INTRODUCTION

In everyday language, the word work covers almost any kind of effort, physical or mental -- studying for an exam, carrying groceries, or straining against an immovable wall are all casually called 'work.' Physics narrows this down to a precise, measurable quantity: work is done by a force only when that force actually displaces the body it acts on. Because doing work is precisely how energy gets transferred into or out of a system, physics defines energy simply as the capacity to do work -- which is also why work and energy share exactly the same SI unit (the joule) and the same dimensional formula, $[ML^2T^{-2}]$.

Energy shows up in a number of different forms -- mechanical, electrical, thermal, nuclear, and more -- and a great many machines exist purely to convert energy from one of these forms into another. This unit focuses specifically on mechanical energy, which itself comes in two varieties: kinetic energy (energy due to motion) and potential energy (energy due to position or configuration).

Once work and energy are pinned down, the natural next question is how fast work gets done or energy gets delivered -- that rate is called power. (A "powerful" strike in cricket, for instance, is simply one where the ball is hit at a high rate of energy delivery.) This unit aims to build a solid understanding of these three closely related quantities -- work, energy and power -- and their physical significance. By the end of it you should be able to:

• state the precise physics definition of work, and compute it for both a constant force and a variable force;
• distinguish kinetic energy from potential energy, including its gravitational and elastic forms;
• apply the law of conservation of energy to mechanical systems, including the special but important case of motion in a vertical circle;
• define power as the rate of doing work, and relate it to force and velocity; and
• classify collisions as elastic or inelastic and analyse them using conservation of momentum (and, where applicable, conservation of kinetic energy).