Work, Energy & Power

Why This Chapter Matters

Work, Energy and Power together supply 2 to 4 questions in almost every SSC and RRB physics paper, and the questions repeat the same handful of ideas year after year: SI units, the difference between kinetic and potential energy, and which simple machine gives what mechanical advantage. This is one of the highest return-per-minute topics in the whole syllabus. You are not learning new physics from scratch here either — you already feel work and energy every day, lifting a bucket, pedalling a cycle uphill, watching a ceiling fan slow down after you switch it off. This chapter turns that lived experience into exam-ready facts.

The single biggest mistake aspirants make is treating "work" the physics way and "work" the everyday way as the same thing. In daily life, holding a heavy school bag on your shoulder for an hour feels like hard work. In physics, if the bag does not move, you have done zero work on it, because physics work needs actual displacement in the direction of the force. Examiners love this exact confusion. Fix it now and you will never lose that mark again.

What Is Work

In physics, work is done only when a force causes displacement. If you push a wall with all your strength and the wall does not budge, you get tired but you have done no work on the wall, because there is no displacement. This single condition, force AND displacement together, is the whole definition.

The formula, kept as simple as possible: Work = Force × Displacement (in the direction of the force). If the force and displacement point the same way, work is positive. If they point opposite ways, like friction acting against a moving box, work is negative. If the force is applied but there is no displacement at all, or the force is perpendicular to the displacement, work is zero.

Exam trap: a coolie carrying a suitcase on his head and walking on level ground does zero work on the suitcase according to physics, because the force he applies (upward, to support the weight) is perpendicular to his displacement (horizontal, as he walks). This exact scenario, coolie or porter carrying a load horizontally, is one of the most repeated "zero work" questions in SSC papers.

SI unit of work: joule (J). One joule is the work done when a force of one newton moves an object by one metre in the force's direction. A joule is a genuinely small amount of energy, roughly what it takes to lift a small apple by about a metre. That is exactly why energy bills are measured in kilowatt-hour, a much bigger practical unit, not in joules.

What Is Energy

Energy is the capacity to do work. Anything that can do work possesses energy, and the amount of energy something has equals the maximum work it can perform. This is the cleanest possible definition and examiners test it directly: "energy is defined as..." questions expect exactly this phrase.

SI unit of energy: joule (J), same as work, because energy and work are two sides of the same coin. You spend energy to do work, and doing work on something gives it energy. A stretched bow has energy stored in it; releasing that energy does the work of launching the arrow.

Analogy: think of energy as money in your pocket and work as the amount you spend on a purchase. You cannot spend more money than you have, and you cannot do more work than the energy you possess. This is why energy is always measured in the same unit as work, the joule, and why "conservation of energy" is really just physics saying nobody can spend money they never had.

Forms of Energy

Energy shows up in many forms, and SSC loves asking you to identify which form is at play in a given everyday scenario.

• Mechanical energy: energy due to motion or position (a moving train, water stored in a dam).
• Heat (thermal) energy: energy due to the random motion of molecules (a hot tawa, boiling water).
• Light energy: energy carried by light, from the sun or a bulb.
• Sound energy: energy carried by vibrations through a medium.
• Chemical energy: stored in chemical bonds, released in reactions (food in your body, petrol in an engine, a battery).
• Electrical energy: energy of flowing charge (current through a wire).
• Nuclear energy: released from the nucleus of an atom, either by splitting it (fission, used in nuclear power plants) or combining nuclei (fusion, the process powering the sun).

Memory hook: remember the seven forms as "Many Hens Lay Small Chicken Eggs, Nightly" — Mechanical, Heat, Light, Sound, Chemical, Electrical, Nuclear. A slightly odd sentence sticks better than a boring list, and that oddness is exactly why you will still recall it in the exam hall.

Every energy conversion device you use daily is really just one form of energy changing into another: a bulb converts electrical energy to light (and unwanted heat), a fan converts electrical energy to mechanical energy, your body converts the chemical energy in food into mechanical energy when you walk. SSC often asks "electrical energy is converted into mechanical energy in a ______" (answer: electric motor, fan, mixer) so learn devices as conversion pairs, not just as forms.

Kinetic Energy and Potential Energy

Mechanical energy itself splits into two types, and this pair is asked constantly, almost every year in some paper.

