In physics, work has a precise meaning that differs from everyday usage. Work is done only when a force causes a displacement in the object on which it acts. Holding a heavy bag stationary — however tiring — does zero work in the physics sense.

The angle θ is between the force vector and the displacement vector. This angle is the most commonly mishandled element in NEET work problems.

When force changes with displacement, the simple formula W = Fs cosθ cannot be directly applied. Instead, the work done is calculated as the area under the Force-Displacement (F-x) graph.

Spring Force – The Classic Variable Force

The spring force follows Hooke’s Law: F = –kx, where k is the spring constant and x is the extension or compression. The work done in stretching a spring from natural length is:

Kinetic energy is the energy possessed by a body by virtue of its motion. It depends on both the mass and the square of the speed — making speed the dominant factor.

Work-Energy Theorem

The net work done on an object by all forces equals the change in its kinetic energy. This is one of the most powerful shortcuts in Work, Energy and Power class 11 problem-solving — it bypasses the need to find acceleration and use kinematics separately.

• If W_net is positive → object speeds up (KE increases)
• If W_net is negative → object slows down (KE decreases)
• If W_net is zero → speed remains constant (KE unchanged)

Potential energy is energy stored in a system due to the position or configuration of its components. It is always defined relative to a reference point and is associated exclusively with conservative forces.

Conservative vs Non-Conservative Forces

The total mechanical energy of a system — the sum of kinetic and potential energy — remains constant when only conservative forces act on it. This is the Law of Conservation of Mechanical Energy, one of the most frequently applied principles in Work, Energy and Power class 11 notes.

Free Fall – The Classic Demonstration

A ball dropped from height h has maximum PE and zero KE at the top. As it falls, PE converts to KE. Just before hitting the ground, all energy is kinetic. At any intermediate height h’, using conservation:

Power measures how quickly work is done. Two machines doing the same amount of work differ in power if they take different amounts of time — the faster one has more power. This concept is central to engineering and real-world physics applications.

• Instantaneous power = dot product of force and velocity vectors
• For a vehicle moving at constant velocity on a rough road: P = f_friction × v
• Efficiency η = (useful power output / total power input) × 100%

A collision is a short-duration interaction between two bodies during which they exert large forces on each other. Momentum is always conserved in collisions (no external net force on system). Kinetic energy conservation depends on the type of collision.

Maximum KE Loss in Inelastic Collision

NEET problems in Work, Energy and Power follow predictable patterns. Use this structured approach to eliminate errors and save time under exam pressure.

1. Identify all forces acting on the object (gravity, friction, applied, spring, normal)
2. Check if conservation of energy applies — are all forces conservative?
3. If yes, apply: KE₁ + PE₁ = KE₂ + PE₂
4. If friction present, use: KE₁ + PE₁ = KE₂ + PE₂ + |W_friction|
5. For power problems, first find work done, then divide by time (or use P = Fv)
6. For collisions, always apply momentum conservation first, then energy conservation if elastic

1. A person carrying a heavy load walks on a horizontal road. How much work does the normal force from the ground do on the person?
2. A body is moved along a closed loop by a conservative force. What is the net work done?
3. Two objects of equal mass collide head-on elastically, one initially at rest. What are the final velocities?
4. A spring is compressed by x₁ and then by 2x₁. Compare the work done in the two cases.
5. A car engine applies force F and the car moves at constant velocity v on a rough road. What is the power of the engine?
6. A ball is thrown upward with velocity v. At what height is the kinetic energy equal to the potential energy?
7. In a perfectly inelastic collision, is it possible for all the kinetic energy to be lost? Under what conditions?