Ch 5Work, Energy and Power — Class 11 Physics, concept-first.

The words work, energy, and power are part of our daily vocabulary. A farmer ploughing a field, a construction worker carrying bricks, a student studying for an exam — all are said to be "working." In…

Vectors can be multiplied in two fundamentally different ways. One way produces a scalar quantity — this is the scalar product (also called the dot product).

The chapter begins with the everyday notion of work — the harder you push and the farther something moves, the more work you do. But physics demands a precise, quantitative definition.

In everyday language, "work" means any physical or mental effort. In physics, the meaning is far more precise and narrow. Work is done only when a force actually causes a displacement.

When a moving object collides with something, it can do work — it can push, deform, or break the target. A fast cricket ball shatters a window; a slow one barely dents it.

In real life, forces are rarely constant. When you stretch a spring, the force you apply increases the further you pull. When a rocket lifts off, the thrust changes as fuel burns.

The work-energy theorem you already know — — was proved for a constant force. But forces in nature are rarely constant.

When you lift a book from the floor to a shelf, you do work against gravity. That work does not vanish — it gets stored in the book-earth system as potential energy.

The most powerful idea in this section is that for a system where only conservative forces do work, the total mechanical energy — the sum of kinetic energy and potential energy — never changes.

When you stretch or compress a spring, you do work against the restoring force that the spring exerts. That work does not disappear — it gets stored in the spring as elastic potential energy.

In everyday language, "power" is often confused with "force" or "energy." In physics, power has a precise meaning: it is the rate at which work is done or the rate at which energy is transferred from…

In physics, we study motion — change in position. But at the same time, we try to discover physical quantities that do not change during a physical process.

Collisions are everywhere — two billiard balls striking each other, a bat hitting a ball, or even molecules bouncing inside a gas.

When two objects collide, the forces they exert on each other are internal to the two-body system. If no external force acts during the collision, the total momentum of the system is conserved.

In the previous section, every collision happened along a single straight line. A ball hits a wall and bounces straight back; two gliders on an air track meet head-on.

- Work is done when a force causes displacement: . Work is a scalar; it is positive if , zero if , and negative if . - Kinetic energy of a body of mass moving with speed is .

1. Saying "calculate the work done" is vague. You must always specify which force (or group of forces) is doing the work, and over which displacement of the body. The context should make this clear.

Higher-order thinking / exemplar-style practice problems.