Kinetic energy (KE) is the energy a body has because of its motion. A moving bus, a flying cricket ball, a running child, all possess kinetic energy. The faster something moves, and the heavier it is, the more kinetic energy it carries. In simple words: KE depends on mass and speed, and it grows very fast with speed, because doubling the speed of an object actually quadruples its kinetic energy, not just doubles it. This is exactly why a car crash at 80 km/h is far more than twice as dangerous as one at 40 km/h, and why speed limits matter so much on highways.

Potential energy (PE) is stored energy due to position or configuration, not motion. Water held behind a dam, a stretched rubber band, a compressed spring, a stone lifted to the roof, all have potential energy because of where they are or what shape they are held in, waiting to be released.

Analogy: think of a cricket bowler at the top of his run-up, standing still, ball in hand, about to sprint in. That pause is potential energy, all stored, nothing spent yet. The moment he starts running in and releases the ball, that stored energy converts into kinetic energy, motion. Every roller coaster ride at a fair works the same way: potential energy at the top of the first hill, converting to kinetic energy as it plunges down.

Exam trap: students often think a body at rest has zero energy. Wrong. A stone sitting on the roof of a building is at rest, its kinetic energy is indeed zero, but it still has potential energy because of its height. "At rest" only rules out kinetic energy, not potential energy.

Feature	Kinetic Energy	Potential Energy
Cause	Motion of the body	Position or state of the body
Depends on	Mass and speed	Mass, height (or deformation)
Example	Moving car, flying bird, flowing river	Water in a dam, stretched bow, raised hammer
Value when body is at rest	Zero	Can still be non-zero
Common device use	Windmill (wind's KE), hydro turbine (falling water's KE)	Dam, wound clock spring

Law of Conservation of Energy

This is the single most important line in the whole chapter: energy can neither be created nor destroyed, it can only be converted from one form to another. The total energy in an isolated system always stays constant.

A swinging pendulum is the textbook favourite example, and worth knowing cold. At the two extreme ends of its swing, the pendulum momentarily stops, so its kinetic energy is zero and its potential energy is maximum. At the lowest point of the swing, it moves fastest, so kinetic energy is maximum and potential energy is minimum. At every point in between, the sum of kinetic and potential energy stays exactly the same (ignoring air resistance and friction at the support). Nothing is lost, energy just keeps changing form between KE and PE as the pendulum swings back and forth.

A falling coconut shows the same idea outside a classroom. High on the tree, it has maximum potential energy and zero kinetic energy. As it falls, potential energy keeps converting into kinetic energy, and just before it hits the ground, kinetic energy is at its maximum. Add up KE and PE at any instant during the fall (ignoring air resistance) and you get the same total throughout.

Exam trap: "energy is lost as heat due to friction" sounds like energy is destroyed, but it is not. That heat energy is simply another form the original energy has converted into. Total energy of the universe never actually decreases; it just spreads into less useful forms like heat. Keep this distinction sharp because SSC sometimes frames trick statements around it.

Power

Power is the rate of doing work, or equivalently, the rate at which energy is transferred or converted. Two people might lift the same 50 kg sack to the same height, doing identical work, but if one does it in 5 seconds and the other takes 50 seconds, the first person has developed ten times more power. Power is about speed of work, not amount of work.

SI unit of power: watt (W), named after James Watt. One watt equals one joule of work done per second. In practical, everyday use, especially for larger machines and vehicles, power is often expressed in horsepower (hp), a unit that literally compares an engine's output to the pulling power of a horse. 1 horsepower is approximately 746 watts, a conversion figure SSC has asked directly.

Exam trap: students confuse energy units and power units constantly. Joule is a unit of work/energy. Watt is a unit of power. Kilowatt-hour, despite having "hour" in the name, is actually a unit of energy (the energy consumed by a 1000-watt appliance running for one hour), not power. Your electricity meter at home literally counts kilowatt-hours, which is why your bill calls them "units" of electricity.

Simple Machines and Mechanical Advantage

A simple machine is a basic device that makes work easier, usually by letting you apply less force over a longer distance, or by changing the direction of the force you apply. Simple machines do not create energy out of nothing; they only make a task more convenient by trading force for distance, or vice versa. This trade-off is measured by a number called mechanical advantage, defined as the ratio of the load (output force, the weight you're moving) to the effort (input force, the force you apply). A mechanical advantage greater than 1 means the machine is helping you, letting a smaller effort lift a bigger load.

Lever: a rigid rod that turns around a fixed point called the fulcrum. A see-saw, a pair of scissors, a crowbar prying open a crate, all are levers. By resting the rod on a fulcrum closer to the heavy load, a lever lets a small effort at the far end move a much heavier load, exactly how a small child can lift an adult on a see-saw if the fulcrum sits closer to the adult's side.

