Conservation of energy
The law of conservation of energy states that energy cannot be created or destroyed, only transformed, including cases with energy loss and efficiency considerations.
Why this shows up in the exam
NEET often asks you to track energy transformations and losses in physical systems.
How NEET tests this
Learn the idea
Conservation of energy means the total energy of an isolated system remains constant – it only changes form. The moment you know how much energy enters, you can find how much leaves by accounting for any losses or efficiencies.
🧠 Memory hook: Energy is like money in a bank – you never destroy it, you only transfer it to another account (with possible fees).
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- Work done by a force = F·s·cosθ
- Power = work done per unit time (W = ΔE/Δt)
- Gravitational potential energy = m g h
- Kinetic energy = ½ m v²
- Mechanical energy is conserved only when no non‑conservative forces act
- Efficiency η = (useful power output / power input) × 100
How to approach it
- 1Identify the type of energy being supplied or taken away (e.g., m g h for falling water).
- 2Calculate the energy per second – this is the input power. Use W = m g h for a mass flow rate ṁ (kg s⁻¹).
- 3If the question mentions losses or efficiency, multiply the input power by the appropriate factor (e.g., 0.9 for 10 % loss).
- 4Convert the final answer to the required unit (W, kW, etc.).
Worked example — watch it click
Water falls from a height of 60 m at the rate of 15 kg/s to operate a turbine. The losses due to frictional force are 10% of the input energy. How much power is generated by the turbine ? (g = 10 m/s²)
- A)7.0 kW
- B)10.2 kW
- ✅8.1 kW
- D)12.3 kW
The concept behind this problem
The worked example asks you to find the rate at which gravitational potential energy of the water is lost (input power) and then apply a 10 % loss, directly using the conservation‑of‑energy idea with efficiency.
Step by step
- 1Potential energy per second = mgh/s = 15 × 60 × 10 = 9000 W.
- 2Losses are 10%, so useful power = 0.9 × 9000 = 8100 W = 8.1 kW.
Watch out
Students often subtract 10 % from the height or mass instead of from the calculated power, giving a wrong answer.
Common slip-ups that cost marks
- •Using g = 9.8 m s⁻² when the problem explicitly gives g = 10 m s⁻².
- •Treating a percentage loss as a reduction in height or mass instead of a reduction in the calculated power.
- •Forgetting to multiply the mass flow rate by g h – the energy per second is not just m g h per kilogram.
- •Mixing up kinetic and potential energy signs when non‑conservative forces are present.
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Practise it
These are real questions from past NEET papers that test this exact idea.
Water falls from a height of 60 m at the rate of 15 kg/s to operate a turbine. The losses due to frictional force are 10% of the input energy. How much power is generated by the turbine ? (g = 10 m/s²)
Push further
More challenging4 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.
A small object of mass 'm' is attached to a light rod of length 'L' and is released from rest when the rod is horizontal. What is the tension in the rod when the object is at the lowest point of its circular path?
More from Work, Energy and Power
Work and its calculation
Work is the energy transferred by a force acting over a distance, and can be calculated using the dot product, area under a force-displacement graph, or for variable and constant forces.
Work-energy theorem
The work-energy theorem states that the net work done on an object equals the change in its kinetic energy, and applies to both constant and variable forces.
Conservative and non-conservative forces
Conservative forces, like gravity and spring force, conserve mechanical energy, while non-conservative forces, like friction, dissipate energy as heat.
Elastic potential energy
Elastic potential energy is the energy stored in a stretched or compressed spring, proportional to the square of its displacement.
Power
Power is the rate at which work is done or energy is transferred, and can be calculated as the product of force and velocity at any instant.
Work by a Constant Force
For a constant force, work is the scalar product of force and displacement, so its sign is set by the angle between those vectors and not by force magnitude alone.