Chemistry NEET Toppers Notes | Download → |
| 📐 Work Energy and Power Handwritten Notes | Download → |
⚡ Master Work, Energy and Power – the key to solving complex mechanics problems efficiently. Bookmark this page for last‑minute revision.