Pulley: a wheel with a grooved rim that a rope runs over, used to lift loads, classically seen at a well where a bucket is hauled up using a rope over a wheel. A single fixed pulley just changes the direction of your pull (you pull down to lift up), which is convenient but gives no mechanical advantage. Combining multiple pulleys (a pulley system) does give real mechanical advantage, letting you lift a heavier load with less effort, though you then have to pull a longer length of rope, a fair trade in physics: nothing is free.

Inclined plane: a sloped surface, like a ramp used to roll a heavy drum onto a truck instead of lifting it straight up. Pushing the drum up the slope needs less force than lifting it vertically, but you push it over a longer distance. This is exactly why hilly roads are built as zigzag switchbacks instead of running straight up the slope: a gentler, longer path needs less force at any moment than a short, steep climb.

Memory hook: remember the three classic simple machines covered here with "Little Puppies Incline", Lever, Pulley, Inclined plane. Each one trades effort for distance in its own way, but the underlying principle, easier work through a longer path, is identical across all three.

Exam trap: a simple machine never lets you do less total work; it only lets you apply less force, spread over a greater distance. Total work done (ignoring friction losses) stays the same with or without the machine. This is a favourite SSC conceptual trap: "does a pulley reduce the work done?" No, it reduces the effort needed, not the work.

Quick Revision — One-Line Facts

• Work is done only when force causes actual displacement.
• SI unit of work is the joule (J).
• Work done by a force perpendicular to displacement is always zero.
• SI unit of energy is also the joule (J), same as work.
• Energy is defined as the capacity to do work.
• Seven common forms of energy: mechanical, heat, light, sound, chemical, electrical, nuclear.
• Kinetic energy is energy due to motion; it depends on mass and the square of speed.
• Doubling an object's speed quadruples its kinetic energy.
• Potential energy is stored energy due to position or configuration.
• A body at rest can still have potential energy even though its kinetic energy is zero.
• Law of conservation of energy: total energy in an isolated system stays constant; energy only changes form.
• In a swinging pendulum, KE is maximum at the lowest point and PE is maximum at the two extreme ends.
• Power is the rate of doing work, or rate of energy transfer.
• SI unit of power is the watt (W), named after James Watt.
• 1 horsepower is approximately 746 watts.
• The joule is a unit of work/energy; the watt is a unit of power; do not mix them up.
• Kilowatt-hour is a unit of energy, not power, despite the "hour" in its name.
• A simple machine makes work easier by trading force for distance or by changing force direction.
• Mechanical advantage = load (output force) divided by effort (input force).
• A lever turns about a fixed point called the fulcrum.
• A crowbar, scissors, and see-saw are all examples of levers.
• A single fixed pulley changes the direction of force but gives no mechanical advantage on its own.
• Combining multiple pulleys can give real mechanical advantage, at the cost of pulling a longer rope.
• An inclined plane (ramp) reduces the force needed to raise a load, but increases the distance travelled.
• Simple machines reduce the effort needed for a task; they do not reduce the total work done.
• A stretched bow and a wound clock spring are both examples of stored potential energy.
• Nuclear fission splits a nucleus to release energy; nuclear fusion combines nuclei, as happens in the sun.
• An electric motor converts electrical energy into mechanical energy.
• A falling object continuously converts potential energy into kinetic energy as it falls.
• Work can be positive, negative, or zero depending on the angle between force and displacement.
• Friction acting against a moving object does negative work on it.

Memory Tables

Quantity	Formula (simple form)	SI Unit	Unit Named After
Work	Force × Displacement	Joule (J)	James Prescott Joule
Energy	Capacity to do work	Joule (J)	James Prescott Joule
Power	Work ÷ Time	Watt (W)	James Watt
Power (practical)	—	Horsepower (hp), 1 hp ≈ 746 W	—

Simple Machine	What It Does	Everyday Example
Lever	Small effort at one end lifts a heavy load using a fulcrum	Crowbar, scissors, see-saw
Pulley (single, fixed)	Changes direction of force, no mechanical advantage alone	Well bucket, flagpole
Pulley (multiple)	Reduces effort needed at the cost of longer rope pulled	Crane, block-and-tackle system
Inclined plane	Reduces force needed by increasing distance travelled	Loading ramp, hill road switchback

