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.
Why this shows up in the exam
You must distinguish between these forces and calculate work done by each in NEET questions.
How NEET tests this
Learn the idea
A conservative force does the same work between two points no matter which path is taken, so its work over any closed loop is zero and it has an associated potential energy. A non‑conservative force (e.g., friction) depends on the actual path and always removes mechanical energy as heat.
🧠 Memory hook: Conservative forces Keep Energy (C‑K‑E), Non‑conservative forces Nix Energy (N‑Friction).
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 conservative force is path‑independent and zero for a closed path
- ΔU = –W_conservative for a conservative force
- Mechanical energy E = K+U remains constant when only conservative forces act
- Work done by non‑conservative forces = ΔK+ΔU (energy lost as heat)
- Friction always does negative work because it opposes motion
How to approach it
- 1Identify whether the force in the question is conservative or non‑conservative
- 2If the motion is along a closed loop, set work of any conservative force to zero; only non‑conservative work may remain
- 3Apply mechanical‑energy conservation (K+U = constant) when only conservative forces act
- 4When non‑conservative forces are present, add their work: ΔK = W_net = W_conservative+W_non‑conservative
- 5Use sign convention: work done against motion (friction) is negative
Worked example — watch it click
Assertion A: Work done by the force of friction moving a body around a closed loop is zero. Reason R: Work done does not depend upon the nature of the force.
- A)If both assertion and reason are true, and reason is the correct explanation of assertion.
- B)If both assertion and reason are true, but reason is not the correct explanations of assertion.
- C)If assertion is true, but reason is false.
- ✅If both assertion and reason are false.
The concept behind this problem
The example asks whether friction does zero work around a loop and whether work depends on the nature of the force, directly testing the definition of conservative vs non‑conservative forces and the path‑dependence of work.
Step by step
- 1Assertion A is false: Friction is a non-conservative force.
- 2Work done by friction around a closed loop is NOT zero; it is always negative (energy is dissipated as heat).
- 3For a conservative force, work around a closed loop would be zero.
- 4Reason R is false: Work done DOES depend on the nature of the force.
- 5Conservative forces (gravity, spring) have path-independent work; non-conservative forces (friction) have path-dependent work.
- 6Both statements are false.
Watch out
Thinking that because the net displacement after a closed loop is zero, friction also does zero work.
Common slip-ups that cost marks
- •Assuming all forces give zero work on a closed loop
- •Treating friction as path‑independent like gravity
- •Missing the negative sign of work done by friction
🌟 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.
The potential energy of a particle of mass 0.5 kg moving in the X-Y plane is given by U = (5.5x-7y) joule, x and y being in metre. If the particle starts from rest, the speed of the particle at time t = 4 s is nearly.
Push further
More challenging3 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.
Consider a system where a block is sliding down an inclined plane and then back up to its original height. Which of the following statements correctly compares the work done by gravity and the work done by friction during this complete motion?
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.
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.
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.
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.