Work, Energy and Power for NEET UG
Energy methods often solve problems faster than forces. Learn the work–energy theorem, conservation of mechanical energy, power and collisions.
PhysicsClass 1170 MCQs4 lessons13 flashcards3 memory tricks
Practise Work, Energy and Power free
What you'll learn
- Work and the work–energy theorem
Calculate the work done by a force and use it to find a change in speed. - Potential energy and conservation of mechanical energy
Use KE + PE = constant to find speeds and heights when only conservative forces act. - Power
Calculate power from work and time, and from force and velocity. - Collisions
Tell elastic from inelastic collisions and find final velocities in one dimension.
Key formulas
| Work by a constant force | W = F s cos θ |
|---|---|
| Kinetic energy | K = ½mv² |
| Work–energy theorem | W_net = ΔK = ½mv² − ½mu² |
| Gravitational PE near Earth | U = mgh |
| Spring PE | U = ½kx² |
| Conservation (no friction) | K₁ + U₁ = K₂ + U₂ |
| Average power | P = W/t |
| Instantaneous power | P = F · v = Fv cos θ |
Memory tricks
W = Fd cos θ. Your upward force is at 90° to the horizontal motion, so cos 90° = 0. The same is true of the centripetal force in circular motion.
In a one-dimensional elastic collision between equal masses, the velocities are exchanged. Momentum and kinetic energy are both conserved.
K = p²/2m and p = √(2mK). With p fixed, K ∝ 1/m; with K fixed, p ∝ √m.
Try 3 NEET-style questions
Q1. A body of mass 2 kg moving at 10 m/s comes to rest. The work done by the retarding force is
- 100 J
- −100 J
- −200 J
- −20 J
Show answer
Answer: (2) −100 J
W = ΔKE = 0 − ½ × 2 × 100 = −100 J.
Q2. A spring of force constant 400 N/m is stretched from an extension of 0.1 m to 0.2 m. The work done on the spring is
- 2 J
- 4 J
- 6 J
- 8 J
Show answer
Answer: (3) 6 J
W = ½k(x₂² − x₁²) = ½ × 400 × (0.04 − 0.01) = 200 × 0.03 = 6 J.
Q3. A spring is stretched by 2 cm, storing 0.4 J. If it is stretched by 4 cm instead, the energy stored is
- 0.8 J
- 1.6 J
- 3.2 J
- 0.2 J
Show answer
Answer: (2) 1.6 J
U ∝ x²; doubling x makes U four times: 1.6 J.
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