Practice MCQs

Q1. What is the SI unit of work? (a) Newton (b) Joule (c) Watt (d) Pascal

Q2. A porter carries a suitcase on his head and walks 10 metres on level ground. According to physics, the work done by him on the suitcase is: (a) Maximum (b) Negative (c) Zero (d) Cannot be determined

Q3. Energy is defined as: (a) The rate of doing work (b) The capacity to do work (c) Force applied over time (d) The speed of an object

Q4. Which form of energy does a stretched rubber band possess? (a) Kinetic energy (b) Chemical energy (c) Potential energy (d) Nuclear energy

Q5. The SI unit of power is the: (a) Joule (b) Newton (c) Watt (d) Horsepower

Q6. One horsepower is approximately equal to: (a) 74.6 watts (b) 746 watts (c) 7460 watts (d) 1000 watts

Q7. In a lever, the fixed point about which the rod turns is called the: (a) Pivot arm (b) Load point (c) Fulcrum (d) Axis point

Q8. According to the law of conservation of energy: (a) Energy can be created but not destroyed (b) Energy can be destroyed but not created (c) Energy can neither be created nor destroyed, only converted (d) Total energy of a system keeps decreasing over time

Q9. A pendulum bob at the lowest point of its swing has: (a) Maximum potential energy, zero kinetic energy (b) Maximum kinetic energy, minimum potential energy (c) Zero kinetic energy and zero potential energy (d) Equal kinetic and potential energy always

Q10. If the speed of a moving car is doubled, its kinetic energy becomes: (a) Two times (b) Three times (c) Four times (d) Unchanged

Q11. Kilowatt-hour is a unit of: (a) Power (b) Force (c) Energy (d) Current

Q12. A single fixed pulley used to draw water from a well: (a) Reduces the effort needed (b) Increases the load automatically (c) Changes the direction of the applied force without giving mechanical advantage (d) Has no practical use

Q13. Mechanical advantage of a machine is defined as: (a) Effort divided by load (b) Load divided by effort (c) Work done divided by time (d) Force divided by displacement

Q14. Two workers lift identical sacks to the same height, but Worker A takes half the time Worker B takes. Which statement is correct? (a) Worker A does more total work than Worker B (b) Worker A develops more power than Worker B (c) Worker B develops more power than Worker A (d) Both do equal work and equal power

Q15. Nuclear fusion, the process that powers the sun, involves: (a) Splitting of a heavy nucleus into lighter nuclei (b) Combining of lighter nuclei into a heavier nucleus (c) Conversion of light energy into heat energy (d) Loss of electrons from an atom

Answer Key

Q	Answer	Reason
Q1	(b) Joule	Work and energy share the same SI unit because energy is simply the capacity to do work.
Q2	(c) Zero	The upward support force on the suitcase is perpendicular to the horizontal displacement, so no work is done in the physics sense.
Q3	(b) The capacity to do work	This is the core definition tested repeatedly; power is a rate, not a definition of energy.
Q4	(c) Potential energy	A stretched rubber band stores energy due to its deformed shape, released when it snaps back.
Q5	(c) Watt	Named after James Watt; one watt equals one joule of work done per second.
Q6	(b) 746 watts	This exact conversion figure is a favourite direct-recall fact in SSC papers.
Q7	(c) Fulcrum	The fulcrum is the fixed pivot point that lets a lever trade effort for distance.
Q8	(c) Energy can neither be created nor destroyed, only converted	This is the exact wording of the law; energy only changes form, the total stays constant.
Q9	(b) Maximum kinetic energy, minimum potential energy	Speed is highest at the lowest point of the swing, so kinetic energy peaks there while height, and hence potential energy, is at its minimum.
Q10	(c) Four times	Kinetic energy depends on the square of speed, so doubling speed quadruples kinetic energy, not just doubles it.
Q11	(c) Energy	Despite containing "hour," a kilowatt-hour measures total energy consumed, exactly what your electricity bill charges you for.
Q12	(c) Changes the direction of the applied force without giving mechanical advantage	A single fixed pulley only lets you pull down to lift up; real mechanical advantage needs multiple pulleys.
Q13	(b) Load divided by effort	Mechanical advantage compares the output force a machine delivers to the input force you apply.
Q14	(b) Worker A develops more power than Worker B	Both do equal work, but Worker A does it faster, and power is the rate of doing work, so less time means more power.
Q15	(b) Combining of lighter nuclei into a heavier nucleus	Fusion joins light nuclei (like hydrogen) into heavier ones, releasing the energy that powers the sun; fission is the opposite, splitting a heavy nucleus.