TNPSC Daily Notes: Work, Energy & Power — Simple Machines, Mechanical Advantage & Conservation of Energy | வேலை, ஆற்றல் மற்றும் திறன் — எளிய இயந்திரங்கள், இயந்திர நலன் மற்றும் ஆற்றல் அழியா விதி — 10 November 2026

1. What physics means by “work”

In everyday speech a student who reads for six hours has “worked hard”. In physics nothing of the kind has happened, because work is done only when an applied force actually moves its point of application. The measure is the product of the force and the displacement taken along the direction of the force: W = F s cosθ, where θ is the angle between the force vector and the displacement vector. Three consequences follow immediately. When the force and displacement point the same way (θ = 0°) the work is positive and maximum, Fs. When they are at right angles (θ = 90°, cos 90° = 0) the work is exactly zero — which is why a porter walking on level ground with a trunk on his head does no work on the trunk, and why the moon, held by a centripetal pull that always points towards the earth, has no work done on it over a full orbit. When the force opposes the motion (θ = 180°) the work is negative, as with friction on a sliding block or gravity acting on a cricket ball rising towards its highest point. Being a product of two vectors in this particular way, work is a scalar: it has magnitude but no direction. Its SI unit, the joule, is the work done when a force of one newton moves a body one metre along its own line of action.

2. Energy: the capacity to do work

Energy and work share the same unit precisely because energy is defined as the capacity to do work. The two mechanical forms examined most often by TNPSC are kinetic and potential. Kinetic energy, the energy a body possesses because it is moving, is KE = ½mv², which can also be written p²/2m in terms of momentum p. The squared dependence on speed is the single most examinable idea in the chapter: tripling the speed of a bus multiplies its kinetic energy nine-fold, so the distance needed to stop it grows nine-fold too. Potential energy is energy of position or configuration; the gravitational form near the earth’s surface is PE = mgh, while a compressed or stretched spring stores elastic potential energy ½kx². The work–energy theorem ties the two concepts together: the net work done on a body equals the change in its kinetic energy. The law of conservation of energy then generalises this — energy is never created or destroyed, only converted. A stone dropped from a tower starts with all its mechanical energy as potential energy and arrives with all of it as kinetic energy, the sum KE + PE staying equal to mgh at every intermediate height. Familiar conversions worth memorising include the electric motor (electrical to mechanical), the dynamo or generator (mechanical to electrical), the microphone (sound to electrical), the loudspeaker (electrical to sound), the solar cell (light to electrical), the battery (chemical to electrical) and the nuclear reactor, where Einstein’s E = mc² shows that even mass is a store of energy.

3. Power and the units that follow from it

Power answers a different question from work: not how much was done, but how fast. P = W/t, and since W = Fs it can also be written P = Fv, a form useful whenever a vehicle moves at steady speed against a constant resistance. The SI unit is the watt, one joule per second; a kilowatt is a thousand watts and the older engineering unit, one horsepower, equals 746 watts. Because the joule is far too small for domestic billing, electricity boards sell energy in kilowatt-hours: one unit of current consumption is one kilowatt drawn for one hour, equal to 3.6 × 10⁶ joules. Candidates should also keep the smaller units in mind — the erg (10⁻⁷ J) from the CGS system, the calorie (4.186 J) used for heat and nutrition, and the electron-volt (1.602 × 10⁻¹⁹ J) used in atomic physics.

4. Simple machines, mechanical advantage and efficiency

A simple machine is a device that lets a small effort overcome a large load, or that changes the direction in which a force is conveniently applied. Three quantities describe its performance. Mechanical Advantage is the ratio of load to effort and tells us how many times the force has been multiplied. Velocity Ratio is the ratio of the distance moved by the effort to the distance moved by the load, and depends only on the geometry of the machine. Efficiency is the useful work output divided by the work input, expressed as a percentage, and equals (MA / VR) × 100. For a hypothetical frictionless machine MA equals VR and the efficiency is 100%; in every real machine some input work is spent against friction and in lifting the machine’s own parts, so MA is less than VR and the efficiency is always below 100%. No machine can ever create energy — it only redistributes force against distance.

5. Levers, pulleys and the inclined plane

The lever is a rigid bar turning about a fulcrum, and it is classified by which of the three points lies between the other two. In a first-class lever the fulcrum is in the middle: a see-saw, a pair of scissors, a crowbar and the beam balance, where MA may be greater or less than one. In a second-class lever the load is in the middle, as in a wheelbarrow, a nutcracker or a bottle opener, and the mechanical advantage is always greater than one. In a third-class lever the effort is in the middle — the human forearm lifting a weight, a pair of sugar tongs, a stapler — and the mechanical advantage is always less than one, the machine being used for speed and control rather than force multiplication. A single fixed pulley has MA = 1 and merely redirects the effort, which is why a flag is raised by pulling down; a single movable pulley doubles the effort, MA = 2; and a block-and-tackle carrying the load on n segments of rope gives MA = n. The inclined plane, used for loading drums onto a lorry, has MA equal to the ratio of its length to its height, so a gentler slope multiplies force more; the screw is an inclined plane wrapped around a cylinder, its pitch being the distance advanced per complete turn; and the wheel and axle has MA equal to the radius of the wheel divided by the radius of the axle.